June 29, 2012
The Hidden Cost of Saving Money
by Hinman Team & Brian Katz
Early this year, the new Department of Defense Anti-terrorism/Force Protection (AT/FP) standards (UFC 4-010-01) were adopted. The stated impact of the document on new and existing construction is “most of the changes should result in cost savings or no change in costs". Over a series of blog posts, I will explore the implications of this new criteria to determine if this intent is indeed the reality. My hope is that these blogs provide a forum to discuss design challenges and solutions encountered when using the new AT/FP criteria.
The most significant change to the UFC standards pertains to modifications of required standoff distances. Previous editions of the AT/FP criteria document correlated building occupancy and design basis level of protection with required setback of a building from perimeter roadways and parking to protect against vehicle-borne explosives. Based on the provided standoff, varying degrees of blast hardening of the structure were required. The two primary standoff benchmarks considered by design were:
- Conventional Construction Standoff Distance – minimum setback distance from the building to vulnerable adjacencies at which conventional construction may be used for buildings without a specific analysis of blast effects
- Minimum Standoff Distance – the smallest permissible standoff distance for new construction regardless of any analysis or hardening of the building
Most commonly known were the 148-foot and 82-foot conventional construction standoffs from the building to the site controlled perimeter (or parking and roadways for sites without a controlled perimeter) and parking and roadways within the controlled perimeter, respectively. Achievement of these setbacks provided the design team with the option to locate a building on a site to preclude blast hardening of the structure and façade (with the exception of windows) and exclude this cost from the construction budget. However, coupling setback with Defense Base Closure and Realignment (BRAC) requirements, many construction projects located on already overcrowded bases were caught in a tug-of-war battle with AT/FP requirements on one end and provision of ample parking for new employees on the other.
The new UFC takes on this obstacle by correlating standoff requirement to building occupancy and function and exterior envelope construction type (wall and roof elements). More robust wall construction types – such as reinforced concrete and masonry, correlate to reduced standoff requirements. As an example, let's consider a building with exterior load bearing reinforced concrete walls. Considering this building to be located on a site without a controlled perimeter, the conventional construction standoff distance is 66 feet to perimeter roadways and parking areas. In contrast to the 148-ft standoff associated with previous UFC editions, the revised setback requirement seemingly improves the opportunity to site a building and minimize the extent of required blast hardening. Finally! a set of criteria that does not unilaterally penalize building design regardless of construction type. However, are the modified standoff requirements going to save the project money? Short Answer: Maybe.
While using the new standoffs and construction requirement tables can be confusing enough, there are plenty of hidden pitfalls where the construction allowance budget can easily be blown. For example, a concrete wall with minimum reinforcement can withstand a high magnitude explosive event at a short standoff in comparison with a metal stud wall with stone or brick veneer. But there is something missing. It seems too convenient to construct the same building required at the old 148foot setback at the new 66-foot conventional construction standoff for the same cost. Unfortunately, the cliché "too good to be true" is only too appropriate in this instance. Although the UFC seems to relax the standoff requirement, hidden costs are buried between the lines of the other AT/FP requirements.
Actually, there are many hidden costs. The most apparent cost-pitfall of the new UFC pertains to window and door design. Assuming typical building design where the doors enter into occupied space, doors suited to satisfy old AT/FP criteria are no longer applicable. Under the new UFC, designers will have to either (1) reach out to door vendors that have strengthened and re-test their products for loads associated with the reduced standoff to bomb locations or (2) pursue design alternatives that minimize doors being dislodged from their frames and propelled into occupied space. The first option is expected to force selection of more robust and costly doors from a limited pool of vendors. If the price of each door increases 10 fold, is it worth the money to get that reduced standoff? Challenges with respect to window design will be similar. Early indications from manufacturers is that construction types associated with reduced standoffs will burden glass and framing system design with higher magnitude air-blast load requirements, resulting in window system cost increases.
So what's the bottom line? The new UFC offers the promise of project-specific standoff requirements that will empower designers to explore exterior wall construction types that are best suited to overcome site constraints. However, the revised standoff requirements are complex and the big picture impact to the building envelope design must be explored to understand the full impact to construction cost. Additionally, the applicability of reduced standoffs are directly tied to specific wall construction. Element configurations that deviate from the prescriptive requirements trigger the need for blast analysis. If not planned for properly, these changes will end up increasing the cost of construction.
In upcoming blogs tied to unraveling the new UFC criteria, I will delve into inherent limitations placed on the architectural aesthetic of the building envelope, updated analysis requirements for the window support systems, master planning challenges, the required analysis of the majority of roof systems, and many more. Stay tuned!