Details
- The wood diaphragm is composed of individual elements, (sheathing,
blocking, bond beam steel, or top plates), acting together as an integral
system. The key to ensuring that the elements work together as a system
is the appropriate connection of elements to each other. Sheathing nailing
requirements were covered in the Using
'97 Table 23-II-H sub-topic. An introduction to anchoring the horizontal
diaphragm to the vertical wall elements is provided in this sub-topic
called Details
- Lateral force details for all-wood frame buildings and wood-masonry
buildings are provided below. In either building type, the connections
must be detailed for lateral forces acting both parallel and perpendicular
to the wall.
- Wood-frame construction:
- Masonry wall/wood roof construciton:
- Typical ledger to masonry wall.
- Connection path for lateral loads acting parallel to the
wall.
- The diaphragm boundary nailing transfers the diaphragm reaction forces into the ledger.
- The ledger bolts need to be designed to simultaneously
carry both the vertical roof load and the parallel to
wall loads as shown in the sketch below. Check all appropriate
load combinations.
- Even though the design of the ledger and bolting is beyond the current scope of this class, it is important that you develop a conceptual knowledge of how these critical elements function.
- Connection path for lateral loads acting perpendicular
to the wall.
- These normal to the wall forces are carried to the
wall through specially designed, light gage steel anchors
that are sometimes referred to as seismic ties or purlin
anchors.
- These anchors are used to avoid cross-grain bending
of the wood ledger which the UBC prohibits ('94
UBC 1631.2.9.5 or '97 UBC 1633.2.9.5).
- In addition, these anchors avoid using nails in withdrawal or toenails.
- You can examine, in further detail, the manufacturer's
information about these anchors by:
- Referring to you Simpson Strong-Tie catalogue, or
- Visiting Simpson Strongtie
- These ties must be capable of resisting the larger
of the lateral design loads or a minimum horizontal
force of:
- 200 lb/ft if '94 UBC (at an allowable stress level)
- 280 lb/ft if '97 UBC (at a strength level where 280/1.4 = 200)
- In addition, these anchors may be spaced no more than 4' o.c. unless the wall is designed to resist bending between the anchors.
- These anchors are used to avoid cross-grain bending
of the wood ledger which the UBC prohibits ('94
UBC 1631.2.9.5 or '97 UBC 1633.2.9.5).
- In the case of buildings with concrete or masonry
walls, '97 UBC 1633.2.9.4 requires continuous ties between
diaphragm chords to distribute these anchorage forces
well into the diaphragm.
- Without special detailing, only those very small buildings with continuous framing members spanning from wall to wall would qualify.
- To get around this problem, the subdiaphragm concept was developed. This topic is beyond the scope of this class. Information on it can be found in Breyer section 15.5.
- These normal to the wall forces are carried to the
wall through specially designed, light gage steel anchors
that are sometimes referred to as seismic ties or purlin
anchors.
- Connection path for lateral loads acting parallel to the
wall.
- Top of wall connection.
- Connection path for lateral loads acting parallel to the
wall.
- Boundary nailing transfers diaphragm shear to blocking.
- Side-mounted framing anchor moves the load from the blocking into the wood plate.
- The plate shear load is then transfered to the wall by the anchor bolt which is embedded in a grouted and reinforced bond beam.
- Connection path for lateral loads acting perpendicular
to the wall.
- The anchor bolt transfers the perpendicular load from the wall into the plate.
- The front-mounted framing anchors move the load from the plate into the framing members, and
- The diaphragm field nailing transfers the load from the framing into the sheathing.
- Connection path for lateral loads acting parallel to the
wall.
- Typical ledger to masonry wall.
- Wood-frame construction:
