8/9/2019 CE 382 L2 - Loads

1/30

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.

1

8/9/2019 CE 382 L2 - Loads

2/30

8/9/2019 CE 382 L2 - Loads

3/30

Table 1. Typical Design Dead Loads

3

8/9/2019 CE 382 L2 - Loads

4/30

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.

8/9/2019 CE 382 L2 - Loads

5/30

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.

5

8/9/2019 CE 382 L2 - Loads

6/30

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.

6

8/9/2019 CE 382 L2 - Loads

7/30

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

.

8/9/2019 CE 382 L2 - Loads

8/30

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)

8

.

8/9/2019 CE 382 L2 - Loads

9/30

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

9

8/9/2019 CE 382 L2 - Loads

10/30

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.

8/9/2019 CE 382 L2 - Loads

11/30

Bridge Loading: (a) HS 20 44Truck; b Lane Loads

11

8/9/2019 CE 382 L2 - Loads

12/30

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.

12

8/9/2019 CE 382 L2 - Loads

13/30

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

13

8/9/2019 CE 382 L2 - Loads

14/30

8/9/2019 CE 382 L2 - Loads

15/30

Since loaded span length,

this simply means that a shorts an brid e will ex eriencegreater dynamic impact than along span bridge.

15

8/9/2019 CE 382 L2 - Loads

16/30

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

8/9/2019 CE 382 L2 - Loads

17/30

2

1.0 A 200ft

2 21 t t

2

R 1.2 0.001A 200ft A 600ft

0.6 A 600ft

= <

8/9/2019 CE 382 L2 - Loads

18/30

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

18

.

8/9/2019 CE 382 L2 - Loads

19/30

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.

19

8/9/2019 CE 382 L2 - Loads

20/30

Variation of Wind Velocitywith Distance Above

20

Ground

8/9/2019 CE 382 L2 - Loads

21/30

p pressure on s op ng roo ; w n

speed on line 2 is larger than line 1 due togreater path length. Increased velocity

21

pressure differential between inside andoutside of the building.

8/9/2019 CE 382 L2 - Loads

22/30

22Wind Speed Map of US

8/9/2019 CE 382 L2 - Loads

23/30

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.

8/9/2019 CE 382 L2 - Loads

24/30

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.

24

8/9/2019 CE 382 L2 - Loads

25/30

(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 .

8/9/2019 CE 382 L2 - Loads

26/30

Lateral Force Distribution due to

26

8/9/2019 CE 382 L2 - Loads

27/30

27

8/9/2019 CE 382 L2 - Loads

28/30

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.

28

8/9/2019 CE 382 L2 - Loads

29/30

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

29

8/9/2019 CE 382 L2 - Loads

30/30