1618.6 Diaphragms Below the Base Level-Downhill Direction.

The following provisions shall apply to the lateral analysis

and design of the connections for all diaphragms below the base level diaphragm in the downhill direction.

1618.6.1 Diaphragm Defined. Every floor level below the base level diaphragm shall be designed as a diaphragm.

1618.6.2 Design Force. Each diaphragm below the base level diaphragm shall be designed for all tributary loads at that level using a minimum seismic force factor not less than the base shear coefficient.

1618.6.3 Design Force Resistance-Primary Anchors. The design force described in Section 1618.6.2 shall be resisted through primary anchors from diaphragm struts provided in each diaphragm to the foundation. Primary anchors shall be provided and designed in accordance with the requirements and limitations of Section 1618.4.

1618.6.3.1 Design Force Resistance-Secondary Anchors.

1618.6.3.1.1 General. In addition to the primary anchors required in Section 1618.4, the design force in the downhill direction shall be resisted through secondary anchors in the uphill foundation connected to diaphragm struts in each diaphragm below the base level.

Exception: Secondary anchors are not required where foundations extending in the downhill direction, spaced at not more than 30 feet (9144 mm) on center, extend up to and are directly connected to each diaphragm below the base level for at least 70% of the diaphragm depth.

1618.6.3.1.2 Secondary Anchor Capacity. Secondary anchors at each diaphragm below the base level diaphragm shall be designed for a minimum force equal to the design force but not less than 300 pounds per lineal foot (4.38 kN/m) based on Allowable Stress Design (ASD) levels. The secondary anchors shall be uniformly distributed along the uphill diaphragm edge and shall be spaced a maximum of 4 feet (1219 mm) on center.

1618.6.3.1.3 Design. Secondary anchors and diaphragm struts shall be designed in accordance with Section 1618.7.