Determining Design Errors in Commercial and Residential Structures
For the forensic investigator, damage assessments require taking a global look at the loss to determine all causes, including possible design and construction deficiencies.
In the world of forensic engineering, the scope of services is usually along the lines of determining the cause and origin of the reported damage. This seemingly straightforward statement requires the assessment of a structure for evidence of damage attributable to a particular event, such as hail, wind, fire, or storm surge.
An implied parallel scope of work is to determine whether the structure or component should have failed, and whether the event’s severity could be expected to damage the structure. Addressing this issue requires a more detailed look at the condition of the facility, the loads that were applied during the event, and an engineering analysis with respect to its expected performance. These expectations can range from governing jurisdiction requirements to the public’s perception of “reasonable” performance.
In building construction, the design process is a convergence of multiple criteria, which include building code and local municipality requirements, industry standards, and satisfactory facilitation of the expected loads and conditions. With the latter, this often takes the form of a building code imposed by a statistical likelihood of reoccurrence during a chosen period of time. For example, snow loads are based on the expected snow levels produced by an event that has a 100-year statistical reoccurrence.
Further, building codes are broken into two basic types: prescriptive and performance-based. The performance-based code dictates the design criterion that establishes how a structure or component of a structure should perform. For example, a floor joist under a certain loading condition should be designed such that it does not deflect more than L/360, where “L” is the length of the joist.
The International Building Code is an example of a performance-based code, while the International Residential Code can be considered prescriptive. There are still performance-based criteria, but this building code dictates how to build things such as stud spacing, reinforcement in basement walls, the thickness of those walls, and more.
Why are there two different types of codes? Basically, the differentiation is commercial versus residential. Typically, commercial structures are somewhat unique and, due to their relative complexity, have design teams that digest the desired use of the facility and produce a code-compliant building design in the form of drawings and specifications.
In most jurisdictions, it is required that the design of commercial facilities be done by registered design professionals. Residential structures, typically one design used for many homes, often do not have the same design professional requirements from a governing jurisdiction. For this reason, the codes that apply to residential structures are prescriptive in their requirements.
It stands to reason that when performing an assessment of the commercial loss, investigators should obtain the design documents for comparison with “as-built” conditions and for further scrutiny with respect to the appropriateness of the design. With residential structures, it is a more abbreviated process; the as-built condition of the structure can simply be compared to the version of the building code applicable at the date of construction. This is only one side of the equation, though. It is equally as important to obtain granular data with respect to the actual loads and conditions at the subject property at the time of the event. With these two components in place, one finally can evaluate the event-specific performance of the structure.
Does the responsibility of the design fall squarely on the shoulders of the registered design professionals? Not at all. Their work is only as good as the information provided to them in the form of the initial design parameters.
Consider a speculative commercial center, such as a strip mall, that is partially built but then remains vacant for a time as all of the units are leased. Partially built often means that the shell (walls, roof, foundation, and parking lot) of these buildings are constructed, then the rest of construction halted until the spaces are leased. This is because a nail salon might require plumbing for only two small bathrooms, while a restaurant would need significantly more plumbing and ventilation.
Now, consider if one of the tenants requires an indoor swimming pool or spa. Facilities that produce high levels of humidity require special construction considerations that probably were not addressed during the design and construction of the generic and speculative shell. Down the road, the corroded bar joists and metal roof deck will need to be addressed. These are design deficiencies but not ones that can necessarily be pinned on the design professionals due to the post-construction change in criteria.
Custom homes provide another example. Contrary to the name, custom homes are frequently tract homes in which the buyer is allowed to select some of the interior and exterior finishes. When stone veneer is selected over an exterior plaster finish, is the size of the lintel over the garage resized? Similarly, when the brick masonry veneer is selected for a chimney, are the rafters, cripple studs, and columns/walls resized? Unfortunately, these homes are left with structural design defects that are attributable to the procurement process.
When asked to determine the cause of loss, a forensic engineer should be looking beyond the obvious event-related distress and on the implied expectation of performance of the structure. Are there design-related defects? Has the usage changed since the building was designed that contributed to the damage? With respect to a residence, does the construction comply with the prescriptive codes? Have the structural members been loaded more than anticipated?
In any case, it is incumbent upon the forensic investigator to take a global look at the loss and determine all causes of loss, including possible design and construction deficiencies.