CIRAS is not the only independent, confidential reporting scheme. Building and civil engineering have their own equivalent, although it is narrower in remit and does not cover quite the same broad spectrum of health and safety issues.

We take a closer look at how it works with a real-life case study of structural collapse.

                           Collapse of the steel framed canopy. © Structural Safety

Introduction

Structural-Safety is an independent umbrella organisation consisting of two entities.

The first is SCOSS (Standing Committee on Structural Safety), a committee which reviews building and civil engineering matters affecting the safety of structures. SCOSS publishes alerts and topic papers to highlight safety issues.

The second is CROSS (Confidential Reporting on Structural Safety), a confidential safety reporting scheme established by SCOSS to capture and share lessons learned which might not otherwise have had formal recognition.

CROSS publishes quarterly newsletters, which contain anonymised reports with comments from a CROSS panel of experts.

As shown in the pyramid of risk image, CROSS aims to capture the precursors with the aim of preventing serious injury or fatalities, just like CIRAS does.

Extracts from a CROSS Report

How does a steel canopy collapse and cause significant injuries?

During the construction of a major new school facility, a 57 metre single span structural steel truss failed.

This resulted in the catastrophic collapse of a steel framed canopy supported by the truss.
At the time, five workers were on top of the canopy, 15 metres above ground level.

All received significant injuries but survived.

The immediate cause of the failure was associated with the fracture of a number of sub-size fillet welds joining paired load bearing tie bars at nodal points.

             Pyramid of risk. © Structural Safety

Mind the gap

The design of the ‘T’ shaped joint required the end of some connection plates to be cut with a bevel angle of 80.7°.

However, to simplify production, the plates were cut at 90° on the contact edge. This resulted in a gap of approximately 4.5 mm to one side of the joint. See Image 1 of the gap in an intact cross section recovered from the collapse.

                                                                               Image 1: Gap at end of shaped plate on intact cross-section. © Structural Safety

Failure had occurred through the weld material at numerous nodes.

The assessment showed that failure of one connection at one node would have been sufficient to unzip the truss.

In other words, a single inadequate connection would make collapse highly likely. There were additional issues with the design that made installation difficult and led to installers cutting and rewelding connections to the building.

Such site alterations are often implicated in collapse incidents. However, in this case the root cause lay elsewhere.

Legal requirements and published guidance and standards are based on ensuring that the designer provides a structural design that is sound and straightforward to build.

Welding standards for structural steelwork specify that visual inspection be carried out before, during, and on completion of welding, to ensure that production quality is being maintained.

To limit the risk of structural collapse, projects need to use sensible quality assurance practices.

These include early and regular discussions and cooperation between designers, fabricators and installers.

Reconstructed building. © Structural Safety

CROSS Panel Comments

This is a classic example of the intimate relationship between design, workmanship and safety.

The failure also illustrates a theme that has surfaced in a number of recent reports - poor workmanship can lead to disaster.

It appears that this event occurred through inadequate workmanship and inadequate quality control.

In this case, the poor workmanship could not be detected by post-fabrication inspection.

Rather, to assure quality, inspection ‘before covering up’ should have been carried out.

Over the years, CROSS has received reports of failure involving tie rods of various types.