1
ELEMENT A: SUBSTRUCTURE
INTRODUCTION
Well-designed foundations are a necessity to building design. A basic understanding of factors that influence facility substructure design—bearing strata, settlement, and the effects of adjacent structures, slopes, and building modification that either physically expand the structure or change the use of a structure—is essential for good building design.
This chapter provides a basic vocabulary for the design team to use to communicate when assisting in the design of the optimum foundation system to satisfy cost, schedule, and building constraints. This chapter also focuses on the work of the geotechnical engineer, who, together with the structural engineer, creates solutions to complex design constraints. When engineering insight is combined with practical construction methods to produce structures that support increasingly larger loads in more efficient way, it reduces the risk that the effectiveness of structures above grade will be diminished by a misunderstanding or lack of attention to an important detail below grade.
In addition to exploring soils and geotechnical investigation, this chapter examines climatic and seismic considerations relating to the facility substructure and provides a review of special foundations, including basement construction. The topic of basement construction addresses basement excavation, soil support, shoring strategies, and basement wall construction, using both concrete and masonry, and methods of waterproofing or dampproofing these elements.
Contributor:
John P. McCarthy, PE, SE, Smith Group Architecture, Engineering, Interiors, Planning, Detroit, Michigan.
ELEMENT A: SUBSTRUCTURE SOILS AND SOILS EXPLORATIONS SOILS AND SOILS EXPLORATIONS
Bringing together project team design professionals, including geotechnical engineers, structural engineers, and architects to discuss the matter of soils and foundations is fundamental to ensure that the foundation selected satisfies the constraints of the project budget as well as the functionality of the structure.
Understanding the vocabulary of geotechnical science (for example, the difference between “cohesive” and “cohesionless” soils) is the first step toward fostering collaborative communication, which becomes increasingly important as the process continues. What should be tested, what the test should be, why it is important, and what the limitations of the test are must be addressed. Likewise, identifying foundation and ground modification alternatives (as well as their pros and cons) will aid in the preliminary design phase, when the building foundations are being developed.
Understanding the geotechnical investigation report and geographical variations such as climate and seismic conditions will assist the design professionals in discussing important foundation issues.
SOILS DEFINITIONS: TERMS AND CLASSIFICATIONS
It is critical that geotechnical and structural engineering information be understood properly; to that end, the following definitions of common soils and other terms are included for reference:
• Clay: Determined by the size of particles and composition, clays are chemically different from their parent materials as a result of weathering. Clays are typically inorganic and have grain sizes less then 0.0002 in. in diameter. This material contains charged particles and has an affinity for water. Because of their size and chemical composition, clays exhibit cohesion and plasticity. Clays can be classified as stiff, medium, or soft, depending on the moisture content, with drier clays typically being stiffer. Clays make a satisfactory bearing material under some conditions. Long-term settlement can sometimes control the allowable bearing pressure. Because of the cohesive nature of clay, excavations can have steep slopes for short periods of time.
• Silt: Silt consists of inorganic particles between 0.003 in. and 0.0002 in. in diameter. These fine-grained particles are similar in composition to the rocks from which they are derived, and are not plastic in nature. Organic silt is found on the bottom of lakes and river deltas.
• Sand: Classifications of sand vary from fine to coarse, these rock particles range in size from 0.003 in. to 0.079 in. in diameter. Adequately compacted, sand makes an ideal bearing material. The coarser the sand, the higher the allowable bearing pressures. Fine sands are susceptible to becoming quick when subjected to unbalanced hydrostatic pressures, and may liquify when they are loose, saturated, or subjected to seismic forces. Settlement is usually immediate, with little long-term settlement.
• Gravel: Classifications of gravel vary from fine to coarse, and these unconsolidated rock fragments range from .75 in. to about 3 in. Except for gravels composed of shale, this material makes a good foundation material. Depending on the compactness and the underling material, very high bearing pressures are allowed by some building codes.
• Cobbles: Ranging in size from about 3 in. to about 10 in., these rock fragments can make reliable foundation-bearing materials, but can be difficult to properly compact when used for fill. Cobble-sized materials can interfere with pile driving and drilled-pier construction causing significant problems.
• Boulders: Typically classified as rock fragments greater than 10 in., boulders can be used as part of a fill mass if the voids between the boulders are filled with finer-grained sands and silts. These materials are generally not considered suitable for direct foundation support because of their size and shape, and the difficulty in excavating the material to desired shapes. As with cobbles, boulders can cause significant problems during construction.
• Bedrock: Unbroken hard rock that is not over any other material is considered bedrock. Depending upon its composition, it can be capable of withstanding extremely high bearing pressure, and is desirable for foundations supporting high loads. If the rock has been weathered or is cracked, its bearing capacity may be compromised. Settlement of buildings on bedrock is primarily limited to the elastic settlement of the foundation.
• Residuum: Residuum consists of soil derived from the in-place decomposition of bedrock materials. In general, these soils are more weathered near the surface, and gradually transition to a more rocklike material with depth. Where residual soils reveal evidence of the stratification and structure of the parent rock, they are known as saprolitic materials.
• Alluvial soils: Because materials are eroded, transported, and deposited through the action of flowing water, these soils are typically loose and saturated, hence often are unsuitable for support of structures or pavements.
• Colluvial soils: Because materials are transported by gravity, typically associated with landslides, these soils are generally irregular in composition and loose. They require improvement prior to being used to support buildings and pavements.
• Aeolian soils: These soils are transported and deposited by the wind. Typically, they consist of silt or sand-sized soils. Loess, one of the more common types of aeolian soils, is composed of fine cemented silt. While this material may be competent in place, it loses much of its strength when disturbed or recompacted.
• Till: Till is a mixture of clay, silt, sand, gravel, and boulders deposited by glaciers. Consolidated tills that are well graded (indicated by a uniform distribution of particle size) are exceptionally strong and make excellent foundation strata. Loose tills can cause differential settlements if used as a bearing material.
• Loam: This organic material, made up of humus and sand, silt, or clay, provides excellent material for agriculture but should not be used for foundations. Organic materials will settle a great deal over time, and even lightly loaded slabs on grade will settle if bearing on loam.
• Cohesionless soils: These types of soils consist of cobbles, gravels, sands, and nonplastic silts. They are generally formed from the mechanical weathering of bedrock brought about by water, ice, heat, and cold. They are typically composed of the same minerals as the parent rock. The strength of cohesionless materials is derived primarily from interparticle friction.
• Cohesive soils: These types of soils contain clay minerals with an unbalanced chemical charge. As a result, they tend to attract water and bond together. The strength of cohesive materials is derived from a combination of these chemical bonds and from interparticle friction.
SOIL TYPES AND THEIR PROPERTIES
1.1
• Consolidation: When soils are subjected to loads, water within the void spaces initially supports the change in stress through an increase in pressure. Excess pressures gradually dissipate in proportion to the permeability of the soil. Coarse-grained materials drain rapidly, while finer-grained silts and clays drain more slowly. As the excess pore pressures dissipate, the void spaces compress and transfer the loads to the soil grains. The resulting reduction in volume over time is known as consolidation.
• Underconsolidated soils: Soils that have built up in river deltas and other water bodies are deposited in a very loose state. These soils are often underconsolidated, in that they have never experienced stresses equal to or greater than current overburden stresses. These materials tend to consolidate under their own weight over time, until all excess pore pressures have been dissipated and the soils become “normally consolidated.” Foundations bearing on underconsolidated soils can typically expect large short- and long-term settlement.
• Overconsolidated soils: Unlike many other types of materials, soils are not elastic. When stresses are applied to soils, they compress. However, when the same stress is removed, they do not rebound to the same height. When reloaded, the soils “remember” previously loaded conditions and compress to their historical level of stress. Soils that have previously been loaded to stresses above those created by the current soil overburden are considered to be overconsolidated. Foundations bearing on overconsolidated soils can typically expect less short- and long-term settlements.
• Desiccation: All soils typically contain some moisture within the voids between soil particles. When soils are dried, capillary tension tends to pull the soil grains together, causing the soil to shrink and lose volume. This action can cause the soil to become overconsolidated, as the capillary tension results in stress.
