Page 340 - Forensic Structural Engineering Handbook

10.28 CAUSES OF FAILURES

Soldier beam

Outlooker
(spacer)
Wale

Timber
lagging

Force component corner brace
Plan

5/16'' (Typ) from

Outlooker
(spacer)

Elevation
(f)
FIGURE 10.3 Washington excavation. (Continued) (f) An “outlooker/spacer” between a sol-
dier column and a wale. (Based on OSHA Investigation of November 19, 1990 Excavation
Collapse at 14th & H Streets, N.W., Washington, D.C. report, May 1991.)

and south walls, and that the external support tieback system did not fail. They also
found that the failure load, the soil pressure immediately preceding the collapse, was
lower than the load for which the temporary structure was designed by the shoring
subcontractor.
It was apparently overlooked in the design and/or in the detailing that the force compo-
nent of a 45° corner brace in the direction of the longitudinal axis of the wale in the north
and south wall did not have a physical reaction in the system. The “outlookers” could trans-
mit the bracing force component (earth pressure) across the plane of the wall, but had no
ability to resist the east-west force component in the plane of the wall (see Fig. 10.3(f)
PLAN) and just “folded” over in a horizontal direction like a two-hinged linkage.