Horizontal Diaphragms of Complex shape

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Horizontal Diaphragms of Complex shape

• by Bart Quimby, P.E., Ph.D
• UAA School of Engineering
• CE 434 - Timber Design

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Multispan Wood Diaphragms

• Considered to be a series of simply supported
  beams.
• Design each diaphragm as separately.

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Another View
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Same Diaphragm, Other Direction

• In this direction, the diaphragm is considered to
  be ONE diaphragm.
• The shear wall on Grid B does not contribute.

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Diaphragm Deflections

• SDPWS 4.2.2
• Must account deformation from
• Bending
• Shear
• Fasteners
• Chord Splice Slip

Applied Technology Council Guidelines for the
Design of Horizontal Wood Diaphragms, 1981
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Drag StrutsGetting the Diaphragm Shear into the
Shear walls

• Drag struts are axial force members that
  collect the horizontal diaphragm shears and
  redistribute them to the shear walls.
• Drag struts are PARALLEL to the direction of
  force.
• Need to draw an AXIAL FORCE DIAGRAM in order to
  compute the force a drag strut.
• The member used for the drag strut is often used
  as a diaphragm chord when the direction of force
  changes to the other principle direction.

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Unit Shears on Drag Strut

• Unit shears (on each side) seen by the drag strut
  equal the total shear force being transferred
  divided by the length (on each side) attached to
  the drag strut.

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Drag Strut Diagrams

• The AXIAL FORCE DIAGRAM show the axial force as a
  function of the length of the member.
• Note that the same member is a chord when the
  direction of force changes. Must also handle
  chord forces.

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Irregular Shaped Diaphrams
Amrhein, James E Reinforced Masonry Engineering
Handbook, 4th edition
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More Irregular Shapes
Amrhein, James E Reinforced Masonry Engineering
Handbook, 4th edition
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L Shaped Diaphragms

• Subdivide into diaphragms according to shear wall
  locations.

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Subdivided Diaphragm, 1

• Along Grid B, Shears are inversely proportional
  to the length of the drag strut.
• Boundary nailing along the strut can be two rows,
  one for each dia. or one row capable of handling
  the sum of the diaphragm shears.

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Drag Strut Diagram, 1

• The peak force equals L1B(vA1vA2) or
  L1A(vwall-vA1).

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The Other Direction

• Drag Strut in this direction also!

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Subdivided Diaphragm, 2

• Note that there are different diaphragms in each
  direction.

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Load Transfer at Vertical Offset
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Vertical Offsets
Applied Technology Council Guidelines for the
Design of Horizontal Wood Diaphragms, 1981
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Diaphragms of Variable Depth

• Length parallel to force varies.
• Unit Shear varies with the depth.

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Chord Extensions

• Chord Force varies with depth.
• Extensions must be designed to anchor the chord
  into the diaphragm.
• Length of chord extension affects shear force in
  diaphragm.
• Diaphragm shear due to extension Chord
  force/length of extension.

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Other Direction

• Consider each leg as a cantilever beam in order
  to determine the moment and resulting chord
  force.
• Anchor the chord into the diaphragm as previously
  described.

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Irregular Examples
Applied Technology Council Guidelines for the
Design of Horizontal Wood Diaphragms, 1981
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Openings In Diaphragms
Applied Technology Council Guidelines for the
Design of Horizontal Wood Diaphragms, 1981