SOIL STUDIES AND REPORTS
READING A SOILS REPORT
A geotechnical report helps the design team understand the site on which the structure is to be built. Most geotechnical reports contain the following information, based on the previously defined scope of exploration:
• Report summary
• Project information
• Exploration methods
• Description of soil and groundwater conditions
• Design recommendations
• Construction considerations
• Appendix
• Location diagram
• Soil-boring or test pit logs
• Soil profiles
• Laboratory test results
The report summary is generally one to two pages long, and provides the most salient information and recommendations of the report. Use the summary as quick reference, but read the entire report for details and qualifications/limitations. Most reports can be read within 30 minutes. Check and verify the project information and criteria (i.e., building height, structural loads, floor/basement levels, and so on). The scope of the evaluation and recommendations are based on this information. Also included in the report would be project information describing the building and site characteristics such as number of stories, building construction materials, foundation loadings, basement data if applicable, and grades. The exploration section defines how the geotechnical engineer obtained the soil information required to describe the foundation this would include number, location and depth of soil borings and test pits, and laboratory and field testing to be performed.
The general soil and groundwater conditions include a general overview of the results of the geotechnical engineer’s tests. More detailed information is contained in the soil-boring and test pit logs, which can be reviewed when required.
The design recommendations section is of greatest interest to the project design team, as it makes specific recommendations concerning the design of foundations, grade slab, walls, drainage requirements, and other key building components. It should be read together with the section on construction considerations, which identifies potential problems during construction that can be avoided or minimized by both the design team and contractor when everyone understands the challenges for the project.
Often reports will provide a transverse section of the soil profile, combining the soil-boring information in a convenient picture. This will enable the reader to better understand approximately how the soil properties vary across the site.
CLIMATIC FOUNDATION ISSUES
DESIGNING FOR COLD AND UNDERHEATED CLIMATES
Cold and underheated climate conditions occur over the northern half of the United States and in mountainous regions. These conditions can be generally quantified as where the frost depth is 12 in. or greater. Designing foundations for these conditions is treated in a more typical manner, such as: providing a foundation below the frost depth, including a basement, and providing insulation on the exterior to reduce the chances of cold ground temperatures reaching the structure.
SLAB-ON-GRADE CONSTRUCTION IN COLD CLIMATES
1.2
BASEMENT CONSTRUCTION IN COLD CLIMATES
1.3
ENERGY-EFFICIENT WALL SECTIONS FOR UNDERHEATED CLIMATES
1.4
FROST ISSUES
Detrimental frost action in soils is obviously limited to those areas of the United States where subfreezing temperatures occur on a regular basis and for extended periods of time. “Frost action,” as used in this context, is the lateral or vertical movement of structures supported on or in the soil. Frozen soil is, in itself, not necessarily detrimental to the supported structures. It becomes detrimental when, through the growth of ice lenses, the soil, and whatever is resting on the soil above the ice lenses, heaves upward. This causes foundations and the structures supported by the foundations to distort and suffer distress. Other common problems are the heaving of sidewalks, pavements, steps, retaining walls, fence poles, and architectural features.
The depth of frost penetration is directly related to the intensity and duration of the freezing conditions, a measure that is termed the freezing degree day index. In milder climates in the United States, the local building codes might stipulate a frost protection depth for foundations of 12 in. In the northern portions of the United States, the frost protection depth might be 42 to 60 in. as required by local building codes. These guidelines are usually conservative, but there are situations where deeper frost protection depths are warranted. For instance, if the emergency entrance to a hospital is on the north side of the hospital, where the sun never warms the pavement adjacent to the building, and the pavement is kept 100 percent snow-free for safety reasons, then the frost penetration can easily exceed the code requirements.
Carefully evaluate exposure conditions to see if a special condition exists. Grass and snow are very effective insulators for the ground below. Avoid the use of sloping exterior faces on grade beams or foundations that give the freezing forces something to push against when the frost heave situation develops.
AVERAGE DEPTH OF FROST PENETRATION (IN.)
1.5
DESIGNING FOR HOT, ARID CLIMATES
CLIMATE IMPLICATIONS
Though classified as arid and overheated, severe desert climates in the United States typically have four distinct periods for determining comfort strategies:
• The hot dry season, occurring in late spring, early summer, and early fall, has dry, clear atmospheres that provide high insulation levels, high daytime air temperatures, very high sol-air temperatures, and large thermal radiation losses at night, producing a 30° to 40°F daily range. Nighttime temperatures may fall below the comfort limits and are useful for cooling. Low humidity allows effective evaporative cooling.
• The hot humid season occurs in July and August. In addition to high insulation, it is characterized by high dew point temperatures (above 55°F), reducing the usefulness of evaporative cooling for comfort conditioning. Cloudiness and haze prevent nighttime thermal reradiation, resulting in only a 20°F or less daily range. Lowest nighttime temperatures are frequently higher than the comfort limits. Thus, refrigeration or dehumidification may be needed to meet comfort standards.
• The winter season typically has clear skies, cold nights, very low dew point temperatures, a daily range of nearly 40°F, and the opportunity for passively meeting all heating requirements from isolation.
• The transitional or thermal sailing season occurs before and after the winter season and requires no intervention by environmental control systems. This season can be extended by the passive features of the building. Other desert climates have similar seasons but in different proportions and at cooler scales.
CONSTRUCTION DETAILS
Capitalize on conditions climatic conditions by incorporating construction practices that respond in beneficial ways to the environment, including :
• Insulate coolant and refrigerant pipes from remote evaporative towers and condensers for their entire length.
• In hot locations, use roof construction similar to the cold climate roof detail.
• Do not use exposed wood (especially in small cross sections) and many plastics, as they deteriorate from excessive heat and high ultraviolet exposure.
• Although vapor retarders may not be critical to control condensation, implement them as a building wrap or wind shield, both to control dust penetration and to avoid convective leaks from high temperature differentials.
• Avoid thermal bridges such as extensive cantilevered slabs.
• Radiant barriers and details appropriate to humid overheated climates are at least as effective as vapor retarders, but avoid holes in assembly where convection would leak their thermal advantage.
• Ventilate building skin (attic or roof, walls) to relieve sol-air heat transfer.
DESIGNING FOR HUMID, OVERHEATED CLIMATES
Humid, overheated conditions are most severe along the Gulf Coast, but occur across the entire southeastern United States. Atmospheric moisture limits radiation exchange, resulting in daily temperature ranges less than 20°F. High insulation gives first priority to shading. Much of the overheated period is only a few degrees above comfort limits, so air movement can cool the body. Ground temperatures are generally too high for the Earth to be useful as a heat sink, although slab-on-grade floor mass is useful. The strategies are to resist solar and conductive heat gains and to take best advantage of ventilation.
Contributors:
Richard O. Anderson, PE, Somat Engineering, Taylor, Michigan; Eric K. Beach, Rippeteau Architects, PC, Washington DC; Stephen N. Flanders and Wayne Tobiasson, U.S. Army Corps of Engineers, Hanover, New Hampshire; Donald Watson, FAIA, Rensselaer Polytechnic Institute, Troy, New York; Kenneth Labs, New Haven, Connecticut; Jeffrey Cook, Arizona State University, Tempe, Arizona; K. Clark and P. Paylore, Desert Housing: Balancing Experience and Technology for Dwelling in Hot Arid Zones, Office of Arid Land Studies, University of Arizona, Tucson, Arizona, 1980. J. Cook, Cool Houses for Desert Suburbs, Arizona Solar Energy Commission, Phoenix, Arizona, 1984.
