Structural Loads Structural Loads Table 1. Typical Design Dead Loads Dead Loads: Gravity loads of constant magnitudes and fixed positions that act permanently on p p y the structure. Such loads consist of the weights of the structural system itself and of all other material and equipment perma- nently attached to the structural system. Weights of permanent equipment, such as heating and air-conditioning systems, are usually obtained from the manufacturer. 1 2 Table 1. Typical Design Dead Loads Dead Load Adjustments Adjustments are made in the dis- tribution of dead loads due to the placement of utility lines under the floor system and fixtures (lights floor system and fixtures (lights, ducts, etc.) on the floor ceiling, which is the floor for the next story if one exists. Rather than worry about the actual weight and location of such routine building additions, the structural engineer g will normally assess an increase in the floor dead load of 10 to 15 lbs/ft 2 (psf) to ensure that the strength of the floor, beams, and columns are adequate. 3 4 1
In addition, designers try to posi- Live Loads: Structural (typically tion beams directly under heavy gravity) loads of varying magni- masonry walls to carry this weight tudes and/or positions caused by directly into the supports or y pp the use of the structure. the use of the structure columns. If this is not possible, then the load is smeared as an Furthermore, the position of a live additional floor load pressure of load may change, so each 10 to 40 lbs/ft 2 , depending on the member of the structure must be masonry wall size. designed for the position of the load that causes the maximum load that causes the maximum stress in that member. 5 6 Building Loads Table 2. Typical Design Live Loads The magnitudes of building design Live Load, lb/ft 2 Occupancy Use ( kN/m 2 ) live loads are usually specified in Assembly areas and theaters building codes. Live loads for Fixed seats (fastened to floor) 60 (2.87) Lobbies Lobbies 100 (4.79) 100 (4.79) b ildi buildings are usually specified as ll ifi d Stage floors 150 (7.18) uniformly distributed surface loads Libraries in pounds per square foot or Reading rooms 60 (2.87) Stack rooms 150 (7.18) kilopascals (kN/m 2 ; 1 Pa = 1 N/ Office buildings m 2 ). Distributed live loads are Lobbies 100 (4.79) Offices 50 (2.40) given in Table 2. Residential Habitable attics and sleeping areas 30 (l.44) Design concentrated live loads Uninhabitable attics with storage 20 (0.96) All other areas 40 (l.92) are given in the USCS (US Schools Customary System) units in Table Classrooms 40 (l.92) 3. Corridors above the first floor 80 (3.83) 7 8 2
Table 3. Typical Concentrated Bridge Loads Live Loads Live loads due to vehicular traffic Area or Structural Concentrated on highway bridges are specified Component Live Load by the American Association of by the American Association of State Highway and Transportation Elevator Machine 300 lbs Room on 4-in 2 Officials (AASHTO) Specification. Since the heaviest loading on Office Floors 2000 lbs highway bridges is usually caused by trucks, the AASHTO Center or Stair Tread 300 lbs on 4-in 2 Specification defines two systems p y of standard loads, HS trucks and Sidewalks 8000 lbs lane loading , to represent the vehicular loads for design Accessible Ceilings 200 lbs purposes as shown in the following figure. 9 10 Bridge Loading: (a) HS 20 – 44 Impact Load Factors Truck; (b) Lane Loads When live loads are applied rapidly to a structure, they cause larger stresses than those that g would be produced if the same loads would have been applied gradually. This dynamic effect of the load is referred to as impact . Live loads expected to cause such a dynamic effect on struc- tures are increased by impact factors. 11 12 3
