Loads are the external forces acting on a structure. Stresses are the internal forces that resist them. Depending on that manner in which the loads are applied, they tend to deform the structure and its components—tensile forces tend to stretch, compressive forces to squeeze together, torsional forces to twist, and shearing forces to slide parts of the structure past each other.
Types of Loads
External loads on a structure may be classified in several different ways. In one classification, they may be considered as static or dynamic.
Static loads are forces that are applied slowly and then remain nearly constant.
One example is the weight, or dead load, of a floor or roof system.
Dynamic loads vary with time. They include repeated and impact loads.
Repeated loads are forces that are applied a number of times, causing a variation in the magnitude, and sometimes also in the sense, of the internal forces. A good example is an off-balance motor.
Impact loads are forces that require a structure or its components to absorb energy in a short interval of time. An example is the dropping of a heavy weight on a floor slab, or the shock wave from an explosion striking the walls and roof of a building.
External forces may also be classified as distributed and concentrated.
Uniformly distributed loads are forces that are, or for practical purposes may be considered, constant over a surface area of the supporting member. Dead weight of a rolled-steel I beam is a good example.
Concentrated loads are forces that have such a small contact area as to be negligible compared with the entire surface area of the supporting member. A beam supported on a girder, for example, may be considered, for all practical purposes, a concentrated load on the girder.
Another common classification for external forces labels them axial, eccentric, and torsional.
An axial load is a force whose resultant passes through the centroid of a section under consideration and is perpendicular to the plane of the section.
An eccentric load is a force perpendicular to the plane of the section under consideration but not passing through the centroid of the section, thus bending the supporting member (see Arts. 5.4.2, 5.5.17, and 5.5.19).
Torsional loads are forces that are offset from the shear center of the section under consideration and are inclined to or in the plane of the section, thus twisting the supporting member (see Arts. 5.4.2 and 5.5.19).
Also, building codes classify loads in accordance with the nature of the source.
Dead loads include materials, equipment, constructions, or other elements of weight supported in, on, or by a building, including its own weight, that are intended to remain permanently in place.
Live loads include all occupants, materials, equipment, constructions, or other elements of weight supported in, on, or by a building and that will or are likely to be moved or relocated during the expected life of the building.
Impact loads are a fraction of the live loads used to account for additional stresses and deflections resulting from movement of the live loads.
Wind loads are maximum forces that may be applied to a building by wind in a mean recurrence interval, or a set of forces that will produce equivalent stresses.
Snow loads are maximum forces that may be applied by snow accumulation in a mean recurrence interval.
Seismic loads are forces that produce maximum stresses or deformations in a building during an earthquake.
Live Loads. These may be concentrated or distributed loads and should be considered placed on the building to produce maximum effects on the structural member being designed. Minimum live loads to be used in building design are listed in Table 5.2. These include an allowance for impact, except as noted in the footnote of Table 5.2b.
Partitions generally are considered to be live loads, because they may be installed at any time, almost anywhere, to subdivide interior spaces, or may be shifted from original places to other places in the future. Consequently, unless a floor is designed for a large live load, for example, 80 lb / ft2, the weight of partitions should be added to other live loads, whether or not partitions are shown on the working drawings for building construction.
Because of the low probability that a large floor area contributing load to a specific structural member will be completely loaded with maximum design live loads, building codes generally permit these loads to be reduced for certain types of occupancy. Usually, however, codes do not permit any reduction for places of public assembly, dwellings, garages for trucks and buses, or one-way slabs. For areas with a minimum required live load exceeding 100 lb / ft2 and for passengercar garages, live loads on columns supporting more than one floor may be decreased 20%. Except for the preceding cases, a reduced live load L, lb/ft2, may be computed from
The reduced live load L, however, should not be less than 0.5Lo for members supporting one floor or 0.4Lo for members supporting two or ore floors.
Roofs used for promenades should be designed for a minimum life load of 60 lb/ft2, and those used for gardens or assembly, for 100 lb / ft2. Ordinary roofs should be designed for a minimum live load L, lb/ft2, computed from
This minimum live load need not be combined with snow load for design of a roof but should be designed for the larger of the two.
Subgrade Pressures. Walls below grade should be designed for lateral soil pressures and the hydrostatic pressure of subgrade water, plus the load from surcharges at ground level. Design pressures should take into account the reduced weight of soil because of buoyancy when water is present. In design of floors at or below grade, uplift due to hydrostatic pressures on the underside should be considered.