TYPICAL WALL SECTIONS FOR HOT, ARID CLIMATES
1.6
ENERGY-EFFICIENT WALL SECTION: VENTED SKIN MASONRY WALL WITH INSIDE INSULATION FOR HUMID, OVERHEATED CLIMATES
1.7
ENERGY-EFFICIENT WALL SECTION: VENTED SKIN WALL WITH RADIANT BARRIER FOR HUMID, OVERHEATED CLIMATES
1.8
SEISMIC FOUNDATION ISSUES
INTRODUCTION TO SEISMIC DESIGN
According to the theory of plate tectonics, the Earth’s crust is divided into constantly moving plates. Earthquakes occur when, as a result of slowly accumulating pressure, the ground slips abruptly along a geological fault plane on or near a plate boundary. The resulting waves of vibration within the Earth create ground motions at the surface, which, in turn, induce movement within buildings. The frequency, magnitude, and duration of the ground motion; physical characteristics of the building; and geology of a site determine how these forces affect a building.
DESIGN JUDGMENT
During a seismic event, buildings designed to the minimum levels required by model codes often sustain damage, even significant structural damage. Early discussions with an owner should explore the need to limit property loss in an earthquake, and the desirability of attempting to ensure continued building operation immediately afterward. To achieve these results, it may be necessary to make design decisions that are more carefully tuned to the seismic conditions of a site than the code requires.
The relationship between the period of ground motion and the period of building motion is of great importance to building design. Fundamental periods of motion in structures range from 0.1 second for a one-story building to 4.0 seconds or more for a high-rise building. Ground generally vibrates for a period of between 0.5 and 1.0 second. If the period of ground motion and the natural period of motion in a building coincide, the building may resonate, and the loads will be increased. Theoretically, one part of the seismic design solution is to “tune” the building so that its own period of motion falls outside the estimated range of ground motion frequency. In practice, this tuning is very seldom carried out. Rather, design professionals rely on increased load effects required by the applicable code to take care of the problem.
SEISMIC CODES
The building code adopted in most jurisdictions in the United States is International Building Code (IBC). There are some significant changes to the seismic forces determined by this code compared to seismic forces determined by previous building codes. The IBC 2006 code seismic provisions are designed around a level of earthquake that is expected to be exceeded only 2 percent of the time in the next 50 years. The level of seismic design for most structures, per the IBC, is based on a “collapse protection” strategy (commonly referred to as a “life safety” level), which assumes that there may be significant damage to the structure up to the point of collapse but that the structure does not collapse.
The structural engineer will design a lateral force-resisting structural assembly to resist a design-level earthquake. These designs are developed from detailed maps that indicate the ground spectral accelerations of buildings, which are based upon known past seismic events, in combination with probability studies. These maps typically include known fault locations, which help to determine the distance of the building from any known fault. The ground accelerations can typically be found down to the county level in the United States. The geotechnical engineer works with the design team to develop the site coefficient, which is dependent on the local soils layers and depths.
The following information is based on the requirements in the IBC 2006 Building Code, which in turn is based on the 2000 National Earthquake Hazards Reduction Program (NEHRP). Detached oneand two-family dwellings are exempt from seismic regulations in areas other than those with high seismicity. (Note: Seismic codes are constantly evolving, so consult the applicable code before beginning a project.)
Contributors:
Donald Watson, FAIA, Rensselaer Polytechnic Institute, Troy, New York; Kenneth Labs, New Haven, Connecticut; Subrato Chandra, Philip W. Fairey, Michael M. Houston, and Florida Solar Energy Center, Cooling with Ventilation, Solar Energy Research Institute, Golden, Colorado. 1982.; K. E. Wilkes, Radiant Barrier Fact Sheet, CAREIRS, Silver Spring, Maryland; P. Fairey, S. Chandra, A. Kerestecioglu, Ventilative Cooling in Southern Residences: A Parametric Analysis, PF-108-86, Florida, Solar Energy Center, Cape Canaveral, Florida 1986; William W. Stewart, FAIA, Stewart-Schaberg Architects, Clayton, Missouri.
MAIN CAUSES OF FOUNDATION FAILURE
1.9
FUNDAMENTAL PERIODS
1.10
SEISMIC ACCELERATION FOR LOW BUILDINGS EXPRESSED AS A PERCENTAGE OF GRAVITY
1.11
Source: Map courtesy of the U.S. Geological Survey, National Seismic Hazard Mapping Project (June 1996)
TERMS
The seismic community has an extensive set of terms that describe common conditions in the field. Here is a short list of these terms and their definitions:
• Base shear (static analysis): Calculated total shear force acting at the base of a structure, used in codes as a static representation of lateral earthquake forces. Also referred to as equivalent lateral force.
BASE SHEAR AND DRIFT
1.12
• Design earthquake: Earthquake ground motion for which a building is designed. This is typically about two-thirds of the maximum considered earthquake (MCE) (defined below) when designing per the IBC codes.
• Drift and story drift: Lateral deflection of a building or structure. Story drift is the relative movement between adjacent floors.
• Ductility: The ability of a structural frame to bend, but not break. Ductility is a major factor in establishing the ability of a building to withstand large earthquakes. Ductile materials (steel, in particular) fail only after permanent deformation has taken place. Good ductility requires special detailing of the joints.
• Dynamic analysis: A structural analysis based on the vibration motion of a building. Dynamic analysis is time-consuming, and normally reserved for complex projects.
• Forces, in-plane: Forces exerted parallel to a wall or frame.
• Forces, out-of-plane: Forces exerted perpendicular to a wall or frame.
• Maximum considered earthquake (MCE): The greatest groundshaking expected to occur during an earthquake at a site. These values are somewhat higher than those of the design earthquake, particularly in areas where seismic events are very infrequent. The code maps are based on earthquakes of this magnitude.
• Reentrant corner: The inside building corner of an L-, H-, X-, or T-shaped plan.
ESTABLISHING SEISMIC FORCES
The equivalent lateral force procedure is the most common method used to determine seismic design forces. In it, the seismic load, V (base shear), is determined by multiplying the weight of the building by a factor of Cs (V=CsW). The value of Cs depends on the size of the design earthquake, the type of soil, the period of the building, the importance of the building, and the response-modification factor (a variable that accounts for different levels of ductility for different types of lateral force-resisting systems used). This force is applied at the base of the structure then is distributed vertically throughout the building according to the mass, and horizontally throughout the building according to the stiffness of the lateral elements of the structure (for a “rigid” diaphragm), or according to tributary width of the lateral elements of the structure (for a “flexible” diaphragm).
Contributors:
Donald Watson, FAIA, Rensselaer Polytechnic Institute, Troy, New York; Kenneth Labs, New Haven, Connecticut; Subrato Chandra, Philip W. Fairey, Michael M. Houston, and Florida Solar Energy Center, Cooling with Ventilation, Solar Energy Research Institute, Golden, Colorado. 1982.; K. E. Wilkes, Radiant Barrier Fact Sheet, CAREIRS, Silver Spring, Maryland; P. Fairey, S. Chandra, A. Kerestecioglu, Ventilative Cooling in Southern Residences: A Parametric Analysis, PF-108-86, Florida, Solar Energy Center, Cape Canaveral, Florida 1986; William W. Stewart, FAIA, Stewart-Schaberg Architects, Clayton, Missouri.
DESIGN FOR RESISTING SEISMIC FORCES AND FOUNDATION ISSUES
A design that resists seismic forces for a structure makes use of the lateral systems’ ductility. Such ductile lateral systems are designed to deflect more under seismic loading than what would be expected from something such as wind loading. This allows for the use of smaller effective seismic design forces and more reasonably sized members. It is important, however, that the overall design still be capable of handling the expected deflections. Story drifts that are too large can result in secondary forces and stresses for which the structure was not designed, as well as increase the damage to the interior and exterior building components, and hinder the means of egress from the building.
Typical means of resisting these forces include the use of moment frames, shear walls, and braced frames. Each of these types of lateral systems can be made up of one of the main structural materials (such as steel or reinforced concrete moment frames; masonry, wood, or reinforced concrete shear walls; or steel or reinforced concrete-braced frames). The building configuration and design parameters will have a major effect on which system to chose and, subsequently, the lateral system chosen will have a major impact on the foundations required to resist the loads.
