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CROSS Safety Report

Rigid wall flexible diaphragm roof collapse in earthquake

Report ID: 877 Published: 1 March 2020 Region: CROSS-US

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Overview

Part of the roof of a one-story building in California collapsed during a 2019 earthquake. The building has rigid masonry bearing walls and a flexible roof diaphragm.  

Cross grain tension failures of wood ledger connections between roof trusses and walls caused the collapse.  Connections of the type that failed have been prohibited by codes since the mid 1970s.

Key Learning Outcomes

For structural and architectural designers and for contractors:

  • Roof to wall diaphragm connections in seismic regions should not employ wood in cross grain tension or bending.

For all built environment professionals:

  • Addressing lessons learned from failures and effecting code improvements, alone, does not completely prevent recurrence of problems.  We must be ever vigilant in quality control and assurance for code compliance in design and construction.

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This case involves the partial collapse of a roof of a one story masonry building in California during a 2019 earthquake. The building structure has concrete masonry unit (CMU) bearing walls with roof trusses of wood chords and steel diagonals.

The roof diaphragm/deck is plywood. Original construction was from the early 1980’s but the building had been expanded several times. The part of the building where the roof collapsed is believed to have been built in the mid 1980’s.

There was a seismic foreshock the day prior to the collapse event. The ground shaking during the collapse event was moderately high by current code standards for the site. In the failure, the roof trusses separated from the bearing walls, and the roof collapsed onto the ground floor. The reporter indicated that the truss bearing failure appeared to be from cross grain tension failure of the timber ledger at the truss to bearing wall connection as illustrated in Figure 1.

Image
Figure 1: Cross grain tension failure of timber ledger on CMU wall

The reporter notes that cross grain tension restrictions did exist in the California Building Code when the part of the building involved in the collapse was built in 1984.

Expert Panel Comments

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Good structural practice for robustness, especially in earthquake zones, is to ensure that parts are well tied together - as here, roof structure to supporting walls. Seismic action will produce large lateral loads from roof to walls, so the viability of the overall load path to ground must be assured by the designer, and that means checking the integrity of all components in the load path.

Problems with cross grain tension and cross grain flexure in timber diaphragm to wall connections were identified in the 1971 San Fernando earthquake. These were addressed in the 1976 Uniform Building Code, Para. 2312.D.3:

3. Special Requirements. A. Wood diaphragms providing lateral support for concrete or masonry walls. ’Where wood diaphragms are used to laterally support concrete or masonry walls the anchorage shall conform to Section 2310.  In Zones No. 2, No. 3 and No. 4 anchorage shall not be accomplished by the use of toe nails, or nails subject to withdrawal; nor shall wood framing be used in cross grain bending or cross grain tension.’

Current codes and design standards continue to prohibit diaphragm to wall connections that place timber in cross grain flexure or tension, and some contain examples of good and bad connections. Examples are the 2015 National Design Specification (NDS) for Wood Construction Para. 4.1.5.1.1 and ASCE 7-16, Para. 12.14.7.5.2.

A second factor in this failure is the large displacements caused by a flexible diaphragm. When flexible diaphragms deflect laterally the tops of the walls to which they attach experience increased inertial forces, placing large demands on wall to diaphragm connections. Computation of demands on such connections must consider increased accelerations caused by the dynamics of flexible diaphragms.

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