Diaphragm flexible: A diaphragm is flexible for the purpose of distribution of story shear and torsional movement where so indicated in Section 12.3.1 of ASCE 7, as modified in this chapter.As you have seen with past chapters, I don’t normally begin with definitions. However there have been changes with these definitions and I have decided the easiest way to point these out is just by giving them their very own section. So, here they are:
Diaphragm flexible: A diaphragm is flexible for the purpose of distribution of story shear and torsional movement where so indicated in Section 12.3.1 of ASCE 7, as modified in this chapter.
Fabric partitions: A partition consisting of a finished surface made of fabric, without a continuous rigid backing, that is directly attached to a framing system in which the vertical framing members are spaced greater than 4 feet on center.
Occupancy category: A category used to determine structural requirements based on occupancy.
Vehicle barrier system: A system of building components near open sides of a garage floor or ramp or building walls that act as restraints for vehicles.
All of these words have changes in definition, either in whole or part. In this chapter you will notice letters and notations, for example “D + F” may be one that you will see. These notations are also listed as definitions. Please see the following list of notations that you will come across:
D = Dead load
E = Combined effect of horizontal and vertical earthquake induced forces as defined in Section 12.4.3 of ASCE 7
Em = Maximum seismic load effect of horizontal and vertical seismic forces as set forth in Section 12.4.3 of ASCE 7
F = Loads due to fluids with well-defined pressures and maximum heights
Fa = Flood load
H = Load due to lateral earth pressures, ground water pressure, or pressure of bulk materials
L = Live load, except roof live load, including any permitted live load reduction
Lr = Roof live load including any permitted live load reduction
R = Rain load
S = Snow load
T = Self-straining force arising from contraction or expansion resulting from temperature change, shrinkage, moisture change, creep in component materials, movement due to differential settlement or combinations thereof
W = Load due to wind pressure.
Are you aware of what information needs to be included in construction documents? There is some important information that cannot be left out such as the size, section, and relative locations of structural members with floor levels, column centers, and offsets dimensioned. The design loads and other information pertinent to the structural design are described in this section and must be indicated on the construction documents. Please see the following exception: Construction documents for buildings constructed in accordance with conventional light-frame construction. Provisions must indicate the following structural design information:
CODE UPDATE |
The design load-bearing values of soils shall be shown on the construction documents. |
Ground snow lead
Basic wind speed (three-second gust), miles per hour, and wind exposure
Seismic design category and site class
Flood design data, if located in flood hazard areas.
The floor live load that has been uniformly distributed and concentrated and used in the design must be indicated for floor areas. If reduction is allowed, that too, must be indicated. Construction documents must include wind design data, regardless of whether wind loads govern the design of the lateral-force-resisting system of the building.
When wind enters a building, internal pressure and external suction create a combined load on the construction causing the building or structure to result in a pile of sticks. The following list contains commonly used wind data:
Basic wind speed, miles per hour
Wind importance factor, and occupancy category
Wind exposure, if more than one wind exposure it used, the wind exposure and applicable wind direction must be indicated
The applicable internal pressure coefficient
Components and cladding.
Earthquake design data is another bit of information that must included. Starting from the seismic importance and occupancy factor to the analysis procedure used, this important information must be included in any construction documents. If your building is located in whole or part of a flood hazard area, the documentation pertaining to design, if required, must be included and the following information, referenced to the datum on the community’s Floor Insurance Rate Map or FIRM, is to be included, regardless of whether flood loads govern the design of the building:
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You must indicate any special loads that are applicable to the design of the building. Keep in mind that it is illegal to allow a load greater than permitted to be placed upon any floor or roof of a building. Also, occupancy permits will not be issued until the floor load signs have been installed. |
In flood hazard areas not subject to high-velocity wave action, the elevation of the proposed lowest floor, including the basement; the elevation to which any nonresidential building will be dry flood-proofed
Elevations to which any nonresidential building will be dry flood proofed
The proposed elevation of the bottom of the lowest horizontal structural member of the lowest floor, including the basement.
Buildings and structures have to be designed and constructed in accordance with the strength design, load and resistance design and allowable stress design as permitted by the applicable material chapters in this book. The structural components of any building or structure must be designed and constructed to safely support the factored loads in load combinations that have been designed in this code. You must be careful not to exceed the appropriated strength limit stated for the materials of construction. You must consult the building official for loads and forces for occupancies or uses that are not covered in the chapter. All structural systems and members must be designed to limit deflections and lateral drift.
