Load Paths and Tributary Area Examples

© T. Bartlett Quimby, 2007

A Beginner’s Guide to Structural Mechanics/Analysis

Alaska State FairgroundsFarm Exhibits Building

Palmer, 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 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 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 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 Framing

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 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.