Diaphragm Basics - The Analysis of Irregular Shaped Structures - page 68

diaphragms. The lessons learned from these failures have prompted significant changes
in the code. The failures occurred partly due to underestimating the magnitude of the
forces that are applied to the connection from the concrete or masonry wall, and partly
due to poor detailing of the connection itself. Detailing of the connection prior to the
code changes was typically done in a way that caused cross-grain bending in the led-
ger that was attached to the wall. The concept of continuous cross ties and subdia-
phragms was developed to safely distribute the out-of-plane wall forces of masonry or
concrete walls into the diaphragm without exceeding the capacity of the diaphragm.
Early discussions of this topic can be found in ATC-7 and ATC7-1. 12 Currently, continu-
ous cross ties and subdiaphragms are required for structures with masonry or concrete
walls tied to flexible diaphragm in Seismic Design Categories C through F. Code
requires the following:
1. Where anchor spacing exceeds 4 ft, the wall shall be designed to span horizon-
tally between the anchors. As an alternate, a beam or other flexural member can
be fastened to or incorporated within the wall and be designed to transfer the
load to the continuous cross ties.
2. Continuous ties shall extend the full distance between the diaphragm chords.
Additional chords may be installed to form subdiaphragms spanning between
and transferring their force into the continuous cross ties. The maximum aspect
ratio of a subdiaphragm shall not exceed 2.5 : 1.
3. In wood diaphragms, the continuous cross ties are in addition to the diaphragm
sheathing. The diaphragm sheathing shall not be considered effective as pro-
viding the ties or struts required by ASCE 7-05 Section 12.11.2.2.3.
4. In steel decking diaphragms, the decking can be used for the cross tie when the
loads are applied parallel to the ribs but not perpendicular to the ribs. When the
steel decking is used as the continuous ties, the decking and its connections
shall be designed to take the out-of-plane tension and compression forces.
5. Anchorage shall not be accomplished by the use of toenails, nails in withdrawal,
or cross-grain bending of a ledger.
6. Connections (anchors) shall extend into the diaphragm a sufficient distance to
develop the force.
The intent of item 6 is not always understood. Anchors that are typically used for
the connection of the walls to the diaphragm or subdiaphragms usually consist of light
gauge straps nailed to flat blocking. It is often interpreted from item 6 that the anchors
and blocking only need extend into the subdiaphragm a short distance. However, in
accordance with item 2 and ATC-7, continuous cross ties (the anchor and its collector)
are also required to extend to the interior chord of the subdiaphragm. This would
ensure that the force is uniformly distributed over the full depth of the subdiaphragm,
preventing localized tearing.
Figure 2.36 shows a typical layout for the cross tie and subdiaphragms where the
structure is loaded in the transverse direction. The subdiaphragms are placed along
grid lines A and C as shown. The subdiaphragms are supported by the end walls located
at grid lines 1 and 5 and by the continuous cross ties located at grid lines 2, 3, and 4.
These cross ties usually consist of beams or girders connected with straps or other con-
nectors. Out-of-plane anchors are connected to collectors that extend the full depth of