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CE A434 – Timber Design. Structural Behavior. 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

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ce a434 timber design

CE A434 – Timber Design

Structural Behavior

classes of systems
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.
gravity load systems1
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!
wood framing system
Wood Framing System

Sheathing supported by joists

Joists supported by beam & wall

more framing
More Framing

Beam supported by columns

Wall consists of columns called studs

continuous load paths
Continuous Load Paths
  • As in all structures, it is critical that there be identifiable continuous load paths.
alaska state fairgrounds farm exhibits building palmer alaska
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.

long span roof load path
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.

mezzanine floor system
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

cantilever loads

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.

hip beam
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.

example framing system house framing plans
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
lateral force resisting systems1
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
lateral force on walls
Lateral Force on Walls
  • See Text Page 3.9
end wall framing
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.

example building
Example Building
  • Lateral Pressures
  • Roof = 20 psf
  • 2nd Flr = 15 psf
  • 1st Flr = 10 psf
example
Example

Roof = 300 sqft

2nd flr = 340 sqft

1st flr = 180 sqft

Roof = 660 sqft

2nd flr = 510 sqft

1st flr = 270 sqft

loads from walls to horizontal diaphragms
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

horizontal diaphragms
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
example1
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

vertical lfrs rigid frames
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
vertical lfrs truss systems aka braced frames
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
vertical lfrs shear walls systems
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.
example2
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

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