As projects go, the challenges were not out of the ordinary. Winter was coming, so it was a race to beat the ﬁrst snow in Colorado with foundation construction and then there were questions about meeting ﬁre code. But what made this project enlightening and pleasantly surprising was the simple reminder that in the end, even in this age of advanced technology and strict prescriptive requirements, common sense can still prevail. The project began in the fall of 2008. Villa Sport in Colorado required an expedited design and construction schedule for its aqua playground expansion. It was imperative that work begin before the onset of winter and freezing of the ground, since remediation work was needed over four-foot-deep expansive soil. Working in parallel with Oculus Architects, in particular Steve Connelly, ZFA empowered the contractor to proceed by providing foundation construction documents for an expedited permit. Further, given the lead time on structural steel at that time, a structural steel package was issued prior to completion of the construction documents – not an unusual project scenario. It was during the finishing of the construction documents, when the design was being optimized, that the question of a ﬁre scenario begged to be scrutinized.
The Project Description
The finished construction photo below depicts the recreational center expansion, which includes a nominal 40-foot-wide by 120-foot-long leisure pool, enclosed and equipped with an environment. The building is integrated with the existing recreation center, utilizing extensive glass between demising walls and the main entrance, which includes a Nana Wall striding and an indoor/outdoor Jacuzzi. The resulting 50-foot roof span tucks under the existing curved roof to maintain the 12-foot clear height while providing snow and water protection.
Although the space has an active ﬁre suppression system (sprinklers), and obviously the compartment has a low ﬁ re fuel source, the roof framing is less than 20 feet above finished floor level and, consequently, requires a one-hour ﬁ re rating in accordance with prescriptive code requirements for the Building Code Occupant classification of A3. Conventional practice to achieve that rating would be to box the columns and beams with a cementious cladding (one that will not degrade in the humid atmosphere), or provide intumescent paint (Albi Clad TF is an excellent product for this environment). Both are passive systems that require material and labor resources. While practical solutions were being developed and compared, there was an underlying common-sense belief by those involved in the project that the wide flange framing that had been used over the trusses for aesthetics and to meet schedule requirements would be capable of maintaining vertical support for at least the one hour that was required. But, the problem was proving it.
Performance based design (PBD) has been a buzz phrase in the industry over the last few years. But, prior to the 20th century’s prescriptive codes and standards, designs were always based on the professional’s knowledge, intuition, and their application of first principles, such as Hooke’s stress-strain theory (developed in the 17th century). The goal was to provide structures that served a purpose: aka PBD. Today we apply the same relationships, though much of it is hidden within the confines of the locked soŌware that we depend upon. Yet, for the most part, we are bound by an evolved and complex set of prescriptive code provisions. The prescriptive-based code products vary vastly in their performance. Fortunately, in the last few years, the code development process and building offcials are embracing PBD, applying a pre-20th century design process to ensure 21st century technology and sustainable design practices are used. To this end, there is more use and acceptance of the Alternate Method, Materials and Construction procedures. And, for ﬁre requirements, there is an explicit provision that permits one to demonstrate code intent is being satisﬁed through PBD.
Performance Based Fire Design
ZFA conducted performance-based testing on the Villa Sport project to demonstrate that the unprotected steel members satisﬁed the required ﬁ re resistivity rating of one hour. The process is outlined below. Based on ASTM E119, the low roof diaphragm (steel deck) stiffness results in a “thermally unrestrained” condition. A restrained condition would permit the beams to sag, working in a catenary fashion, and maintain vertical load carrying capability – an alternate load path not relied upon for this project. Rather, steel beams spanning the width of the building were assessed from a “residual bending strength” perspective, which is a function of the temperature of the steel. The steps of the assessment are broadly deﬁned below.
- Establish a credible design ﬁre scenario based on geometry and openings and fuel load in the structure from which the time-dependent temperature of the “compartment” is determined.
- Determine the temperature of the steel over time based on the compartment temperature as well as the steel section properties.
- Determine the residual stress capacity of the steel due to temperature effects, and conﬁrm that the component has the residual strength to maintain the load demand to prevent collapse for the design event time.
Using the inherent steel strength in a low-fuel source environment yielded the ﬁ re resistivity required by code, thus the achievement of performance based engineering design. Specifically, using a performance based ﬁre design that strategically utilized steel with a relatively low surface-to area ratio eliminated the need for conventional passive ﬁre protection. The design resulted in a significant reduction in materials and labor resources, and provided a system that confidently achieves the target performance intent. Not only that, but the expressed steel finish looks better, too!