Exclusive: Why the Austin Scaffolding Fell

On June 10 of this year, three construction workers died when a section of scaffolding collapsed at a construction site near the campus of the University of Texas at Austin. Because the accident resulted in deaths, personnel from the U. S. Occupational Safety and Health Administration began an investigation shortly thereafter. While it is the normal policy of OSHA not to divulge information on an investigation that is incomplete, leaks do occur. If you keep reading, you will be the beneficiary of one such leak. But first, some details about the accident.

Photographs published at the time showed that the scaffolding was a cantilevered box frame that extended several yards on either side of an elevator-type mechanism that allowed the whole thing to move up and down the side of the high-rise apartment complex under construction. Photos of units on the manufacturer's website show that the scaffolding can extend as far as 25 or 30 feet either side of the mast that supports it. The workers using the scaffolding walk along the frame to do whatever operations the construction job requires.

There's no particular mechanical-engineering magic to cantilevering. Like a branch of a tree that sticks out sideways, the box frame's unbalanced weight has to be supported by the central elevator. In particular, the bolts that keep the top of the box frame attached to the elevator unit are under tremendous tension, since the whole weight of the frame tends to pull the bolts away from the elevator. If these bolts aren't strong enough and fail, the scaffolding will fall away like a branch cracking off a dead tree. That is apparently what happened on June 10.

The following information is what the newspeople call an unconfirmed report from an anonymous source. But I believe it to be reliable. According to the source, the bolts used in the scaffolding that failed were not the required Grade 5 type. If you have ever bought bolts at a hardware store, you may have noticed little patterns of lines on the hexagonal bolt head. These are not just random forging marks. They indicate the quality of steel in the bolt, and in particular, the maximum stress the bolt can withstand. Grade 5 bolts have three embossed lines in a "Y" pattern on the bolt head, indicating that they can take up to 120,000 pounds per square inch of stress under certain defined conditions. Cheaper bolts with no head markings are available. For example, Grade 1 bolts have only about half the stress capacity of Grade 5.

Assuming my source is correct, how did the wrong bolts get there? To answer that question requires that we move from the realm of science to the realm of human behavior. The problem could lie anywhere from the manufacturer of the scaffolding, to the general contractor for the site, to the subcontractor who installed the scaffolding, to the subcontractor's employees who put it together, to someone else who might have lost the Grade 5 bolts and substituted others without knowing the danger it would cause. Presumably the OSHA investigators are also working on this angle of the problem, which will require extensive interviews and inquiries which may never reach a satisfactory conclusion. But this shows the vital importance of apparently minor details, such as three little lines on a bolt head, to the safety of construction workers who probably had nothing to do with the assembly of the scaffolding.

A subsequent news article in the Austin American-Statesman pointed out that Texas has one of the worst records in the U. S. for construction-related fatalities. Pay rates are low, some construction employers opt out of workers' compensation insurance, and since OSHA primarily investigates complaints lodged by workers, the workers have to be their own safety inspectors. Since many speak only Spanish and some are undocumented, their reluctance to undertake this role is understandable. Simply identifying the next of kin of the workers killed in this accident was a challenge to authorities, since their families lived in small towns in Mexico.

How can accidents like this be prevented in the future?

This incident shows how vital the "soft" technologies of training, inspection, and good management can be to safety improvements. Even if we give the benefit of the doubt to the contractor and subcontractors by assuming the use of the cheaper bolts was accidental, the accident reveals a grave deficiency in the way supplies are inventoried and assembly procedures are carried out. No human-operated process can be made 100% foolproof, but if certain bolts have to be Grade 5 in order for a scaffold to be assembled safely, there must be a way to make sure that only Grade 5 bolts go into the system. Simple things like matching spray-paint colors or other wordless guidelines can go far to prevent tragedies like this, especially where non-English speaking employees are involved.

Such measures come too late for the three workers who died when the less expensive bolts apparently failed. But I hope as more details emerge, the lessons of how to avoid this kind of accident won't be lost on those who are in a position to make sure their employees and subcontractors have a safe working environment.

Sources: My source for the bolt information is anonymous for the simple reason that I do not know his name. Published information on the scaffolding collapse can be found on the Austin American-Statesman website at http://www.statesman.com/search/content/news/stories/local/2009/06/17/0617construction.html. Information on the grades of bolts available can be found at http://www.engineershandbook.com/Tables/boltgrades.htm.