Chord Design
- After the diaphragm web, (the sheathing), has been designed, you must
also consider the chord members which act as diaphragm flanges resisting
axial forces induced by the bending moment.
- At any point along the diaphragm length, you can de-couple the bending
moment to obtain chord forces:
T = C = M/b
- Building elements that serve as axially loaded chords include:
- The double top plate for a wood frame wall.
- The reinforced bond beam of a masonry wall, which is shown below.
- In masonry bond beams:
- The axial compression is resisted by the grouted masonry.
- The axial tension is resisted by the steel reinforcement.
- Generally, the strength of masonry in compression is more than adequate to resist the axial stresses.
- The steel, however, is a critical element and should be explicitly designed.
- To determine bond beam steel:
- The allowable stress increase factor of 1.33 is applied if using
the alternate basic load combinations, '97 UBC 1612.3.2.
- It should not be used if using the basic combinations, '97 UBC 1612.3.1
- In this case, the transient load could be reduced by .75, if eqn 12-11 applies.
- Given that chord steel is being designed for lateral effects only (no Lr of S or D) in this application, the strict application of '97 UBC 1612.3.1 suggests that the .75 value is not applicable either.
- Careful detailing of the steel is required to ensure a fully anchored
and continuous chord along the entire length of the diaphragm:
- This requires the lapping of tensile reinforcement at splices,
intersections, and corners.
- Lapping occurs because it is not reasonable or possible to use a single continuous rebar for the entire length of a building.
- Code specified lapping requirements are shown below. These are given in terms of ld, known as development length.
- This requires the lapping of tensile reinforcement at splices,
intersections, and corners.
