CE A434 Timber Design

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CE A434 Timber Design

• Structural Behavior

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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.

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Gravity Load systems
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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!

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Wood Framing System
Sheathing supported by joists
Joists supported by beam wall
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More Framing
Beam supported by columns
Wall consists of columns called studs
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Sample Floor Framing System
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Gravity Load Paths
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Continuous Load Paths

• As in all structures, it is critical that there
  be identifiable continuous load paths.

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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.
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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.
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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
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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.
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.
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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.
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Example Framing SystemHouse Framing Plans

• Check out the drawings for the House found on the
  website for the Beginners 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

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Lateral force resisting systems
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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

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Lateral Force on Walls

• See Text Page 3.9

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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.
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Example Building

• Lateral Pressures
• Roof 20 psf
• 2nd Flr 15 psf
• 1st Flr 10 psf

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Example
Roof 300 sqft 2nd flr 340 sqft 1st flr 180
sqft
Roof 660 sqft 2nd flr 510 sqft 1st flr 270
sqft
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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
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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

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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
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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

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Example Rigid Frame
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Knee Brace
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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

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Example Trussed Systems
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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.

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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