Moment frames typically are distributed more evenly over the building footprint and have little or no uplift; they also generally have large base moments that can be difficult to resist. In addition, moment frames will tend to have greater lateral deflections than other stiffer systems (such as shear walls or braced frames). Concrete shear walls and steel-braced frames are more localized, not only concentrating lateral shear at the base but also having a high potential for net uplift forces to be resisted. These forces are difficult to resist with some foundation systems and should be reviewed extensively before selecting the lateral load-resisting system.
Tall, narrow structures tend to have overturning issues before they will face sliding issues, whereas short structures face sliding problems rather than overturning problems. Seismic motion rocks the building, increasing overturning loads, and can act in any direction. Thus, resistance to overturning is best achieved at a building’s perimeter, rather than at its core.
Building foundations must be designed to resist the lateral forces transmitted through the earth and the forces transmitted from the lateral load-resisting system to the earth. In general, softer soils amplify seismic motion.
SHEAR WALLS AND DIAPHRAGMS
1.14
OUT-OF-PLANE VERTICAL OFFSETS
1.15
TORSION IN PLAN
1.16
REENTRANT CORNERS
1.17
MASS IRREGULARITY
1.18
SOFT STORY
1.19
IN-PLANE DISCONTINUITY
1.20
NOTES
1.16 The lateral force-resisting system for a symmetrical building is much easier to design than that for an asymmetrical building. Because the source of an earthquake cannot be known, symmetry in both directions should be considered.
1.17 This is a variation of the symmetry issue. When the notch gets too big, the building tends to tear at the inside corner.
1.18 Not all floors have to be the same; nevertheless, it is important that no floor has much more mass than those adjacent.
1.19 When a taller (inherently softer) first floor is desired, anticipate using much heavier first-floor framing to equalize the stiffness with that of the floors above.
1.20 Although both drawings illustrate shear walls in the same plane, one arrangement is discouraged because the load path is not direct.
Contributor:
William W. Stewart, FAIA, Stewart-Schaberg Architects, Clayton, Missouri; Scott Maxwell, PE, SE, Adrian, Michigan.
FOUNDATION STRATEGIES FOR HIGH - SEISMIC LOADS
UPLIFT
Braced buildings typically end up with high-tension loads at the foundations. Shallow foundations are difficult to design with high-tension loads. Some strategies are available to resist these uplift forces:
• Increase the dead load by removing adjacent columns, increasing the tributary area.
• Deepen the footing, increasing the soil load.
• Increase the footing dimensions, to increase the soil and concrete loads.
• Decrease column spacing, to decrease the brace forces.
• Change the foundation type (typically, to a deep foundation that can resist the uplift more effectively).
SHEAR
Braces and shear walls tend to collect the lateral forces and concentrate the loads in a few locations. Shallow and deep foundations have limited lateral load-resisting capability. By combining several foundations together, it is possible to effectively increase the lateral load resistance. The concrete tie beams are typically designed to distribute the lateral loads through tension or compression of the beam.
BRACE WITH UPLIFT
1.21
FOUNDATIONS
GENERAL
Foundations, because they are hidden below the surface, are often overlooked and their importance minimized by the design team. A great deal of scientifically guided creativity is often necessary to produce a foundation that supports the loads of the structure in such a way as to economically maintain the aesthetics and function of a facility. The wide variety of soil types and conditions across the United States—from the bedrock near the surface in New York to the sink holes and coral in Florida to the deep soft clays in the Midwest and South, and expansive soils in the Southwest to the seismically active areas of the country such as the West Coast—pose a challenge to the design team. The most popular and economical foundation solution is the spread footing. Spread footings are typically shallow, simple to design and construct, and perform well under many conditions. When properly designed, load is spread from a column to the soil at a bearing pressure that causes neither excessive settlement nor failure of the soil.
Should the soil conditions near the surface be weak, poorly compacted, filled with debris or organic material, or too compressible, deep foundations are warranted. The effect is to extend the foundation through the weak strata to a soil type that can withstand the loadings with tolerable settlement. Deep foundations come in several types and, depending on the soil conditions, may include driven piles, bored piles (bored, augered or drilled) or caissons. Deep foundations resist imposed loads either by end bearing or side friction or some combination. It is not necessary to drive or drill a deep foundation to rock, only to the depth required to reach a suitable stratum.
A spread footing is not always appropriate, such as when the property line limits the extent of the foundation in one direction, or when the soil conditions are very weak and suitable soil strata too deep to reach with a deep foundation. In these cases, other types of special foundations (such as combined footings, strap footings, and raft foundations) are sometimes required.
Two basic criteria should be met for all foundations:
• Soil strength (bearing capacity): The ability of a soil to support a load without experiencing failure is known as the bearing capacity and is a function of the foundation size as well as the inher-ent strength properties of the soil. If the pressures exerted by a foundation exceed the strength of a soil, the soil mass experiences a shear failure leading to gross movements of both the soil and the supporting foundation element.
• Limitations of settlement: Settlement can happen either immediately (foundations on sands), or over period of time, short or long term (foundations on clays). Some settlement is expected, over various parts of the country, typical and acceptable settlement is usually less then 1 in. Settlement is not as important with a solitary structure, but becomes more important when: (1) buildings adjacent to an existing structure need to be interconnected, (2) long utility runs need to be connected to the structure, or (3) there is sensitive equipment in the building. Uniform settlement is somewhat better tolerated than differential settlement that is uneven across several columns. Differential settlement distorts the structure and causes cracking of the exterior skin and interior partitions, broken windows, and doors that don’t open. Allowable differential settlement may be dependant on the material of the skin and structure; for example, brick and concrete masonry buildings tolerate less differential settlement than curtain wall buildings. Differential settlement of 1/4 in. is typically considered tolerable for most building types.
The importance of proper foundation design and detailing cannot be overemphasized. Working with the geotechnical engineer, familiar with the soil conditions in the area, and a structural engineer, familiar with the proposed design and detailing of the foundation, will help ensure the building functions as intended for its life cycle.
SETTLEMENT AND DIFFERENTIAL SETTLEMENT
Often, settlement governs the allowable bearing pressure, which is set at an intensity that will yield a settlement within tolerable levels for the building type. Allowable settlement is typically building and use-specific. Total and differential settlement, as well as the time rate of the occurrence of the settlement, must be considered when evaluating whether the settlement is tolerable. For example, in the case of a conventional steel frame structure, in typical practice a total maximum settlement of 1 in. is usually acceptable, and differential settlement of one-half of the total settlement is also usually tolerable.
ANGULAR DISTORTION
Settlement tolerance is commonly referred to in terms of angular distortion in the building or settlement between columns. Typically, an angular distortion of 1:480 is used for conventional structures. This equates to 1 in. in 480 in., or 1 in. in 40 ft. Depending on the type of structure, the allowable angular distortion might vary from 1:240 for a flexible structure (such as a wood frame, single-story structure) to 1:1000 for a more “brittle” or sensitive structure.
EFFECTS OF SOIL TYPES
When load is applied to granular soils, the grains of soil are able to respond almost immediately, and they will densify as the packing of the grains becomes tighter.
Clay soils exhibit a time-dependent relationship associated with the consolidation of the clay soil. In order for the clay to consolidate, and the overlying soil or structure to settle, the excess pressures that are induced in the water in the clay must dissipate, and this takes time because of the low permeability of the clay. Depending on the drainage characteristics of the clay, the time required for 90 percent of the consolidation (and settlement) to occur may vary from a few months to several years. If there is a high frequency of sand layers or seams within the clay mass, then the consolidation will be quicker, because the excess pore water pressure can be dissipated faster.
Both sand and clay soils have a built-in “memory” that, in effect, remember the maximum load that was applied to the soil at some time in the past. This memory is referred to as the preconsolidation pressure. If 10 ft of soil has been removed (by excavation or erosion) from a soil profile then the equivalent weight of that 10 ft of soil (approximately 1250 lbs per square foot) could be reapplied to the soil profile without the soil below sensing any difference. Depending on the process that deposited the soil, weathering processes, past climatological changes, or human activities, the preconsolidation pressure of the soil may be far in excess of the pressures induced by the current soil profile. When that is the case, settlement of conventional structures is rarely a significant concern. But when the soil has not been preconsolidated, the addition of any new load may result in excessive settlement.
