First Look - Protecting Buildings From Structural Impacts
By: Richard Donnally, principal, DVA Architects
Besides a three-month shorter construction schedule, a key savings in the SecureTilt® prototype is the elimination of exterior columns or beams due to the use of load-bearing concrete panels.
Over the last decade, increased attention has been given to shielding facilities from blast damage. The federal office building bombing incident in Oklahoma, in particular, revealed that current standards were inadequate, especially regarding progressive collapse. Progressive collapse is where a localized blast results in immense collateral damage due to structural failure.
The 9-11 tragedy further demonstrated the need to provide much higher levels of protection. In response, the Anti-Terrorism Force Protection (ATFP) guidelines issued by the Department of Defense (DoD) have been upgraded several times. The resultant building standards have usually been met with large increases in steel member sizes and stronger connections, more robust precast wall panels and higher levels of blast-resistant glass and framing. While perfectly acceptable, the cost associated with these systems tends to make the space much more expensive.
DVA Architects, having completed numerous conventional blast-resistant structures as well as several multi-story tilt-up concrete office buildings (i.e., Riverside 5), realized the potential of site cast concrete in achieving blast-resistant construction with significant cost savings. They set out to design a prototype (later named SecureTilt® by Morgan Keller) that reduced costs while providing higher protection.
The process started with DVA providing full drawings for a blast precast and steel building they had done to contractor Morgan Keller who bid the project to subcontractors for actual structure costs. DVA then produced a new set of architectural and structural plans for the prototype which were analyzed by Hinman Consulting Engineers and bid to subcontractors by Morgan Keller. The resulting SecureTILT® prototype the team designed reached a savings of 20 percent below a traditional precast model. Ten percent of the savings came from faster erection of the structure which meant less overhead costs. One of the key savings is the elimination of exterior columns or beams due to the use of load-bearing concrete panels. This is always a savings on tilt-up design, but even more so in blast-resistant construction, where steel sizes and connections are increased significantly.
Other cost factors were:
- The simple cost savings of texture-coated site cast concrete over factory architectural precast;
- The reduced cost of transportation; and
- The three-month shorter construction schedule due to the speed of construction inherent in tilt up.
DVA Architects used Riverside 5, a tilt-up concrete office building, as the basis for design of a blast resistant structure that reduced costs while providing higher protection against collapse.
SecureTILT® - A Model for Blast Resistant Construction
The team redesigned the façade and structure of Riverside 5 to accommodate a medium level of blast protection based on DoD Unified Facilities Criteria (UFC) 4-010-01 and assumed a minimum 82-foot setback to parking and a minimum 148-foot setback to a controlled perimeter, which also matched the original precast blast structure. In redesigning the façade, focus was given to a structure that would compete successfully in the marketplace. While the windows shrank in dimension slightly to accommodate additional reinforcing steel, the glass curtainwall accents on the corner, at the fourth floor and above the entry, were retained.
LJB Inc., in consultation with Hinman Consulting Engineers, a nationally recognized leader in blast-resistant design, then undertook the structural redesign. One of the key components of the new structural model was a moderate thickening of the concrete panels and widening of the legs of the panels, both to accommodate additional reinforcing steel. Much stronger and more numerous connections between panels were added to meet progressive collapse requirements. Hinman performed a blast analysis of the resulting structural design to confirm its accordance with the UFC 4-010-01 standards. The SecureTILT® design package was then competively priced by Morgan-Keller Construction, the original contractor of Riverside 5. Great care was taken to make the comparison equal in all aspects, such as amount and type of blast-resistant glazing, framing and anchorage.
As more and more government agencies require high levels of protection for their facilities, SecureTILT® site cast concrete may be a cost effective way to save money, but more importantly, lives.