CE A434 – Timber Design

1. CE A434 – Timber Design Structural Behavior

2. Classes of Systems • Gravity Load 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 FairgroundsFarm Exhibits BuildingPalmer, 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.

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

13. 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. Cantilever Loads 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.

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 SystemHouse Framing Plans • Check out the drawings for the House found on the website for the Beginner’s Guide to Structural Engineering: www.bgstructuralengineering.com • 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 Roof = 660 sqft 2nd flr = 510 sqft 1st flr = 270 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 2ndflr = 4,200 # = 105 plf 1stflr = 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 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 # 2ndflr = 3,150 # --------------------------- 2nd Story Shear = 6,000 lb = 150 plf 1st Story Shear = 9,150 lb = 229 plf Direction #2 Forces Roof = 2,600 # 2ndflr = 2,100 # --------------------------- 2nd Story Shear = 2,600 lb = 43.3 plf 1st Story Shear = 4,700 lb = 118 plf