Contributor:
James W. Niehoff, PE, Chief Engineer, PSI, Wheat Ridge, Colorado.
MINIMUM SPECIFIED COMPRESSIVE STRENGTH AT 28 DAYS F’C, AND MAXIMUM SLUMP OF CONCRETE
1.22
Source: Based on ACI 332 Requirements for Residential Concrete Construction and Commentary, Table 4.1, Reprinted with permission of the American Concrete Institute.
AIR CONTENT FOR TYPE 2 AND TYPE 3 CONCRETE UNDER MODERATE OR SEVERE WEATHERING PROBABILITY
1.23
Source: Based on ACI 332 Requirements for Residential Concrete Construction and Commentary, Table 4.2, Reprinted with permission of the American Concrete Institute.
| NOMINAL MAXIMUM AGGREGATE SIZE (IN.) | AIR CONTENT (TOLERANCE ±0.015) |
|---|
| MODERATE | SEVERE |
|---|
| 3/8 | 0.06 | 0.075 |
| 1/2 | 0.055 | 0.07 |
| 3/4 | 0.05 | 0.07 |
| 1 | 0.045 | 0.06 |
| 1-1/2 | 0.045 | 0.055 |
FOOTING MINIMUM DIMENSIONAL REQUIREMENTS
1.24
DOWEL AND KEYWAY REQUIREMENTS FOR FOOTINGS
1.25
Source: Based on ACI 332 Requirements for Residential Concrete Construction and Commentary, Figures R6.5 and R 6.6. Reprinted with permission of the American Concrete Institute.
SHALLOW FOUNDATIONS
Shallow foundations are typically the most economical foundations to construct where soil and loading conditions permit. Coordination with local codes for frost depth and with the underground utilities is required. The thickness of the footing has to be coordinated with anchor bolt and dowel embedment. Typically, one layer of steel in the bottom of the footing is required to resist the bending of the footing caused by soil-bearing pressures.
AXIALLY LOADED SPREAD FOOTING
1.26
Axial loads are distributed in a uniform manner under the footing. The allowable bearing pressure necessary to resist the load determines footing size.
SPREAD FOOTING RESISTING MOMENT, SHEAR, AND AXIAL LOADS
1.27
Axial loads, combined with shear and overturning forces can be resisted by spread footings. The combination of axial load and moment forces on the foundation need to be balanced to keep the calculated loads on the footing less than the allowable bearing pressure of the soil as determined by the geotechnical engineer.
SPREAD FOOTING SIZE LIMITATIONS
1.28
Minimum sizes of spread footings are specified by the geotechnical engineer, to reduce the possibility of local soil failures by punching shear of an overall movement of soil mass. Maximum sizes of spread footings keep the nonuniform bearing pressure from becoming extreme and overstressing the soil.
NOTES
1.22 Maximum slump refers to the characteristics of the specified mixture proportion based on water cement ratio only. Midrange and high-range water-reducing admixtures can be used to increase the slump beyond these maximums.
1.23 American Concrete Institute (ACI) and International Building Code (IBC) have requirements for the minimum footing dimensions.
Contributor:
SPREAD FOOTING—CONCRETE COLUMN
1.29
SPREAD FOOTING—STEEL COLUMN
1.30
TYPICAL FOUNDATION WALL AND SPREAD FOOTING DETAIL
1.31
FOUNDATIONS AT GRADE
1.32
FOUNDATIONS AT BASEMENT
1.33
NOTES
1.29 a. Soil below footing should not be disturbed after excavation.
b. Footing size based on allowable bearing pressure.
c. Thickness based on bending and shear requirements.
d. Reinforcing steel based on bending and minimum steel requirements.
e. Dowels as required to transfer load. 1.30 a. Provide concrete fill after all dead load has been applied to column.
b. Thickness of nonshrink grout to accommodate unevenness of footing surface and leveling nuts.
c. Anchor bolts designed to resist moments and shears from axial loads, as well as lateral loads.
Contributors:
Anthony L. Felder, Concrete Reinforcing Steel Institute, Schaumburg, Illinois; Kenneth D. Franch, AIA, PE, Phillips Swager Associates, Inc, Dallas, Texas; Donald Neubauer, PE, Neubauer Consulting Engineers, Potomac, Maryland; Mueser Rutledge Consulting Engineers, New York City, New York; SmithGroup, Architecture, Engineering, Interiors, Planning, Detroit, Michigan.
SPECIAL FOUNDATIONS
PILE TYPES
1.34
GENERAL PILE DATA
1.35
DRILLED PIER WITH BELL
1.36
NOTES
1.35 a. Applicable material specifications: Concrete ACI 318; Timber ASTM D 25; Structural Sections ASTM A 36, A 572, and A 690. For selection of type of pile, consult a geotechnical engineer.
b. A mandrel is a member inserted into a hollow pile to reinforce the pile shell while it is driven into the ground.
c. Timber piles must be treated with wood preservative when any portion is above the groundwater table.
1.36 a. Test soils to determine their allowable bearing capacity.
b. “H” (depth of shaft reinforcing, below concrete cap) is the function of the passive resistance of the soil, generated by the moment applied to the pier cap.
c. Piers may be used under grade beams or concrete walls. For very heavy loads, pier foundations may be more economical than piles.
Contributors:
Mueser Rutledge Consulting Engineers, New York City, New York; John P. McCarthy, PE, SE, SmithGroup, Architects, Engineers, Interiors, Planners, Detroit, Michigan; AB Chance, Centralia, Missouri.
DRILLED PIER WITH SOCKET
1.37
CONSTRUCTION ISSUES WITH DRILLED PIERS
1.38
REINFORCING AXIALLY LOADED DRILLED PIERS
1.39
PILE CAPS
Typically, more than one deep foundation element is required to resist the gravity and lateral loads; in order to distribute the loads from the single point column to the multiple foundation elements, a pile cap is required. Pile caps are thick, reinforced concrete blocks that distribute the load from the column to the foundations through a combination of flexure and shear.
Other applications of the pile caps include providing a method of connecting the columns to the foundations, and easing the construction tolerance issues that occur when installing deep foundations. These pile caps are designed and detailed to encase a small portion of the deep foundation, and transition to the column support elevation, thus providing a convenient location to position anchor bolts and column dowels.
PILE SUPPORTED FOUNDATIONS
1.40
NOTES
1.37 a. Set pier into a socket in rock to transmit high compression or tension lads into rock by side friction and end bearing.
b. Pier shaft should be poured in dry conditions if possible, but tremie pours can be used.
TYPICAL JOINT DETAILS
1.41
Source: Armored Construction Joint Detail based on ACI 302.1R Guide for Concrete Floor Construction and Slab Construction, Figure 3.15, reprinted with permission of the American Concrete Institute.
DIAMOND-SHAPED LOAD PLATES AT SLAB CORNER
1.42
DOWELED JOINT DETAIL FOR MOVEMENT PARALLEL AND PERPENDICULAR TO THE JOINT
1.43
Source: Based on ACI 302.1R Guide for Concrete Floor Construction and Slab Construction, Figure 3.14, reprinted with permission of the American Concrete Institute.
FINISH AND FLOOR FLATNESS
In general, concrete floor slabs are monolithically finished by floating and troweling, to achieve a smooth and dense surface finish. ACI 302 provides guidance for appropriate finishing procedures to control achievable floor flatness. ACI 302, ACI 360, and ACI 117 provide guidance for flatness selection, as well as techniques by which flatness and levelness are produced and measured.
Floor finish tolerance is measured by placing a freestanding 10-ft straightedge on the slab surface, or by F-Numbers. The preferred method of measuring flatness and levelness is the F-Number System. Special finishes are available to improve appearance, as well as surface properties. These include sprinkled (shake) finishes or high-strength toppings, either as monolithic or separate surfaces.
TYPICAL ISOLATION JOINTS AT TUBE COLUMNS
1.44
Source: Based on ACI 302.1R Guide for Concrete Floor Construction and Slab Construction, Figure 3.3, reprinted with permission of the American Concrete Institute.
