2 Classes of Systems Gravity Load System Lateral Force System Supports Dead, Live, Roof Live, Snow, and other loads that result from gravitational pull.Lateral Force SystemSupports Wind, Seismic, Fluid, Soil loads that push laterally on the structureBoth systems must provide a COMPLETE and IDENTIFIABLE load pathPrinciples of Statics and Structural Analysis are used to trace the loads through the structure.
4 Gravity Load SystemsGravity Loads are generally supported by systems of beams and columns.In Timber systems:Loads are applied to sheathing which acts as a continuous beam supported by closely spaced beams known as JOISTS or by TRUSSESThe JOISTS are generally supported by BEAMS or TRUSSES or WALLSBEAMS are generally supported by other beams or COLUMNSTimber Walls consist of a series of closely spaced columns know as STUDSBEAMS, COLUMNS, and WALLS can be supported by other BEAMS, COLUMNS, WALLS, or FOUNDATIONSYou must always be able to identify the support for each structural element all the way to the ground!
5 Wood Framing System Sheathing supported by joists Joists supported by beam & wall
6 More Framing Beam supported by columns Wall consists of columns called studs
9 Continuous Load PathsAs in all structures, it is critical that there be identifiable continuous load paths.
10 Alaska State Fairgrounds Farm Exhibits Building Palmer, Alaska Long Span Roof Truss GirdersMezzanine AreaAwning RoofAwning Roof with Hip BeamA large open exhibit building with long span truss girders.
11 Long Span Roof Load Path 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 deckThe 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.Long Span Roof Load Path
12 Mezzanine Floor System The girders are not single span so the tributary area for the columns cannot be graphically determinedThe 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 GirdersGirders Support JoistsMetal Deck/Slab System Supports Floor Loads AboveJoists Support Floor DeckMezzanine Floor System
13 Exterior joist carried load to the supporting cantilever beam ends 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.Exterior joist carried load to the supporting cantilever beam endsThe 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.Cantilever Loads
14 Hip BeamThis 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.
15 Example Framing System House Framing Plans Check out the drawings for the House found on the website for the Beginner’s Guide to Structural Engineering:For each member:Identify what the member supportsDraw a load diagram for the memberIdentify what supports the memberCompute the reactions for the member and identify where they appear on the supporting member
17 Lateral Force Resisting Systems Lateral forces are applied to wall/roof systems which generally transfer the forces to horizontal diaphragmsHorizontal diaphragms are used to transfer forces to the vertical components of the LFRSThe three most common types of vertical LFRS components are:Rigid FramesVertical TrussLateral forces are resisted by axial forces in the membersBracing is used to create a trussConnections are generally assumed to be pinnedShear Walls
19 End Wall FramingThe 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.
20 Example Building Lateral Pressures Roof = 20 psf 2nd Flr = 15 psf 1st Flr = 10 psf
25 Vertical LFRS: Rigid Frames Lateral forces are resisted by bending in the membersMoment resisting connections are requiredDifficult to do in timberMoment connections can be approximated with KNEE BRACINGLots of indeterminate analysis!Rigid frames are actually very flexible compared to the other systemsCalled RIGID because the connections are rigid
30 Vertical LFRS: Shear Walls Systems SHEAR WALLS act as vertical cantilever beamsShear walls carry the forces via shear in the wall and chord forces to handle the momentThis is the most common LFRS in timber structures.
31 Example Direction #1 Forces Roof = 6,000 # 2nd flr = 3,150 # 2nd Story Shear = 6,000 lb = 150 plf1st Story Shear = 9,150 lb = 229 plfDirection #2 ForcesRoof = 2,600 #2nd flr = 2,100 #2nd Story Shear = 2,600 lb = 43.3 plf1st Story Shear = 4,700 lb = 118 plf
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