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Structural Loads
Dead Loads: Gravity loads ofcons an magn u es an xepositions that act permanently on
.of the weights of the structuralsystem itself and of all othermaterial and equipment perma-
nently attached to the structuralsys em. e g s o permanenequipment, such as heating and
- ,
usually obtained from themanufacturer.
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Table 1. Typical Design Dead Loads
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Dead Load Adjustments
Adjustments are made in the dis-tribution of dead loads due to theplacement of utility lines under thefloor system and fixtures (lights,
, . ,which is the floor for the next storyif one exists. Rather than worrabout the actual weight andlocation of such routine buildingadditions, the structural engineerwill normally assess an increase
lbs/ft 2 (psf) to ensure that thestren th of the floor beams and
4columns are adequate.
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In addition, designers try to posi-tion beams directly under heavy
directly into the supports or
then the load is smeared as anadditional floor load pressure of10 to 40 lbs/ft 2, depending on the
masonry wall size.
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Live Loads: Structural (typicallyravit loads of var in ma ni-
tudes and/or positions caused bythe use of the structure.
Furthermore, the position of a live,
member of the structure must bedesigned for the position of theload that causes the maximumstress in that member.
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Buildin Loads
The magnitudes of building design
building codes. Live loads forbuildings are usually specified asuniformly distributed surface loadsin pounds per square foot or
=m2). Distributed live loads are
iv n in T l 2.
Design concentrated live loadsare given in the USCS (USCustomary System) units in Table
7
.
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Table 2. T ical Desi n Live Loads
Occupancy Use Live Load, lb/ft 2
(kN/m 2)
ssem y areas an t eatersFixed seats (fastened to floor) 60 (2.87)Lobbies 100 (4.79)
Stage floors 150 (7.18)Libraries
Reading rooms 60 (2.87)Stack rooms 150 (7.18)
Office buildingsLobbies 100 (4.79)Offices 50 (2.40)
ResidentialHabitable attics and sleeping areas 30 (l.44)Uninhabitable attics with storage 20 (0.96)
o er areas .
SchoolsClassrooms 40 (l.92)
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Table 3. Typical Concentrated
Live Loads
rea or tructuraComponent
oncentrateLive Load
eva or ac neRoom on 4-in 2
300 lbs
ce oors s
Center or Stair Treadon 4-in 2
Sidewalks 8000 lbs
Accessible Ceilings 200 lbs
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r ge oa s
Live loads due to vehicular trafficon highway bridges are specifiedby the American Association of
Officials (AASHTO) Specification.Since the heaviest loadin onhighway bridges is usually causedby trucks, the AASHTOSpecification defines two systemsof standard loads, HS trucks and
,vehicular loads for design
ur oses as shown in the10following figure.
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Bridge Loading: (a) HS 20 44Truck; b Lane Loads
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Im act Load Factors
When live loads are appliedrap y o a s ruc ure, ey causelarger stresses than those that
loads would have been appliedgradually. This dynamic effect ofthe load is referred to as impact .
ve oa s expec e o causesuch a dynamic effect on struc-
factors.
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Buildin Load Im act
Building load impact factors are.
impact loads are added to thedesign loads to approximate thedynamic effect of load on a staticanalysis ( I impact factor).Loading Case % IElevator Supports & Machinery 100Light machinery supports 20Reciprocating machine supports 50Hangers supporting floors & balconies 33Crane support girders 25
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Since loaded span length,
this simply means that a shorts an brid e will ex eriencegreater dynamic impact than along span bridge.
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Roof Live Loads
Largest roof loads typically caused
pitch rise/spanLr = 20 R 1 R2
12 < L 20
Lr horizontal projection roof live
R1, R 2= live load reduction factors
R1 accounts for size of tributaryarea of roof column A t
16R2 effect of the roof rise
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2
1.0 A 200ft
2 21 t t
2
R 1.2 0.001A 200ft A 600ft
0.6 A 600ft
= <
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loads caused by the environmentin which the structure is locatedspecial examples of live loads.Rain, snow, ice, wind and earth-qua e oa ngs are examp es oenvironmental loads.
Rain Loads: Ponding wateraccumulates on roof faster than itruns off thus increasing the roofloads. Typically, roofs with slopeso . n t or greater are notsubjected to ponding unless roof
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Wind loads are produced by theflow of wind around structures.Wind load magnitudes vary inproportion to the distance from
e ase o e s ruc ure, peawind speed, type of terrain,
,building and roof slope.
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Variation of Wind Velocitywith Distance Above
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Ground
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p pressure on s op ng roo ; w n
speed on line 2 is larger than line 1 due togreater path length. Increased velocity
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pressure differential between inside andoutside of the building.
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22Wind Speed Map of US
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Roof loading on the windward
side is a suction load for small.ing for a fixed value of h/L willlead to the windward roof load
being a pressure load. Con-versely, increasing h/L for a fixed
23w resu t n a suct on roo oa
on the windward side.
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Earth uake Forces
An earthquake is a sudden un-
surface . Although the groundsurface moves in both horizontaland vertical directions during anearthquake, the magnitude of thev umotion is usually small and does
most structures.
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(N TE: This last statement isbeing vigorously reconsidered in
California and Japan. )
ground motion that causes struc-
considered in designs of struc-tures located in earthquake-prone areas.
Vertical motions that result in
differential upward move-ments do cause large stresses
25n s ruc ures .
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Lateral Force Distribution due to
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Snow Loads
Design snow load for a structureis based on the round snow loadfor its geographic location, expo-sure to wind, and its thermal, geo-metric, and functional charac-teristics. In most cases, there is
ground.
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Hydrostatic and Soil Pressures
Hydrostatic pressure acts normalto the submerged surface of thestructure, with its magnitudevarying linearly with height, ass own n e gure e ow.
= unit
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