Account equilibrium, general stability, and geometric compatibility must be taken into account to determine the load effects on structural members and connections. Residual deformations that tend to accumulate on members under repeated service loads must be included in the analysis in regards to added eccentricities expected to occur during their service life. Any system or method of construction to be used must be based on a rational analysis. This analysis must result in a system that provides a complete load path capable of transferring loads from the origin point to the load-resisting elements.
Any rigid elements that are assumed not to be a part of the lateral-force-resisting system are allowed to be incorporated into buildings provided their effect on the action of the system is considered and provided for in the design. In instances where diaphragms are flexible, or are allowed to be analyzed as flexible, provisions will be made for the increased forces induced on resisting elements of the structural system resulting from torsion due to eccentricity between the center of application of the lateral forces and the center of rigidity of the lateral-force-resisting system.
Buildings and structures that have a relatively low hazard to people if there is an emergency issue are categorized as occupancy I. The categories continue down to IV, with the nature of the occupancy in which these buildings or structures fit. As you can see this doesn’t give any information for buildings that have two or more occupancies. In such cases, the structure must be assigned to the classification of the highest nature.
For example, if one part of the building is a storage facility and the other is a jail or detention center, the building must be categorized as occupancy III because there are people who occupy the jail. If the building or structure has two separate portions with separate entrances, they must be categorized separately. The building official is authorized to require an engineering analysis or a load test, or even both, of any building when there is a question of safety regarding the construction of the occupancy in question.
CODE UPDATE |
Each building and structure shall be assigned an occupancy category. |
Handrails and guards shall be designed to resist a load of 50 pounds per linear foot applied in any direction at the top and to transfer this load through the supports to the structure. |
To resist the uplift and sliding forces that result from the use of prescribed loads you must anchor the roof to walls and columns, and in turn, anchor the walls and columns to foundations. If your building has concrete and masonry walls, these must be anchored to floors, roofs, and other structural elements that are used to provide lateral support for the wall. Realize that the required anchors that you use in masonry walls of hollow units or cavity walls must be embedded in a reinforced grouted structural element of the wall.
Structural elements, components, and cladding must be designed to resist forces from earthquakes and wind with consideration to overturning, sliding, and uplift. As a contractor you must provide continuous load paths for transmitting these forces to the foundation.
In cases where allowable stress or working stress design is allowed, and used by this code, structures and portions thereof must resist the most critical effects.
Please note these exceptions:
Crane hook loads do not need to be combined with roof live load or with more than three-fourths of the snow load or one-half of the wind load; and flat roof loads of 30 psf or less do not need to be combined with seismic loads, unless the flat roof snow loads exceed 30 psf, 20 percent must be combined with seismic loads.
Keep in mind that increases in allowable stresses specified with the referenced standards cannot be used with the load combination listed above, except that a duration of load increase is permitted in accordance with code requirements.
There may be times when you will be allowed to use alternative basic load combinations that include wind or seismic loads. In these cases, allowable stresses can be increases or load combinations reduced where permitted by the material chapter of this book or the referenced standards.
Be sure you are confident that you can make these changes and understand the referenced standards carefully.
Heliports and helistops landing areas are occupancies where the load factors are of a serious nature.
Landing areas must be designed in accordance to the following:
Dead load, D, plus the gross weight of the helicopter, Dh, plus snow load, S
Dead load, D, plus two single concentrated impact load, L, approximately 8 feet apart applied anywhere on the landing area, having a magnitude of 0.75 times the gross weight of the helicopter
Both loads acting together total one and one-half times the gross weight of the helicopter
Dead load, D, plus a uniform live load, L, of 100 psf.
Please note that there is an exception to this, which is also a change in the code, as follows:
Landing areas designed for helicopters with gross weights not exceeding 3000 pounds in accordance with bullets one and two (above list) must be allowed to be designed using a 40 psf uniform live load in bullet three (above list), provided the landing area is identified with a 3000 pound weight limitation.
The 40 psf uniform live load cannot not be reduced and the landing area weight limitation must be indicated by the numeral 3.
Dead loads: The weight of materials of construction incorporated into the building, including but not limited to walls, floors, roofs, ceiling, stairways and other similarly incorporated architectural and structural items, and the weight of fixed service equipment, such as cranes, plumbing stacks and risers, electrical feeders, heating, ventilating and air-conditioning systems. |
The appropriate location would be in the bottom right corner of the landing area as viewed from the primary approach.
The landing area weight limitation must be a minimum of 5 feet in height.
