1. CE A434 – Timber Design
Structural Behavior

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 System
Supports Wind, Seismic, Fluid, Soil loads that push laterally on the structure
Both systems must provide a COMPLETE and IDENTIFIABLE load path
Principles of Statics and Structural Analysis are used to trace the loads through the structure.

3. Gravity Load systems

4. Gravity Load Systems
Gravity 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 TRUSSES
The JOISTS are generally supported by BEAMS or TRUSSES or WALLS
BEAMS are generally supported by other beams or COLUMNS
Timber Walls consist of a series of closely spaced columns know as STUDS
BEAMS, COLUMNS, and WALLS can be supported by other BEAMS, COLUMNS, WALLS, or FOUNDATIONS
You 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

7. Sample Floor Framing System

8. Gravity Load Paths

9. Continuous Load Paths
As 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 Girders
Mezzanine Area
Awning Roof
Awning Roof with Hip Beam
A 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 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.
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 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

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 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.
Cantilever Loads

14. 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.

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 supports
Draw a load diagram for the member
Identify what supports the member
Compute the reactions for the member and identify where they appear on the supporting member

16. Lateral force resisting systems

17. Lateral Force Resisting Systems
Lateral forces are applied to wall/roof systems which generally transfer the forces to horizontal diaphragms
Horizontal diaphragms are used to transfer forces to the vertical components of the LFRS
The three most common types of vertical LFRS components are:
Rigid Frames
Vertical Truss
Lateral forces are resisted by axial forces in the members
Bracing is used to create a truss
Connections are generally assumed to be pinned
Shear Walls

18. Lateral Force on Walls
See Text Page 3.9

19. End Wall Framing
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.

20. Example Building Lateral Pressures Roof = 20 psf 2nd Flr = 15 psf
1st Flr = 10 psf

21. Example Roof = 300 sqft 2nd flr = 340 sqft 1st flr = 180 sqft

22. Loads from Walls to Horizontal Diaphragms
Direction #1
Roof = 12,000 # = 200 plf
2nd flr = 6,300 # = 105 plf
1st flr = 2,700 # = 45 plf
Direction #2
Roof = 5,200 # = 60 plf to 200 plf
2nd flr = 4,200 # = 105 plf
1st flr = 1,800 # = 45 plf

23. Horizontal Diaphragms
Wood diaphragms are considered to be flexible
Horizontal diaphragms transfer load collected from the walls by beam action to the supporting vertical LFRS components

24. Example Direction #1 Reactions Direction #2 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

25. Vertical LFRS: Rigid Frames
Lateral forces are resisted by bending in the members
Moment resisting connections are required
Difficult to do in timber
Moment connections can be approximated with KNEE BRACING
Lots of indeterminate analysis!
Rigid frames are actually very flexible compared to the other systems
Called RIGID because the connections are rigid

26. Example Rigid Frame

27. Knee Brace

28. Vertical LFRS: Truss Systems (aka Braced Frames)
Lateral forces are resisted by axial forces in the members
Bracing is used to create a truss
Connections are generally assumed to be pinned

29. Example Trussed Systems

30. Vertical LFRS: Shear Walls Systems
SHEAR WALLS act as vertical cantilever beams
Shear walls carry the forces via shear in the wall and chord forces to handle the moment
This 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 plf
1st Story Shear = 9,150 lb = 229 plf
Direction #2 Forces
Roof = 2,600 #
2nd flr = 2,100 #
2nd Story Shear = 2,600 lb = 43.3 plf
1st Story Shear = 4,700 lb = 118 plf