Archive for June, 2009

The Challenges of Forensic Structural Investigations

Friday, June 12th, 2009

Causes of structural damage

VCE structural engineers are often tasked with determining the cause of a structural collapse. Such structural failures may result from design errors, construction errors, natural events, or other man-caused events. Often, more than one cause contributes to a structural failure. A windstorm can cause collapse of an inadequately braced masonry wall during building construction. Roof trusses which are not laterally braced during erection can roll over and collapse under their own weight. The structural investigator must evaluate the circumstances and make educated judgments as to the failure mechanisms, the trigger mechanism initiating the failure, and the causes of individual and progressive failure events. As may be seen in Photo1, determination of these issues can sometimes be a formidable task.

Photo 1. Light gauge steel structure collapsed during construction. Cause was determined to be Inadequate lateral bracing of steel roof trusses during erection.

Windstorm and Water Damage

After severe windstorms, a claimant will often scrutinize his building looking for damage. It is not uncommon for claimants to assume that a storm has caused newly discovered drywall cracks, foundation cracks, or other defects which may have actually resulted from prior foundation settlement or other long-term building construction or normal wear and tear issues. This phenomenon is also common in blasting and vehicle-building impact claims. Sometimes, cracking in masonry buildings and foundations, attributed to storm damage by a claimant, is found to have been previously caulked or contains mold in the cracks. A trained eye can distinguish between new and prior building damage.

Water damage to building interiors also requires skilled investigators to distinguish between damage from prior rainwater leakage through roofs and that which would have occurred during a storm for which the insured has filed a claim. Mold develops on building components and finishes after a period of time, not within several days. Dry-rot of wood takes much longer. The investigator must be keenly aware of these nuances when determining causes of damage from rainwater intrusion.

Figure 1. Tornado Track

The structural investigator is not so much concerned as to whether a windstorm is a tornado or straight line wind because the insured’s coverage is typically not dependent on which type of windstorm caused damage to his property. Tornado tracks such as shown in Figure 1 may be obtained from the local weather bureau. Additional information such as linear tornado velocity and estimated fastest 3-second ground wind speeds are sometimes available from weather agencies, but quite often this information is not available until several weeks after a storm event. Qualified structural investigators must have the skills to accurately distinguish one storm type from another by the type of wind damage and, to a high degree of accuracy, estimate the ground wind speed which caused structural damage and damage to nearby trees or man-made features.

Whether covered damage or not?

Photo 2. An insured claimed this retaining wall partially collapsed as the result of a windstorm. Investigation showed that failure resulted from improper design and construction of the un-reinforced block & brick veneer wall and foundation supporting the wall. Roots of a nearby maple tree contributed significantly to tilting of the retaining wall.

VCE’s structural engineers are guided by principles of ethics which require their reports to truthfully represent the facts, to the best of their knowledge and belief. Insured claimants should not be presumed to be making false claims. The purpose of insurance is to protect the insured from losses resulting from covered perils. A claimant is entitled to compensation for damage caused by an insured risk event. Although it is natural for investigators to sympathize with a claimant regarding the losses he has incurred, the structural investigator must use his skills to accurately determine the cause of the damage and render a fair and impartial judgment in his report. In one case a claimant stated that a 4 inch deep snow had caused his roof and ceiling to bow permanently downward. Given the worst case scenario, wherein the snow may have blown into drifts and been rained upon and refrozen, the maximum roof load which the investigating engineer calculated was less than the 20 pounds per square foot, 7-day roof live load for which roofs in the United States are required by codes to be designed. In addition, the residual deflection of the roof was found to be less than the limits for roof dead load deflection permitted by current and previous building codes.

Primary duty of the investigating structural engineer is public safety

Photo 3. Laterally buckled wood roof/ceiling trusses were found in the attic of an old church sanctuary building. Had the laterally unsupported wood board top chord been further stressed by roof loads, the ceiling and framing would have likely collapsed onto the church congregation. The investigating engineer warned the church trustees of the impending collapse of the roof and occupancy was terminated until remedial construction of the roof/ceiling framing was completed.

It is not uncommon during structural investigations to discover structural conditions which constitute a hazard to building occupants. In such cases, it is the duty of the structural investigator to inform the owner of the risk to life if the building is not evacuated or is occupied before remedial construction is undertaken to correct the structural defects in the building. In some cases this has resulted in VCE engineers making on-the-spot decisions to inform the owner that a building should not be occupied, or that limited entry is advisable only for removal of valuable assets which may be damaged by building collapse, remedial construction, or rainwater intrusion. In cases where a threat to life or the possibility of injury to occupants exists, it is the duty of the investigating engineer to give prompt notice to the owner of the potential consequences of entering a building. It is also the duty of the investigating engineer to be certain of the facts in making such a judgment.

VCE has investigated many damaged buildings, the owners of which have felt certain could not be repaired. Structural investigators are often pressed by owners to render an on-the-spot opinion as to whether a building can or cannot be repaired. When requested to do so by an insurer, VCE may, if the evidence is obvious and incontrovertible, render an on-site opinion of feasibility of repair after the investigation. More often than not, however, careful study of the investigative photography to determine potential causes of the building failure, and the application of sound principles of structural mechanics in the investigation are warranted before a proper opinion can be rendered.

Jim Waller, P. E.