Wind Loads. Horizontal pressures produced by wind are assumed to act normal to the faces of buildings for design purposes and may be directed toward the interior of the buildings or outward (Arts. 3.2.1 and 3.2.2). These forces are called velocity pressures because they are primarily a function of the velocity of the wind striking the buildings. Building codes usually permit wind pressures to be either calculated or determined by tests on models of buildings and terrain if the tests meet specified requirements (see Art. 3.2.2). Codes also specify procedures for calculating wind loads, such as the following:
Velocity pressures due to wind to be used in building design vary with type of terrain, distance above ground level, importance of building, likelihood of hurricanes, and basic wind speed recorded near the building site. The wind pressures are assumed to act normal to the building facades.
The basic wind speed used in design is the fastest-mile wind speed recorded at a height of 10 m (32.8 ft) above open, level terrain with a 50-year mean recurrence interval.
Unusual wind conditions often occur over rough terrain and around ocean promontories.
Basic wind speeds applicable to such regions should be selected with the aid of meteorologists and the application of extreme-value statistical analysis to anemometer readings taken at or near the site of the proposed building. Generally,
however, minimum basic wind velocities are specified in local building codes and in national model building codes but should be used with discretion, because actual velocities at a specific sites and on a specific building may be significantly larger.
In the absence of code specifications and reliable data, basic wind speed at a height of 10 m above grade may be approximated for preliminary design from the following:
Coastal areas, northwestern and southeastern
United States and mountainous area 110 mph
Northern and central United States 90 mph
Other parts of the contiguous states 80 mph
For design purposes, wind pressures should be determined in accordance with the degree to which terrain surrounding the proposed building exposes it to the wind. Exposures may be classified as follows:
Exposure A applies to centers of large cities, where for at least one-half mile upwind from the building the majority of structures are over 70 ft high and lower buildings extend at least one more mile upwind.
Exposure B applies to wooded or suburban terrain or to urban areas with closely spaced buildings mostly less than 70 ft high, where such conditions prevail upwind for a distance from the building of at least 1500 ft or 10 times the building height.
Exposure C exists for flat, open country or exposed terrain with obstructions less than 30 ft high.
Exposure D applies to flat unobstructed areas exposed to wind blowing over a large expanse of water with a shoreline at a distance from the building or not more than 1500 ft or 10 times the building height.
For design purposes also, the following formulas may be used to determine, for heights z (in feet) greater than 15 ft above ground, a pressure coefficient K for converting wind speeds to pressures.
For Exposure A, for heights up to 1500 ft above ground level,
where V = basic wind speed, mi/hr, but not less than 70 mi/hr.
For important buildings, such as hospitals and communication buildings, for buildings sensitive to wind, such as slender skyscrapers, and for buildings presenting a high degree of hazard to life and property, such as auditoriums, qz computed from Eq. (5.7) should be increased 15%.
To allow for hurricanes, qz should be increased 10% for ordinary buildings and 20% for important, wind-sensitive or high-risk buildings along coastlines. These increases may be assumed to reduce uniformly with distance from the shore to zero for ordinary buildings and 15% for the more important or sensitive buildings at points 100 mi inland.
Wind pressures on low buildings are different at a specific elevation from those on tall buildings. Hence, building codes may give different formulas for pressures for the two types of construction. In any case, however, design wind pressure should be a minimum of 10 psf.
Multistory Buildings. For design of the main wind-force resisting system of ordinary, rectangular, multistory buildings, the design pressure at any height z, ft, above ground may be computed from
The negative sign indicates suction. Table 5.3 lists values of Cp for pressures on roofs.
Flexible Buildings. These are structures with a fundamental natural frequency less than 1 Hz or with a ratio of height to least horizontal dimension (measured at mid-height for buildings with tapers or setbacks) exceeding 5. For such buildings, the main wind-force resisting system should be designed for a pressure on windward walls at any height z, ft, above ground computed from
In ASCE-7-95 and 98, the basic wind speed changed from fast mile wind to 3- second gust wind speed in miles per hour. The wind speed values on the basic wind speed map has changed. This change should not have any big impact on the wind pressure. However, confusion is easily created because all the major building codes including the IBC 2000 are still using old basic wind speed map based on fast mile wind, and they repeatedly refer to ASCE-7 95 or 98. It is to be noted that the reference from the building codes to the ASCE-7 are either adoption of ASCE- 7 as an alternative approach or for certain factors that are not related to the basic wind pressure.
In ASCE-7-95 and 98, new factors such as wind directionality factor, topographic factor were introduced, and gust effect factors were updated for rigid structures as well as for flexible /dynamically sensitive structures. The calculation became much more complicated than the approach in this book and the results should be more accurate. We suggest that for complicated structures it is necessary to use ASCE-7-98 method to check the results.
