CE A434 – Timber Design. Structural Behavior. Classes of Systems. Gravity Load System Supports Dead, Live, Roof Live, Snow, and other loads that result from gravitational pull. Lateral Force System Supports Wind, Seismic, Fluid, Soil loads that push laterally on the structure
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Sheathing supported by joists
Joists supported by beam & wall
Beam supported by columns
Wall consists of columns called studs
Long Span Roof Truss Girders
Awning Roof with Hip Beam
A large open exhibit building with long span truss girders.
Roof deck transfers load to supporting joists.
Each joist supports an area equal to its span times half the distance to the joist on either side.
Load rests on roof deck
The joists transfer their loads to the supporting truss girders.
The pier supports half the area supported by the truss girder plus area from other structural elements that it supports.
Each truss girder supports an area equal to its span times half the distance to the girder on either side.
The truss girders transfer their loads to the supporting piers and columns.
The girders are not single span so the tributary area for the columns cannot be graphically determined
The area tributary to a joist equals the length of the joist times the sum of half the distance to each adjacent joist.
The area tributary to a girder equals the length of the girder times the sum of half the distance to each adjacent girder.
Columns Support Girders
Girders Support Joists
Metal Deck/Slab System Supports Floor Loads Above
Joists Support Floor Deck
The point load consists of the reaction from the two supported joists which equals the tributary area (1/2 the cantilever span times the spacing of the cantilevers) times the pressure load on the floor plus the self weight of the joist.Cantilever Loads
Exterior joist carried load to the supporting cantilever beam ends
The load diagram for the cantilever (excluding self wt) consists of a single point load at the end of the cantilever.
Deck carries load to edge joist and wall.
This beam picks up load from joists of varying lengths. In this case the resulting load distribution would have a linearly varying component. The illustrated area is part of the tributary area at the roof deck level.
The hip beam also picks up a point load reaction from a pair of the roof girders.
The beam-columns do not support any roof load, they are here to resist lateral forces that they receive from the girts. They support an area that extends from locations half way to the adjacent beam-columns on each side and from floor to roof as shown.
For lateral pressures, the siding spans between the horizontal girts (yet another fancy word for a beam!)
The girts support half the siding to the adjacent girts. This is the tributary area for one girt.
The girts transfer their lateral load to the supporting beam-columns.
The beam-columns transfer their lateral loads equally to the roof and foundation.
Roof = 300 sqft
2nd flr = 340 sqft
1st flr = 180 sqft
Roof = 660 sqft
2nd flr = 510 sqft
1st flr = 270 sqft
Roof = 12,000 # = 200 plf
2nd flr = 6,300 # = 105 plf
1st flr = 2,700 # = 45 plf
Roof = 5,200 # = 60 plf to 200 plf
2ndflr = 4,200 # = 105 plf
1stflr = 1,800 # = 45 plf
Direction #1 Reactions
Roof = 6,000 lb = 150 plf
2nd flr = 3,150 lb = 78.8 plf
Direction #2 Reactions
Roof = 2,600 lb = 43.3 plf
2nd flr = 2,100 lb = 35 plf
Direction #1 Forces
Roof = 6,000 #
2ndflr = 3,150 #
2nd Story Shear = 6,000 lb = 150 plf
1st Story Shear = 9,150 lb = 229 plf
Direction #2 Forces
Roof = 2,600 #
2ndflr = 2,100 #
2nd Story Shear = 2,600 lb = 43.3 plf
1st Story Shear = 4,700 lb = 118 plf