Dead loads are the weights of various structural members and objects that are permanently attached to the structure. There are two types of dead loads—building dead loads and collateral dead loads. A building dead load is the actual building system, such as a roof or floor and materials used for covering such as decking, felt, and hinges. Collateral dead loads are the weight of the permanent materials, such as drywall, sprinklers, and electrical systems. Collateral dead loads do not include the weight of the actual building system. In the absence of definite information, values will be subject to the approval of the building official. We call these dead loads because they are unable to be moved.
One can’t have a dead load without a live load, so let’s explore this subject a bit more. Live loads are not permanent and can change in magnitude. Live loads include items that can be found inside of a building such as furniture, safes, people, or stored materials. Environmental effects, such as earthquakes, wind, and snow, that have the power to change and cause potential damage or failure to a building, are also considered live loads. However, the live load will be determined in accordance with a method approved by the building official.
Concentrate loads—force localized over a relatively small area such as floors and similar surfaces—must be designed to support live loads that are uniformly distributed. In office buildings and in other buildings where partition locations are subject to change, requirements for partition weight must be made. Whether these partitions are shown on the construction documents or not, you must allow for them unless the specified live load is more than 80 psf. In case you’re wondering, the partition load cannot be less than a uniformly distributed live load of 15 psf.
Truck and bus garages are among the many buildings which require a minimum live load. Both the uniform and concentration load must be uniformly distributed over a 10-foot width and placed within their individual lanes. This will produce the maximum stress in each structural member, but not on both at the same time. Keep in mind that all garages accommodating trucks and buses must also be designed with an approved method that contains provisions for traffic railings.
Apartments, which include residential, have requirements for loads. Handrail assemblies and guards must be able to resist 200 pounds in a single concentrated load. This means in any direction at any point along the top, and have attachment devices and supporting structures that can transfer the loading.
With this said and with the exception of roof uniform, live loads and all other minimum uniformly distributed live loads are allowed to be reduced in accordance with certain parts of this section. It is up to you to read and understand these exceptions if they pertain to your building or structure. Note that live loads over 100 psf cannot be reduced unless the following are true:
The live loads for members supporting two or more floors are permitted to be reduced by a maximum of 20 percent, but the live load cannot be less than the reduced design live load per square foot.
For uses other than storage, where approved, additional live load reductions are permitted where shown by the registered design professional that a rational approach has been used and that the reductions are necessary.
As an alternative to the section above, floor live loads are permitted to be reduced in accordance with the following provisions. These reductions apply to slab systems, beams, girders, columns, piers, walls, and foundations.
A reduction is not permitted in Group A occupancies.
A reduction is not permitted where the live load exceeds 100 psf except that the design live load for members supporting two or more floors is permitted to be reduced by 20 percent.
A reduction will not be permitted in passenger parking garages except that the live loads for members supporting two or more floors may be reduced by a maximum of 20 percent.
For all live loads that are not more than 100 psf, the design live load for any structural member supporting 150 square feet or more is allowed to be reduced in accordance with the same code exceptions.
Construction crews must consider not only how loading condition might affect a structure, but also how loads are distributed. This affects both floor and roof loads. Where the uniform floor live load is involved with the design of structural members the minimum applied loads must be the full dead loads on all spans in combination with the floor live loads. This is to create the greatest effect at each location under consideration.
Where uniform roof loads are reduced to less than 20 psf and are involved in the design of structural members, the minimum applied loads must also be the full dead load on adjacent spans or on alternate spans, or whichever produces the greatest effect.
Minimum uniformly distributed roof live loads are allowed to be reduced, but only to the following provision: ordinary flat, pitched, and curved roofs are allowed to be designed for a reduced roof live load. However, you must be careful with this because if your workers are using scaffolding for a work surface for themselves and materials during maintenance and repair jobs, a lower roof load than specified (in the equation below) cannot be used unless approved by the building official. Human safety must always be at the top of your mind.
There are times when cranes are used in construction. The crane live load must always be the rated capacity of the crane. Design loads of moving bridge cranes and monorail cranes must include the maximum wheel loads of the crane and the vertical impact, lateral, and longitudinal forces made by the moving crane. The design load includes connections and support brackets as well.
Wheel load: The vertical force without impact produced on a crane wheel bearing on a runway rail or suspended from a runway beam. Maximum wheel load occurs with the crane at rated capacity and the trolley positioned to provide maximum vertical force at one set of wheels. |
Wheel loads of a crane have a maximum load and can be increased by the percentages shown below to determine the induced vertical impact or vibration force.
