Download - Trib Are a Examples 2
Load Paths and Load Paths and Tributary Area ExamplesTributary Area Examples
© T. Bartlett Quimby, 2007© T. Bartlett Quimby, 2007
A Beginner’s Guide to Structural A Beginner’s Guide to Structural Mechanics/AnalysisMechanics/Analysis
Alaska State FairgroundsAlaska State FairgroundsFarm Exhibits BuildingFarm Exhibits Building
Palmer, AlaskaPalmer, Alaska
A large open exhibit building with long span truss girders.
Long Span Roof Truss Girders Mezzanine Area Awning Roof
Awning Roof with Hip Beam
Long Span Roof Load PathLong Span Roof Load Path
Load rests on roof deck
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.
The joists transfer their loads to the supporting truss
girders.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 pier supports half the area supported by the truss girder plus area from other structural elements that it supports.
Mezzanine Floor SystemMezzanine Floor System
Metal Deck/Slab System Supports Floor Loads Above
Joists Support Floor Deck
Girders Support JoistsColumns Support Girders
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.
The girders are not single span so the
tributary area for the columns cannot be
graphically determined
Cantilever LoadsCantilever Loads
Deck carries load to edge joist and wall.
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.
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.
End Wall End Wall FramingFraming
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 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.
The beam-columns transfer their lateral
loads equally to the roof and foundation.
Hip BeamHip Beam
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.