PROTECTIVE AND DECORATIVE COATINGS
Concrete surfaces may require a sealer or coating for the following:
• To protect against severe weather, chemicals, or abrasions.
• To prevent dusting of the surface layer.
• To harden the surface layer.
• To add a decorative finish.
Sealers are usually clear and are expected to penetrate the surface without leaving a visible film. Coatings are clear or opaque, and, though they may have some penetration, they leave a visible film on the surface. Sealers and coatings should allow vapor emission from the concrete but, at the same time, prevent moisture from penetrating after curing.
Decorative coatings usually protect as well, and are formulated in a wide selection of colors. Decorative coatings include the following:
• Water-based acrylic emulsion
• Elastomeric acrylic resin
• Liquid polymer stain
• Solvent-based acrylic stain
• Portland cement-based finish coating
• Water-based acidic stain (a solution of metallic salts)
NOTE
1.42 Alternate to round or square dowels.
Contributor:
BASEMENT CONSTRUCTION
BASEMENT EXCAVATION
TRENCHING
Trenches are narrow vertical cuts in soil used to place utilities and to construct continuous foundations. For short periods of time, many trenches may appear to be stable, but then collapse suddenly. Unsupported trenches can be dangerous to workers, as the early-warning signs of a trench collapse cannot be seen from the bottom of the trench. Moreover, in a narrow trench, there is nowhere to escape, if a sudden collapse of the trench wall occurs. And, because 1 to 2 cu ft of soil weighs as much as 100 to 250 lbs., even a relatively small collapse can severely injure or kill a worker. For safety purposes, therefore, OHSA requires trenches more than 5 ft deep be supported with shoring, or protected with a trench box, before allowing worker access.
TIMBER-BRACED TRENCH
1.45
HYRAULICALLY BRACED TRENCH
1.46
TRENCH BOX
1.47
INTERNALLY BRACED EXCAVATIONS
1.48
LATERAL SUPPORT
Walls that extend to depths of 10 to 15 ft can generally be self-supported or cantilevered and do not require additional lateral support. This is accomplished by extending the shoring sufficient depth below the excavation. These “cantilevered” walls resist the lateral pressures from the soil and groundwater, although some deflection at the top of the wall should be expected. However, walls that extend deeper or have significant surcharge loads, or where deflection is a concern, will require some type of additional lateral support (known as “braced walls”).
INTERNAL SUPPORT
The lateral support can be internal to the excavation, in the form of wales, struts, or rakers. Internal bracing is very effective but interferes with the construction of the permanent structure.
INTERNALLY BRACED EXCAVATION SCHEMES
1.49
EXTERNALLY BRACED EXCAVATION DETAILS
1.50
EXTERNAL SUPPORT
External bracing of the excavation is usually in the form of drilled tieback (ground) anchors. The use of tieback anchors to support the wall enables a clear and unencumbered excavation, but requires access to areas beyond the building, which may extend outside the property line. For this reason, tieback anchors are almost always the most desirable method of lateral support if the owner controls the property around the excavation, or if temporary easements can be obtained from private owners or public municipalities.
EXTERNALLY BRACED EXCAVATION
1.51
TEMPORARY TIEBACK SECTION
1.52
PERMANENT TIEBACK AT FOUNDATION WALL
1.53
SOIL STABILIZATION
Soil stabilization is sometimes called soil modification or soil conditioning. For deep excavations in very soft soils, the construction of earth support systems can be quite massive and expensive because of the higher lateral soil and hydrostatic pressures. Furthermore, in some situations, wall installation can be difficult to accomplish because of space requirements.
Various techniques have been developed to improve the effective strength or resistance of the soils around and below the excavations. Improving the in-situ soil strength or resistance reduces or eliminates the net pressure at the face of the excavation. The underground construction industry has developed the following techniques for improving or reinforcing soil.
SOIL-MIXING STABILIZATION
Soil mixing stabilization is the mixing the soils in the ground with asphalt, cement, lime, fly ash, or lime-fly ash to stabilize and strengthen the soil. Often, only a selected portion of soil mass must be treated to provide a significant increase in overall soil strength or resistance.
PRESSURE GROUTING SOIL STABILIZATION
Pressure grouting soil stabilization is the injection of cement or chemicals into the pores of the soil. Soil particles are then “glued” together to form a hardened solid mass of grouted soil. The spacing of the injection points and the grout mixture is varied to achieve a specific pattern and grout coverage, depending on the requirements of the project.
GROUND FREEZING
Ground freezing uses the natural moisture in the soil and artificial cooling methods to freeze and harden the soil. The ambient temperature of nearly all soil is above freezing (except for permafrost regions). Lowering the soil temperature to below freezing causes the water in the pores of the soil to freeze and “cement” the soil particles together. This results in a very hard, impermeable condition. However, soil has a relatively low thermal conductivity. Liquid Nitrogen may be used to freeze the soil in a timely manner. A continual flow of a brine solution through a series of embedded refrigeration pipes must be used to maintain the soil in a frozen state.
SOIL NAILING
Soil nailing is a method used for both temporary excavation bracing and for permanent retaining walls. This technique employs closely spaced, high-strength steel anchors grouted into the soil, and may include a reinforced shotcrete facing. The major advantages of soil nail wall construction over more traditional excavation bracing methods include relatively low cost and the ability to construct the wall system from the top down, as the excavation proceeds. A typical installation sequence for grouted soil nailing is as follows:
1. Excavation begins by exposing a cut about 3 to 5 ft in depth.
2. A borehole (typically 4 to 6 in. in diameter) is drilled into the face of the excavation at a downward angle of approximately 15° to the horizontal. The length of the borehole is dependent on the height of the cut and the nature of the material exposed in the excavation. Typical lengths range from 60 to 70 percent of the wall height.
3. A high-strength, threaded reinforcing bar is inserted into the borehole, then the borehole is grouted to the excavation face.
4. After the grout has cured, a wire mesh is placed over the exposed face of the cut, and reinforcement is placed to span over the borehole and reinforcing bar. When groundwater is a concern, a geosynthetic drainage mat is typically placed against the soil face to intercept water and direct it to the base of the wall.
5. The exposed excavation face is sprayed with shotcrete, typically 8 to 10 in. thick.
6. A bearing plate is fitted over the reinforcing bar, and a nut is screwed into place to tension the soil nail.
7. If the method is to be used as a permanent wall, a second application of shotcrete is used to cover the soil nail head and bearing plate.
8. After completion of the first level, the excavation extends downward an additional 4 to 6 ft, and the process is repeated.
SLOPE STABILITY WITH SOIL NAILS
1.54
EXCAVATION SUPPORT WITH SOIL NAILS
1.55
SOIL NAIL WALL DETAIL
1.56
BACKFILL AND COMPACTION
FILL AND BACKFILL
Fill is typically used to raise or level site grades. Backfill is used to fill in spaces around below-grade structural elements, such as around basement walls, The fill must have sufficient strength or resistance and low compressibility to support its own weight and any other overlying structures pavements, floor slabs, foundations, etc.) without excessive settlement. When soils are excavated, they become loosened and disturbed. If they are suitable for reuse as structural fill or backfill, the soils must be placed in thin layers and compacted to achieve the required strength, resistance and stability.
COMPACTION
Compaction is the process by which mechanical energy is applied to a soil to increase its density. The degree to which soil can be densified depends on the amount and type of compactive effort, type of soil, and moisture content. Soil is made up of solids and the void spaces between the solid particles. The void space almost always contains some water. If the water completely fills the void, the soil is considered to be totally saturated. During compaction, the total volume of the soil is decreased by reducing the volume of voids, while the volume of solids remains essentially unchanged. If the soil is saturated or nearly saturated during compaction, water must be expelled to decrease the void space.
MOISTURE-DENSITY RELATIONSHIP
Nearly all soil exhibits a defined moisture-density relationship for a specific level of compactive effort.
These relationships can be graphed in a nearly bell-shaped curve, with the maximum density at the apex, corresponding to the optimum moisture content.