Monorail cranes (powered) … 25 percent
Cab-operated or remotely operated bridge cranes (powered)…25 percent
Pendant-operated bridge cranes (powered) … 10 percent
Bridge cranes or monorail cranes with hand-geared bridge, trolley, and hoist…0 percent
Other loads include elemental or weather related loads such as snow and wind. Design snow loads are determined in accordance with ASCE 7. You must use the extreme value statistical analysis to determine ground snow loads that are found in the vicinity of the site using a value with a 2-percent annual probability of being exceeded. The ASCE 7 is used to determine wind loads on every building or structure.
The type of opening protection, the basic wind speed, and the exposure category for a site is determined in this chapter. Wind can be assumed to come from any horizontal direction and wind pressures can be assumed to act normal to the surface that is being considered.
There are several areas that are defined as hurricane-prone regions. These regions include the U.S. Atlantic Ocean and Gulf of Mexico coasts where the basic wind speed is greater than 90 mph, and Hawaii, Puerto Rico, Guam, Virgin Islands, and American Samoa. Portions of these areas that are within 1 mile of the coastal mean high water line where the basic wind speed is 110 mph or greater; or portions of hurricane-prone regions where the basic wind speed is 120 mph or greater; or Hawaii are also considered wind-borne debris regions.
For each wind direction considered, an exposure category that adequately reflects The characteristics of ground surface irregularities must be determined for the site at which the building or structure is to be constructed. You must take into account any variation in ground surface roughness that arises from natural topography and vegetation as well as constructed features. A ground surface roughness within each 45-degree sector is determined for a distance upwind of the site as defined from the categories provided below.
Surface Roughness B: Urban and suburban areas, wooded areas, or other terrain with numerous closely spaced obstructions having the size of single-family dwellings or larger.
Surface Roughness C: Open terrain with scattered obstructions having heights generally less than 30 feet. This category includes flat open country, grasslands, and all water surfaces in hurricane-prone regions.
Surface Roughness D: Flat, unobstructed areas and water surfaces outside hurricane-prone regions. This category includes smooth mud flats, salt flats, and unbroken ice.
Roof systems, such as roof decks, are affected by wind loads and must be designed to withstand the wind pressures determined in accordance with ASCE 7. If you are like most people, you’ve applied or intend to apply asphalt shingles to your roof deck for design. Make sure that any asphalt shingles that you are using haVE been tested to determine the resistance of the sealant to uplift forces using ASTM D 6381. You can find yourself in a bit of a mess if you disregard this. All it takes is a heavy wind to come along and blow those shingles off the roof of your building or structure. If you’ve decided on using rigid tile for your roof covering make sure you note that the code requirement is not the same.
Handrails and guards shall be able to resist a single concentrated load of 200 pounds applied in any direction at any point along the top, and to transfer this load through the supports to the structure. |
Concrete and clay roof tiles complying with the following limitations must be designed to withstand the aerodynamic uplift moment:
The roof tiles must be loose laid on battens, mechanically fastened, or mortar/adhesive set.
The roof tiles must be installed on solid sheathing which has been designed as components and cladding.
An underlayment must be installed in accordance with code requirements.
The tile must be single lapped interlocking with a minimum head lap of no less than 2 inches.
The length of the tile must be between 1.0 and 1.75 feet.
The maximum thickness of the tail of the tile cannot be more than 1.3 inches.
Roof tiles using mortar set or adhesive set systems must have at least two-thirds of the tile’s area free of mortar or adhesive contact.
This section is short, but not necessarily unimportant and refers to basements, foundations, and retaining walls that have to be designed to resist lateral soil loads. Soil loads must be used as the minimum design lateral soil loads unless specified otherwise in a soil investigation report. Make sure you get the investigation report approved by your building official. Basement walls with restricted horizontal movement have to be designed for at-rest pressure. Note that basement walls that do not extend more than 8 feet below grade and supporting flexible floor systems will be allowed to be designed for active pressure.
When designing a building or structure, engineers and architects must consider the loads, both external and internal, that a building must endure. The building is then designed to resist these loads. A type of external load is rain. When designing a building roof the goal is to have each portion of the roof sustain the load of rainwater that will accumulate on it if the primary drainage system is not working properly. This is also true for uniform load caused by water that has risen above the inlet of the secondary drainage system.