Bridge Impact Load Multiplier Building Load Impact AASHTO specifies the following Building load impact factors are expression for highway bridges: given in the table below. These 50 impact loads are added to the = = ≤ ≤ I I 03 0.3 (U.S. Units) (U S Units) design loads to approximate the d i l d t i t th + L 125 dynamic effect of load on a static 15 analysis ( I ≡ impact factor). = ≤ I 0.3 (SI Units) + L 38.1 Loading Case % I Elevator Supports & Machinery 100 ≡ impact factor p I Light machinery supports g y pp 20 20 Reciprocating machine supports L ≡ length in feet (or meters) of the 50 Hangers supporting floors & balconies 33 portion of the span loaded to Crane support girders 25 cause the maximum stress in the member 13 14 Since loaded span length Roof Live Loads inversely affects bridge impact, this simply means that a short Largest roof loads typically caused span bridge will experience by repair and maintenance greater dynamic impact than a pitch ≡ rise/span pitch ≡ rise/span long span bridge. long span bridge L r = 20 R 1 R 2 ≤ 12 < L r 20 L r ≡ horizontal projection roof live load R 1 , R 2 = live load reduction factors R 1 – accounts for size of tributary area of roof column A t R 2 – effect of the roof rise 15 16 4
Environmental Loads: Structural loads caused by the environment in which the structure is located; special examples of live loads. Rain, snow, ice, wind and earth- Rain snow ice wind and earth ⎧ ⎧ ≤ 2 2 1.0 A 200ft t ⎪ quake loadings are examples of = − < < ⎨ 2 2 R 1.2 0.001A 200ft A 600ft 1 t t environmental loads. ⎪ ≥ 2 ⎩ 0.6 A 600ft t Rain Loads: Ponding – water ≤ ⎧ 1.0 F 4 accumulates on roof faster than it ⎪ = − < < ⎨ ⎨ R 1.2 0.05F 4 F 12 runs off thus increasing the roof runs off thus increasing the roof 2 2 ⎪ ≥ loads. Typically, roofs with slopes ⎩ 0.6 F 12 of 0.25 in/ft or greater are not F = rise in inches per foot of span subjected to ponding unless roof drains become clogged. = pitch x 32 – dome or arch roof 17 18 Wind loads are produced by the flow of wind around structures. Wind load magnitudes vary in proportion to the distance from proportion to the distance from the base of the structure, peak wind speed, type of terrain, importance factor, and side of building and roof slope. Variation of Wind Velocity with Distance Above Ground 19 20 5
Uplift pressure on sloping roof; wind speed on line 2 is larger than line 1 due to greater path length. Increased velocity reduces pressure on top of roof creating a pressure differential between inside and 21 Wind Speed Map of US 22 outside of the building. Earthquake Forces An earthquake is a sudden un- dulation of a portion of the earth’s surface. Although the ground surface moves in both horizontal f i b th h i t l and vertical directions during an earthquake, the magnitude of the vertical component of ground Roof loading on the windward motion is usually small and does side is a suction load for small θ not have a significant impact on angles and h/L ratios. Increas- θ θ most structures most structures. ing for a fixed value of h/L will i f fi d l f h/L ill lead to the windward roof load being a pressure load. Con- versely, increasing h/L for a fixed θ will result in a suction roof load 23 24 on the windward side. 6
(NOTE: This last statement is being vigorously reconsidered in light of recent earthquakes in California and Japan.) It is the horizontal component of ground motion that causes struc- tural damage and that must be considered in designs of struc- tures located in earthquake- prone areas. p Vertical motions that result in differential upward move- Lateral Force Distribution due to ments do cause large stresses Lateral Earthquake Motion in structures . 25 26 Snow Loads Design snow load for a structure is based on the ground snow load for its geographic location, expo- sure to wind and its thermal geo sure to wind, and its thermal, geo- metric, and functional charac- teristics. In most cases, there is less snow on the roof than on the ground. 27 28 7
Hydrostatic and Soil Pressures Miscellaneous Loads Hydrostatic pressure acts normal • Ice loads to the submerged surface of the • Flooding structure, with its magnitude • Blast loads varying linearly with height as varying linearly with height, as • Thermal forces shown in the figure below. • Centrifugal forces • Longitudinal loads due to brak- ing of large trucks or trains on bridges, ships entering a harbor or cranes on a rail harbor, or cranes on a rail γ = unit weight 29 30 8
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