Standard laboratory tests, such as the Standard Proctor (ASTM D 698) and Modified Proctor (ASTM D 1557), use a standard-size mold and a specific level of compactive energy to develop the moisture density curve for a specific soil. The maximum density from these curves defines the 100 percent level of compaction for a given soil. Compaction requirements for fill and backfill are generally specified as a percentage of the maximum density, typically between 90 to 95 percent, as determined using one of the standard laboratory tests mentioned above. The low and high moisture contents are usually represented as a horizontal line connecting opposite sides of the Proctor curve for a given density. This represents the range of moisture content within which the soil can be compacted most readily.
MOISTURE-DENSITY RELATIONSHIP
1.57
FIELD DENSITY TESTS
Field density tests are performed on compacted soil to verify that a specific level of compaction has been achieved. There are several methods for determining the in-place density of soil. Today, the most commonly used method involves the nuclear density gauge, a device that measures the reflection of atomic particles from a tiny radioactive source material to determine the soil density and moisture content. The test is performed at the surface without any excavation, and results can be obtained faster than with other test methods.
MOISTURE CONTROL
Moisture content of the fill and backfill should be near the optimum moisture content. Otherwise, the minimum required field density is very difficult (or impossible) to obtain, no matter how much energy is used for compaction. If the soil is too wet, the water in the pores cannot be expelled fast enough to allow for a sufficient decrease in volume. If the soil is too dry, the capillary forces around the soil particles are too large to be broken down by the compactive energy. Therefore, controlling the moisture of the fill and backfill to within specific limits near the optimum moisture content is necessary to achieve the required level of compaction.
SOIL TYPE AND COMPACTION EQUIPMENT
To be most effective, the compaction equipment must match the type of soil to be compacted. In general, compaction equipment can be divided into two basic groups: rollers or plates. Rollers come in large variations in size, but all use a weighted wheel or drum to impart energy to the soil. In addition, some rollers use an electric rotor to vibrate the drum, thereby increasing the energy to the soil. Other drums have protrusions called sheepsfoots, which impart a kneading action to the soils. Some plate compactors also use vibratory energy to compact the soils, while other plate compactors, called tampers, move up and down, imparting a vertical dynamic load to the soil.
In general, coarse-grained granular soils such as sands and gravels are more easily compacted than fine-grained soils such as clays and clayey silts. Vibratory energy is very effective in densifying sands and gravels, since the interparticle bonds are relatively weak. When the granular soils are vibrated at the correct frequency, the soil particles rearrange themselves into a denser state under their own weight and the weight of the compactor. Vibratory steel drum rollers and vibratory plate compacters are considered the most effective compaction equipment for granular soils.
Fine-grained soils hold more moisture and have higher internal interparticle forces. Vibratory energy is much less effective for these soils. Clayey soils require more mechanical energy to break down the internal forces during compaction. The kneading action of a sheepsfoot roller is very effective in this regard.
Fine-grained soils generally have a narrower range of moisture contents for optimum compaction. Often, the clayey soils are too wet to compact and the moisture content must be reduced. Reducing the moisture content in clay is typically done by allowing water to evaporate from the surface of the clay. The rate of evaporation is dependent on the ambient air temperature and wind conditions. However, the drying process can be enhanced by using a process called aeration in which steel discs are used to periodically turn the soil, thus exposing more of the soil to the atmosphere.
BASEMENT WALLS
Basement walls may be constructed of various materials, including, concrete, masonry, and wood.
BASEMENT WALL CONSTRUCTION
CONCRETE BASEMENT WALLS
Concrete basement walls may be either cast-in-place or precast. Cast-in-place concrete basement walls provide a cost-effective means of supporting a floor and resisting soil pressures. Commercial and residential applications of cast-in-place concrete basement walls are prevalent. Forms are easily placed in the excavation on the footings. Reinforcing steel may be tied on or off-site, and is placed within the wall formwork. Depending on the soil and groundwater conditions, dampproofing should typically be used on foundations walls and waterproofing is generally required on basement walls prior to backfilling. Unless lateral bracing is utilized, the top of the basement wall must be supported by the first floor and the base of the wall by the footing or slab-on-grade before backfilling against the wall can begin. Keeping the wall heights uniform, as well as reducing the number of penetrations and maintaining a simple plan configuration, will help reduce the final cost of the wall.
Precast concrete basement walls enable basement construction in less time than conventional cast-in-place concrete. In addition to time and construction methods other advantages of precast concrete include the ability of the precast supplier to utilize concrete admixtures that focus on ultimate strength, rather than cure time and temperature. Precast concrete manufacturers are able to produce mixes that cure to 5000 psi, which is stronger than concrete unit masonry or cast-in-place concrete walls. Additionally, better control of the concrete mixture and curing environment allows the use of low water/cement ratios, which results in a dense material that reduces water penetration.
NOTE
1.57 Reinforcing is based on unbraced backfill height, soil pressure, and groundwater conditions.
Contributors:
Donald Neubauer, PE, Neubauer Consulting Engineers, Potomac, Maryland; Mueser Rutledge Consulting Engineers, New York City, New York; James W. Niehoff, PE, PSI, Wheat Ridge, Colorado; Timothy H. Bedenis, PE, Soil and Materials Engineers, Inc., Plymouth, Michigan; American Concrete Institute; Grace S. Lee, Rippeteau Architects, PC, Washington DC; Stephen S. Szoke, PE, National Concrete Masonry Association, Herndon, Virginia; Daniel Zechmeister, PE, Masonry Institute of Michigan, Livonia, Michigan; Paul Johnson, AIA, Senior Architect, SmithGroup, Detroit, Michigan.
CONCRETE BASEMENT WALLS
1.58
Source: Based on ACI 332 Requirements for Residential Concrete Construction and Commentary, Figure R7.1. Reprinted with permission of the American Concrete Institute.
MASONRY BASEMENT WALLS
Masonry walls have long served as foundations for structures. Today, most masonry basement walls consist of a single wythe, or hollow, solid concrete masonry units, depending on the required bearing capacity. The walls are reinforced as necessary to resist lateral loads. Generally, such reinforcing should be held as close to the interior face shell as possible, to provide the maximum tensile strength.
Basement walls should protect against heat and cold, insect infestation (particularly termites), fire, and penetration of water and soil gases.
If radon is a major concern, the top course of the masonry and the course of masonry at or below the slab should be constructed of solid units or fully grouted hollow units using foundation drain to collect and drain condensation moisture from basements, should be avoided in areas where soil-gas entry is a concern.
Architectural masonry units may be used to improve the appearance of the wall. Masonry units with architectural finishes facing the interior can be used for economical construction of finished basement space.
Masonry easily accommodates any floor plan, and returns and corners increase the structural performance of the wall for lateral load resistance.
TYPICAL MASONRY BASEMENT WALL
1.59
THICKNESS OF CMU BASEMENT WALLS
1.60
| BASEMENT WALL CONSTRUCTION | NOMINAL THICKNESS (IN.) | MAXIMUM DEPTH OF UNBALANCED FILL (FT) |
|---|
| CMU - hollow units, ungrouted | 8 | 5 |
| 10 | 6 |
| 10 12 | 6 7 |
| CMU - solid units | 8 | 5 |
| 10 | 7 |
| 12 | 7 |
| CMU - hollow or solid units, fully grouted | 8 | 7 |
| 10 | 8 |
| 12 | 8 |
NOTES
1.58 a. Drainage must be provided on surface and below grade to remove groundwater from the basement wall. The backfill must be granular, and the soil conditions nonexpansive.
b. Backfill pressure on wall is assumed to be 30 psf/ft of depth of wall. Soil pressures may be higher, and greater thicknesses required at a given location. Consult with local code officials or geotechnical engineers.
