Out-of-Plane Anchorage to Diaphragms

Diaphragms shall be defined as horizontal elements that transfer earthquake-induced inertial forces to vertical elements of the lateral-force-resisting systems through the collective action of diaphragm components including chords, collectors, and ties.

Walls shall be positively anchored to all diaphragms that provide lateral support for the wall or are vertically supported by the wall. Walls shall be anchored to diaphragms at horizontal distances not exceeding eight feet, unless it can be demonstrated that the wall has adequate capacity to span horizontally between the supports for greater distances. Anchorage of walls to diaphragms shall be designed for forces calculated using Equation (1), which shall be developed in the diaphragm. If sub-diaphragms are used, each sub diaphragm shall be capable of transmitting the shear forces due to wall anchorage to a continuous diaphragm tie. Sub-diaphragms shall have length-to-depth ratios not exceeding 3:1.  Where wall panels are stiffened for out-of-plane behavior by pilasters or similar elements, anchors shall be provided at each such element and the distribution of out-of-plane forces to wall anchors and diaphragm ties shall consider the stiffening effect and accumulation of forces at these elements. Wall anchor connections shall be considered force-controlled.

Fp = c S_XS W            (1)                

Where:
 
Fp = Design force for anchorage of walls to diaphragms

c = Factor from Table below for the selected Structural Performance Level. Increased values of χ shall be used when anchoring to flexible diaphragms

S_XS  = Spectral response acceleration parameter at short periods for the selected hazard level and damping adjusted for site class

W = Weight of the wall tributary to the anchor

Exceptions:

1. Fp shall not be less than the minimum of 400 lb/ft or 400 S_XS  (lbs/foot).

Coefficient c  for Calculation of Out-of-Plane Wall Forces
Structural Performance Level	c
	Flexible diaphragms	Other diaphragms
Collapse Prevention	0.9	0.3
Life Safety	1.2	0.4
Immediate Occupancy	1.8	0.6