A change in the code has a provision for roofs equipped with a slope that is less than ¼ inch per foot. The design calculations must include verification of adequate stiffness to preclude progressive deflection in accordance with Section 8.4 of ASCE 7. One drainage system is simply not enough.
Roofs that are equipped with hardware to control the rate of drainage must be equipped with a secondary drainage system. All secondary drainage systems must be at a higher elevation that limits any accumulation of water on the roof above that elevation.
Flooding is when excess water overflows from bodies of water onto adjacent land. Flood areas are established by local government bodies with a flood hazard map and other supporting data. This flood hazard map has to include any areas of special flood hazards that have been identified by the Federal Emergency Management Agency and presented as an engineering report. This report has to be written in the following manner: The Flood Insurance Agency for (NAME OF JURISDICTION), dated (INSERT DATE OF ISSUANCE), as amended. Be sure to include the Flood Insurance Rate Map (FIRM) and the Flood Boundary and Flooding Map (FBFM) and any supporting data relating to the flood area.
Floods kill people and destroy homes in parts of the world every year. Obviously, any structures or buildings built in floor-prone areas are permanently at risk. Design and construction of buildings located in flood hazard areas, including those areas that are at risk for high velocity wave action, must be in accordance with ASCE 24. See the following list for documentation that must be prepared and sealed by a registered design professional and submitted to the building official. The first list is for construction in flood hazard areas not to subject high-velocity wave action:
The elevation of the lowest floor, including the basement, as required by the lowest floor elevation inspection
For fully enclosed areas below the design flood elevation where provisions to allow for the automatic entry and exit of flood-waters do not meet the minimum requirements in Section 2.6.2.1 of ASCE 24, construction documents must include a statement that the design will provide for equalization of hydrostatic flood forces in accordance with Section 2.6.2.2 of ASCE 24.
For dry flood-proofed nonresidential buildings, construction documents must include a statement that the dry flood-proofing is designed in accordance with ASCE 24.
For construction in flood hazard areas subject to high-velocity wave action:
The elevation of the bottom of the lowest horizontal structural member must be as required by the lowest floor elevation inspection.
Construction documents must include a statement that the building is designed in accordance with ASCE 24, including that the pile or column foundation and building or structure to be attached is designed to be anchored to resist flotation, collapse and lateral movement due to the effect of wind and flood loads acting simultaneously on all building components, and other load requirements.
CODE UPDATE |
When using the alternative all-heights method, wind pressures shall be applied simultaneously on, and in a direction normal to, all building envelope wall and roof surfaces. |
For breakaway walls designed to resist a nominal load of less than 10 psf or more than 20 psf, construction documents must include a statement that the breakaway wall is designed in accordance with ASCE 24.
The 2009 International Building Code contains a provision that every structure, and portion thereof, including non-structural components that are permanently attached to structures and their supports and attachments, must be designed and constructed to resist the effects of earthquake motions in accordance with ASCE 7. This does not apply to Exterior Walls, and Appendix 11A of the 2009 International Building Code book. You may use this chapter or ASCE 7 to determine the seismic design category for your structure. Please see the following list of exceptions:
Detached one- and two-family dwellings that are categorized as A, B, or C of the Seismic Design Category.
The seismic-force-resisting system of the wood-frame building that conforms to the provisions of the code is not required to be analyzed as specified in this section.
Agricultural storage structures intended only for incidental human occupancy
Structures that require special consideration of their response characteristics and environment that are not addressed by this code or ASCE 7 and for which other regulations provide seismic criteria, such as vehicular bridges, electrical transmission towers, hydraulic structures, buried utility lines and their accessories and nuclear reactors.
Any existing buildings that you plan on adding to, making alterations to, or modifying must be done in accordance to Existing Structures. Please note that this applies to any changes in occupancy as well.
Site class: A classification assigned to a site based on the types of soils present and their engineering properties as defined in this chapter. |
Where required by Special Inspections specifies that seismic requirements include the statement of special inspections and must identify the designated seismic systems and seismic-force-resisting systems and any additional special inspections and testing. This is to include the applicable standards referenced by this code.
Site class definitions are provided in the code. This is the classification assigned to a site based on the types of soils present and their engineering properties. Based on site soil properties, the site is classified as Site Class A, B, C, D, E, or F. But what if you’re not sure of the soil properties? If you are unsure of the soil properties of your building or structure site and you do not have sufficient detail to determine such information, use the classification Site Class D until the building official or geotechnical data determines which site class that the soil belongs in.
Consult your local code book for maps of maximum earthquake ground motion for various regions of the world.
