CROSS Safety Report
Street sign collapse causes fatality
This report is over 2 years old
This report highlights the potential dangers of street signs on fixed vertical supports subject to wind loading.
Key Learning Outcomes
For asset owners:
Street signs, posts, advertisement hoardings, lamp posts and other ancillaries to our streets should be of a high standard to ensure fatalities do not occur
Specifications should be of a high standard to give confidence that the components in question will give long, reliable, and safe service
If an alteration is to be made to an element, its condition should be accurately assessed to ensure it is suitable to accommodate the alteration. Assessment should be carried out by a person who is competent to do so.
This report highlights the importance of carrying out regular inspections of assets, including those that are ubiquitous and seemingly low risk
For producers/fabricators of street furniture:
Quality control and competent inspections of fabricated elements can help to ensure that they are built in accordance with the design
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This report highlights the potential dangers of street signs on fixed vertical supports subject to wind loading and draws attention to a possible failure mechanism in welded section support posts with defective joints.
The findings arise out of an investigation following the death of a pedestrian who was walking past a sign that failed during high winds in 2015.
Sign support system
The sign was located adjacent to a public footpath and highway. The support was a single steel hollow square section post measuring 130mm by 130mm at the base, tapering to 62mm by 62mm at the top. The post was 4,460mm in height and a rectangular steel plate had been welded in place on top. A plastic box section sign measuring 1,200mm wide by 1,800mm high was attached to another steel plate, which was then bolted to the rectangular support plate
Fabrication of post
The support post had been formed with sections of steel U-channel which had been butt welded end to end to create the full length of the post. The square section was formed with longitudinal welds between the two full length channels. The transverse butt welds to each U-channel were offset so that no full cross section transverse weld occurred in the post.
The post had been embedded approximately 600mm into the ground and into a concrete foundation. The 1,200mm by 1,800mm box sign had been in place for two months, replacing a sign that had measured 1,009mm wide by 725mm high. The age of the post is not known.
Post failure during storm event
The incident occurred during a named storm force event. During the event, a gust wind speed was measured at 68mph some 17miles away from the sign. The exact speed at the sign location is not known. On the morning of the incident, a pedestrian was walking past the sign when the support post broke apart (Figure 1), striking and fatally injuring the passer-by.
A metallurgical examination was carried out on the support post which had fractured approximately 2,600mm above ground level at one of the transverse weld sections. The transverse butt weld at the fracture location exhibited regions of incomplete weld penetration through the post wall and internal corrosion.
Fatigue crack initiation
A fatigue crack was present in the full length U-channel adjacent to the butt weld and had initiated at the unfused edge of the transversely welded channel. The unfused areas would have raised the local stresses in the post, reducing resistance to fatigue crack initiation, propagation, and an increased susceptibility to failure.
Did the larger sign lead to failure?
Whilst the larger sign would have increased the forces acting upon the post and were a contributory factor in the post failure, calculations indicate that the support post would not have structurally failed had it been free from defects.
Whilst the larger sign would have increased the forces acting upon the post and were a contributory factor in the post failure, calculations indicate that the support post would not have structurally failed had it been free from defects
Whilst it is acknowledged that the incomplete weld and internal corrosion may not have been obvious as part of a routine inspection, this should not detract from the importance placed on the inspection and maintenance of all assets, including those that are ubiquitous and seemingly low risk. This tragic event also highlights the need to accurately assess the condition and suitability of elements that may be affected when alterations are made.
Expert Panel Comments
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A fatality that occurred because of faults in a mundane piece of street furniture and should not have happened.
In the same category are garden walls that fall on children and signs above shops that fall on pedestrians. These are not the result of deliberate acts but of the importance of structural matters not being understood.
Those responsible for the procurement of signs, posts, advertisement hoardings, lamp posts and other ancillaries to our streets must be aware that they are dealing with potentially life-threatening issues and act accordingly.
Specifications must be of a high enough standard to give confidence that the components in question will give long, reliable, and safe service. The same standards must be followed through for construction, inspection, and maintenance
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