Snow, Ice, and Rain Loads. These, in effect, are nonuniformly distributed, vertical, live loads that are imposed by nature and hence are generally uncertain in magnitude and duration. They may occur alone or in combination. Design snow loads preferably should be determined for the site of the proposed building with the advice of meteorologists and application of extreme-value statistical analysis to rain and snow records for the locality.
Rain loads depend on drainage and may become large enough to cause roof failure when drainage is blocked (see Art. 3.4.3).
Ice loads are created when snow melts, then freezes, or when rain follows a snow storm and freezes. These loads should be considered in determining the design snow load. Snow loads may consist of pure snow or a mixture of snow, ice, and water.
Design snow loads on roofs may be assumed to be proportional to the maximum ground snow load pg, lb/ft2, measured in the vicinity of the building with a 50- year mean recurrence interval. Determination of the constant of proportionality should take into account:
1. Appropriate mean recurrence interval.
2. Roof exposure. Wind may blow snow off the roof or onto the roof from nearby higher roofs or create nonuniform distribution of snow.
3. Roof thermal conditions. Heat escaping through the roof melts the snow. If the water can drain off, the snow load decreases. Also, for sloped roofs, if they are warm, there is a tendency for snow to slide off. Insulated roofs, however, restrict heat loss from the interior and therefore are subjected to larger snow loads.
4. Type of occupancy and uses of building. More conservative loading should be used for public-assembly buildings, because of the risk of great loss of life and injury to occupants if overloads should cause the roof to collapse.
5. Roof slope. The steeper a roof, the greater is the likelihood of good drainage and that show will slide off.
In addition, roof design should take into account not only the design snow load uniformly distributed over the whole roof area but also possible unbalanced loading.
Snow may be blown off part of the roof, and snow drifts may pile up over a portion of the roof.
For roofs sheltered from the wind, increase pƒ computed from Eq. (5.16) by 20%, and for windy sites, reduce pƒ 10%. For a poorly insulated roof with heated space underneath, decrease pƒ by 30%.
Increase pƒ 10% for large office buildings and public-assembly buildings, such as auditoriums, schools, factories. Increase pƒ 20% for essential buildings, such as hospitals, communication buildings, police and fire stations, power plants, and for structures housing expensive objects or equipment. Decrease p.ƒ 20% for structures with low human occupancy, such as farm buildings.
The ground snow load pg should be determined from an analysis of snow depths recorded at or near the site of the proposed building. For a rough estimate in the absence of building-code requirements, pg may be taken as follows for the United States, except for mountainous regions:
0–5 lb/ ft2—southern states from about latitude N32 southward 10–15 lb/ ft2—Pacific coast between latitudes N32 and N40 and other states between latitudes N32 and N37
20–30 lb/ ft2—Pacific coast from latitude N40 northward and other states between latitudes N37 and N40
40–50 lb/ ft2—north Atlantic and central states between latitudes N40 and N43
60–80 lb/ ft2—northern New England between latitudes N43 and N45 and central states from N43 northward 80–120 lb / ft2—Maine above latitude N45
For sloping roofs, the snow load depends on whether the roof will be warm or cold. In either case, the load may be assumed to be zero for roofs making an angle of 70 or more with the horizontal. Also, for any slope, the load need not be taken greater than pƒ given by Eq. (5.16). For slopes , deg, between 0 and 70, the snow load, lb / ft2, acting vertically on the projection of the roof on a horizontal plane, may be computed for warm roofs from
loaded with ps, and also with the windward wide unloaded and the leeward side carrying 1.5ps.
For curved roofs, the snow load on the portion that is steeper than 70p may be taken as zero. For the less-steep portion, the load ps may be computed as for a sloped roof, with taken as the angle with the horizontal of a line from the crown to points on the roof where the slope starts to exceed 70. Curved roofs should be designed with the whole area fully loaded with ps. They also should be designed for the case of snow only on the leeward side, with the load varying uniformly from 0.5ps at the crown to 2ps at points where the roof slope starts to exceed 30 and then decreasing to zero at points where the slope starts to exceed 70.
Multiple folded-plate, sawtooth, and barrel-vault roofs similarly should be designed for unbalanced loads increasing from 0.5ps at ridges to 3ps in valleys.
Snow drifts may form on a roof near a higher roof that is less than 20 ft horizontally away. The reason for this is that wind may blow snow from the higher roof onto the lower roof. Drifts also may accumulate at projections above roofs, such as at parapets, solar collectors, and penthouse walls. Drift loads accordingly should be taken into account when:
1. The ground snow load pg exceeds 10 lb / ft2.
2. A higher roof exists (or may be built in the future) within 20 ft of the building, if the height differential, ft, exceeds 1.2pƒ /, where pƒ is computed from Eq. (5.16) and is the snow density, lb/ ft3.
