Load Paths and Tributary Area Examples © T. Bartlett Quimby, 2007 A Beginner’s Guide to Structural Mechanics/Analysis Alaska State Fairgrounds Farm Exhibits Building Palmer, Alaska Long Span Roof Truss Girders Mezzanine Area Awning Roof with Hip Beam A large open exhibit building with long span truss girders. Awning Roof 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. Roof deck transfers load to supporting joists. Load rests on roof deck The pier supports half the area supported by the truss Each truss supports girdergirder plus area from an other area equal to itselements span times structural that it half the distance to the girder supports. The truss girders on either side. transfer their loads to the supporting piers and columns. Long Span Roof Load Path 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 Mezzanine Floor System The point load consists of the reaction from the two supported joists which equals the tributary area (1/2 the cantilever span times the Exterior spacing joist of the carried cantilevers) load to the times supporting the pressure cantilever load on the floor beam plus ends the self weight of the joist. The load diagram for the cantilever (excluding self wt) consists of a single point load at the end of the Deck carries load to edge cantilever. joist and wall. Cantilever Loads End Wall Framing The beam-columns do not support any roof For lateral pressures, the load, they are here to siding spans between the resist lateral forces that horizontal girts (yet they fancy receive from another word forthe a girts. They support an beam!) The girts support the area that extendshalf from siding to the adjacent girts. locations half way to the This is the tributary area adjacent beam-columns one girt. on each sidefor and from floor to roof as shown. The girts transfer their lateral load to the The beam-columns supporting beamtransfer their lateral columns. 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.