CE 382 L2 - Loads

1 1 Dead Loads :Gravity Loads of constant magnitudesand fixed positions that act permanently on Structural Loadsppythe 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 1equipment, such as heating and air-conditioning systems, are usually obtained from the 1. Typical Design Dead Loads2 Table 1. Typical Design Dead Loads3 Dead Load AdjustmentsAdjustments are made in the dis-tribution of dead Loads due to the placement of utility lines under the floor system and fixtures (lightsfloor system and fixtures (lights, ducts, etc.))

2 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 4gwill normally assess an increase in the floor dead load of 10 to 15 lbs/ft2(psf) to ensure that the strength of the floor, beams, and columns are addition, designers try to posi-tion beams directly under heavy masonry walls to carry this weight directly into the supports or yppcolumns. If this is not possible, then the load is smeared as an additional floor load pressure of 10 to 40 lbs/ft2, depending on the masonry wall Loads :Structural (typically gravity) Loads of varying magni-tudes and/or positions caused by the use of the structurethe use of the structure.

3 Furthermore, the position of a live load may change, so each member of the structure must be designed for the position of the load that causes the maximum6load that causes the maximum stress in that LoadsThe magnitudes of building design live Loads are usually specified in building codes. Live Loads for b ildillifi dbuildings are usually specified as uniformly distributed surface Loads in pounds per square foot or kilopascals (kN/m2; 1 Pa = 1 N/ m2). Distributed live Loads are given in Table 2. 7 Design concentrated live Loads are given in the USCS (US Customary System) units in Table 2.

4 Typical Design Live LoadsOccupancy UseLive Load, lb/ft2(kN/m2)Assembly areas and theatersFixed seats (fastened to floor)60 ( )Lobbies100 ( )Lobbies100 ( )Stage floors150 ( )LibrariesReading rooms60 ( )Stack rooms150 ( )Office buildingsLobbies100 ( )Offices50 ( )Residential8 Habitable attics and sleeping areas30 ( )Uninhabitable attics with storage20 ( )All other areas40 ( )SchoolsClassrooms40 ( )Corridors above the first floor80 ( )3 Table 3. Typical Concentrated Live LoadsArea or Structural ComponentConcentrated Live LoadElevator Machine Room on 4-in2300 lbsOffice Floors2000 lbsCenter or Stair Tread on 4-in2300 lbs9 Sidewalks8000 lbsAccessible Ceilings200 lbsBridge LoadsLive Loads due to vehicular traffic on highway bridges are specified by the American association ofby the American association of State Highway and Transportation Officials (AASHTO) Specification.

5 Since the heaviest loading on highway bridges is usually caused by trucks, the AASHTO Specification defines two systems 10pyof standard Loads , HS trucks and lane loading, to represent the vehicular Loads for design purposes as shown in the following Loading: (a) HS 20 44 Truck; (b) Lane Loads11 Impact Load FactorsWhen live Loads are applied rapidly to a structure, they cause larger stresses than those that gwould be produced if the same Loads would have been applied gradually. This dynamic effect of the loadis referred to as impact. Live Loads expected to cause 12such a dynamic effect on struc-tures are increased by impact Load ImpactBuilding load impact factors are given in the table below.

6 These impact Loads are added to the di ldti tthdesign Loads to approximate the dynamic effect of load on a static analysis (I impact factor).Loading Case% IElevator Supports & Machinery100 Light machinery supports2013gypp20 Reciprocating machine supports50 Hangers supporting floors & balconies33 Crane support girders25 bridge Impact Load MultiplierAASHTO specifies the following expression for highway bridges:(U S Units)50I03= ( Units)(SI Units)I impact + +14pL length in feet (or meters) of the portion of the span loaded to cause the maximum stress in the memberSince loaded span length inversely affects bridge impact, this simply means that a short span bridge will experience greater dynamic impact than a long span bridgelong span Live LoadsLargest roof Loads typically caused by repair and maintenancepitch rise/spanpitch rise/spanLr= 20 R1R212 < Lr 20Lr horizontal projection roof live load 16R1.

7 R2= live load reduction factorsR1 accounts for size of tributary area of roof column AtR2 effect of the roof rise52 F 12 = << ftA600 ft = << 12 F = rise in inches per foot of span= pitch x 32 dome or arch roofEnvironmental Loads :Structural Loads caused by the environment in which the structure is located; special examples of live Loads . Rain snow ice wind and earthRain, snow, ice, wind and earth-quake loadings are examples of environmental Loads : Ponding water accumulates on roof faster than it runs off thus increasing the roof18runs off thus increasing the roof Loads .

8 Typically, roofs with slopes of in/ft or greater are not subjected to ponding unless roof drains become loadsare produced by the flow of wind around structures. Wind load magnitudes vary in proportion to the distance fromproportion to the distance from the base of the structure, peak wind speed, type of terrain, importance factor, and side of building and roof slope. 1920 Variation of Wind Velocity with Distance Above Ground621 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 outside of the Speed Map of USRoof loading on the windward side is a suction load for small angles and h/L ratios.

9 Increas-iffi dlf h/L ill 23ing for a fixed value of h/L will lead to the windward roof load being a pressure load. Con-versely, increasing h/L for a fixedwill result in a suction roof load on the windward side. Earthquake ForcesAn earthquakeis a sudden un-dulation of a portion of the earth s surface. Although the ground fibthhitlsurface moves in both horizontal and vertical directions during an earthquake, the magnitude of the vertical component of ground motion is usually small and does not have a significant impact on most structures24most structures.

10 7(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 motions that result in differential upward move-ments do cause large stresses in Force Distribution due to Lateral Earthquake Motion27 Snow LoadsDesign snow load for a structure is based on the ground snow load for its geographic location, expo-sure to wind and its thermal geosure to wind, and its thermal, geo-metric, and functional charac-teristics.