3. A projection extends a distance, ft, exceeding 1.2pƒ / above the roof and is more than 15 ft long.
In computation of drift loads, the snow density , lb/ft3, may be taken as follows:
The drift may be assumed to be a triangular prism with maximum height, located adjacent to a higher roof or along a projection, taken as hd 2pg /, modified by factors for risk and exposure, described for flat roofs. Width of the prism should be at least 10 ft and may be taken as 3hd for projections up to 50 ft long and as 4hd for projections more than 50 ft long. Accordingly, the load varies uniformly with distance from a projection, from hd at the projection to zero. For drifts due to snow load from a higher roof at a horizontal distance S, fit, away horizontally (S 20 ft), the maximum drift intensity may be taken as hd (20 S) / 20.
Rain-Snow Load Combination. In roof design, account should be taken of the combination of the design snow load with a temporary water load from an intense rainstorm, including the effects of roof deflection on ponding. The added water load depends on the drainage characteristics of the roof, which, in turn, depend on the roof slope. For a flat roof, the rain surcharge may be taken as 8 lb/ ft2 for slopes less 1⁄4 in / ft and as 5 lb/ ft2 for steeper slopes, except where the minimum allowable design snow load p exceeds p computed from Eq. (5.16). In such cases, these min ƒ water surcharges may be reduced by p p . min ƒ (W. Tobiasson and R. Redfield, ‘‘Snow Loads for the United States,’’ Part II, and S. C. Colbeck, ‘‘Snow Loads Resulting from Rain on Snow,’’ U.S. Army Cold Regions Research and Engineering Laboratory, Hanover, N.H.)
Seismic Loads. These are the result of horizontal and vertical movements imposed on a building by earth vibrations during an earthquake. Changing accelerations of the building mass during the temblor create changing inertial forces. These are assumed in building design to act as seismic loads at the various floor and roof levels in proportion to the portion of the building mass at those levels. Because analysis of building response to such dynamic loading generally is very complex, building codes permit, for design of ordinary buildings, substitution of equivalent static loading for the dynamic loading (see Art. 5.18.6).
(‘‘Minimum Design Loads for Buildings and Other Structures,’’ ASCE 7-98, American Society of Civil Engineers, 345 E. 47th St., New York, NY 10164-0619; ‘‘International Building Code 2000,’’ 1998.)
Structural members must be designed with sufficient capacity to sustain without excessive deformation or failure those combinations of service loads that will produce the most unfavorable effects. Also, the effects of such conditions as ponding of water on roofs, saturation of soils, settlement, and dimensional changes must be included. In determination of the structural capacity of a member or structure, a safety margin must be provided and the possibility of variations of material properties from assumed design values and of inexactness of capacity calculations must be taken into account.
Building codes may permit either of two methods, allowable-stress design or load–and–resistance factor design (also known as ultimate-strength design), to be used for a structural material. In both methods, design loads, which determine the required structural capacity, are calculated by multiplying combinations of service loads by factors. Different factors are applied to the various possible load combinations in accordance with the probability of occurrence of the loads.
In allowable-stress design, required capacity is usually determined by the load combination that causes severe cracking or excessive deformation. For the purpose, dead, live, wind, seismic, snow, and other loads that may be imposed simultaneously are added together, then multiplied by a factor equal to or less than 1. Load combinations usually considered in allowable-stress design are
Building codes usually permit a smaller factor when the probability is small that combinations of extreme loads, such as dead load plus maximum live load plus maximum wind or seismic forces, will occur. Generally, for example, a factor of 0.75 is applied to load-combination sums (2) to (6). Such factors are equivalent to permitting higher allowable unit stresses for the applicable loading conditions than for load combination (1). The allowable stress is obtained by dividing the unit stress causing excessive deformation or failure by a factor greater than 1.
In load–and–resistance factor design, the various types of loads are each multiplied by a load factor, the value of which is selected in accordance with the probability of occurrence of each type of load. The factored loads are then added to obtain the total load a member or system must sustain. A structural member is selected to provide a load-carrying capacity exceeding that sum. This capacity is determined by multiplying the ultimate-load capacity by a resistance factor, the value of which reflects the reliability of the estimate of capacity. Load criteria generally used are as follows:
For floors in places of public assembly, for live load in excess of 100 psf, and for parking garage live load, the load factor is taken as 1.0 for L. Em is the maximum seismic effect of horizontal and vertical forces.