VERTICAL REINFORCEMENT SPACING
1.61
CMU BASEMENT WALL REINFORCEMENT (BAR SIZE AND MAXIMUM BAR SPACING)
1.62
CMU BASEMENT WALL: HORIZONTAL JOINT REINFORCEMENT
1.63
NOTES
1.63 a. The empirical design method of the Building Code Requirements for Masonry Structures, ACI 530/ASCE 5, Chapter 9, allows up to 5 ft of backfill on an 8-in., nonreinforced concrete masonry wall.
b. As an alternate, W1.7 joint reinforcement placed in joints numbers 3, 4, 5, 7, 8, and 11 may be used.
c. Use of vapor retarders should be verified by proper analysis.
d. Backfill pressure on wall is assumed to be 30 psf/ft of depth of wall. Soil pressures may be higher, and greater thicknesses required at a given location. Consult with local code officials or geotechnical engineers.
TREATED WOOD BASEMENT WALLS
The construction of treated wood foundations and basements is similar to the construction of standard wood light-frame walls except for two factors:
• The wood used is pressure-treated with wood preservatives.
• The extra loading and stress requirements caused by below-grade conditions must be accommodated in the design and detailing of the fasteners, connections, blocking, and wall corners.
As with standard masonry or concrete foundations, treated wood foundations require good drainage to maintain dry basements and crawl spaces. However, the drainage system typically used with treated wood foundations is different from that used with masonry or concrete systems. The components of a drainage system suitable for use with a treated wood foundation include:
• A highly porous backfill material, which directs water down to a granular drainage layer.
• A porous granular drainage layer under the entire foundation and floor to collect and discharge water.
• Positive discharge of water by means of a sump designed for the soil type. This drainage system (developed for treated wood foundations) takes the place of a typical porous backfill over a perimeter drainage pipe.
Benefits of a treated wood foundation include system:
• All framing is standard 2 by construction.
• Treated wood foundations can be erected in any weather and when site access for other methods is difficult.
• Deep wall cavities allow use of high R-value thermal insulation without loss of interior space.
• Wiring and finishing are easily achieved.
Considerations when working with treated wood foundations:
• Treated wood foundations are not appropriate for all sites. Selection of the proper foundation for a project depends on site conditions, including soil types, drainage conditions, groundwater, and other factors. Wet sites in low areas (especially areas with coarse-grained soil) should be avoided if a full basement is desired, although a crawl space type foundation can be used in these cases. Consult a geotechnical engineer to determine the viability of any foundation system. Also, refer to the wood deterioration zones indicated in Figure 1.150. Lumber and plywood used in treated wood foundations must be grade-stamped for foundation use. These are typically pressure treated with chromated copper arsenate. Treated wood products used in foundation construction are required to contain more preservatives than treated wood used in applications such as fencing and decking. Codes generally call for hot-dipped, galvanized fasteners above grade and stainless steel fasteners below grade.
• Avoid skin contact and prolonged or frequent inhalation of sawdust when handling or working with any pressure-treated wood product.
• Consult applicable building codes and the American Forest & Paper Association’s Permanent Wood Foundation System—Design, Fabrication, Installation Manual for requirements and design guidelines. In the early stages of a project, consult with the building code officials for the area or jurisdiction to assess their familiarity with and willingness to approve this type of construction.
• The vertical and horizontal edge-to-edge joints of all plywood panels used in these systems should be sealed with a suitable sealant.
• Correct materials and details of construction are very important for treated wood foundations. If the contractor to be used for the installation is unfamiliar with this foundation type, the design should include the use of shop-fabricated foundation panels. Most problems with treated wood foundations can be traced to improper installation by inexperienced workers.
• This type of foundation depends especially on the first-floor deck to absorb and distribute any backfill loads; therefore, backfilling cannot occur until the first floor deck is complete unless lateral bracing is utilized.
WOOD DETERIORATION ZONES
1.64
Source: Based on AWPA Book of Standards 1997.
TYPICAL TREATED WOOD BASEMENT WALL
1.65
TREATED WOOD BASEMENT WALL WITH EXTERIOR KNEE WALL
1.66
WOOD SLEEPER FLOOR SYSTEM
1.67
NOTES
1.65 a. Geotextile material may be used under and around drainage layers and backfill, if soil conditions warrant.
b. Stud size and spacing vary with material grade and backfill depth. In general, 42-in. backfill requires 2 by 4 at 12 in. o.c., 64-in. requires 2 by 6 at 16 in. o.c., and 84-in. requires 2 by 6 at 12 in. o.c.
1.67 a. Joists to be butted end to end over pressure-treated wood sleepers.
b. Floor stiffness will be increased by blocking between each joist above each sleeper.
c. Check with applicable code for underfloor ventilation requirements.
BASEMENT—WATERPROOFING/ DAMPPROOFING/INSULATION
Consult a geotechnical engineer to determine soil types and groundwater levels, as well as their effect on drainage and waterproofing methods. Consult a waterproofing specialist to determine a specific design approach for problem soils and conditions. Sites may have groundwater contamination that will degrade the durability of the waterproofing materials. Generally, waterproofing will be necessary if a head of water is expected against the basement wall or under the slab. Because groundwater levels can vary with seasons, it is important to understand these seasonal fluctuations and design for the maximum expected head.
Foundation drainage is recommended when the groundwater level may rise above the top of the floor slab or when the foundation is subject to hydrostatic pressure after heavy rain. Geosynthetic drainage material conveys water to the drainage piping, thus reducing hydrostatic pressure. It is important to understand the hydrostatic pressures exerted on the floor slab and wall systems if the drainage system is not adequate to remove all the water.
Special negative-side coatings on interior face of foundation wall, such as metallic oxide, are recommended only when the exterior is not accessible (such as pits and trenches, and in particular, elevator pits).
BASEMENT WALL VERTICAL WATERPROOFING
The grading around the building is an important part of the overall water management plan. The backfilling operation usually results in a more porous material than the adjacent undisturbed soil, which makes it easier for water to collect next to the building. The finished grade should slope away from the building, and an impervious layer of soil placed on top of the backfill against the building. Drainage from downspouts should be diverted away from the foundations.
Types of waterproofing include built-up bituminous, sheet, fluid-applied, cementitious and reactive, and bentonite.
• Built-up bituminous: Composed of alternating layers of bituminous sheets and viscous bituminous coatings. Bituminous waterproofing includes built-up asphalt and cold-tar waterproofing systems.
• Sheet waterproofing: Formed with sheets of elastomeric, bituminous, modified bituminous or thermoplastic materials. Sheet waterproofing may be either mechanically attached or self-adhered. Sheet waterproofing provides an impermeable surface to water penetration.
• Fluid-applied: Applied in a hot or cold viscous state. Includes hot fluid-applied rubberized asphalt. As with sheet waterproofing, fluid-applied waterproofing will bridge minor cracks in a concrete surface.
• Cementitious and reactive: Types of waterproofing that achieve waterproof qualities through chemical reaction and include polymer modified cement, crystalline, and metal-oxide waterproofing systems. Metal oxide is recommended for use when the exterior surface is not accessible, as in the case of an elevator pit.
• Bentonite: Formed from clay into panels and composite sheets. When moistened, the clay swells and takes on a gel-like consistency, forming an impermeable retarder when confined. Bentonite clay works well only when moistened. For applications where the water table fluctuates, there may be a time lag between the rising water table and when the bentonite takes effect, during which time there is the possibility of water infiltration. Therefore, when the water table varies, caution is in order when relying on bentonite clay for waterproofing. Proper coordination between the wall construction details and the waterproofing termination is required.
At the interface of the foundation wall and slab, waterstops are placed on top of the footing, at vertical concrete keyed wall joints.
Most waterproofing materials require a stable, rigid, and level substrate. Generally, a mud slab (subslab that is nonreinforced and nonstructural) is used when the waterproofing material is placed below the structural slab and/or when a solid working surface is needed on unstable soils. When waterproofing materials are placed on top of the structural slab, a protective cover, such as another concrete slab, is required.
WATERPROOFING APPLICATIONS AT BASEMENT CONDITIONS
1.68
WATERPROOFING AT FOOTINGS
1.69
WATERPROOFING UNDERSLAB
1.70
NOTE
1.68 Place 12-in. neoprene strips over joints in sheet piling.
