By Aerik Carlton
Structural fire consideration has been taking some large steps recently, with several codes and standards having added or altered structural fire sections. At Nishkian Dean, we have examined these structural fire design codes and methods from a structural engineering prospective for our clients and readers below.
A summary of these structural fire design codes includes:
ASCE 7-16 Minimum Design Loads and Associated Criteria for Buildings and Other Structures has added a new appendix to address structural fire considerations.
AISC 360-16 Specification for Structural Steel Buildings has refined and made additions to Appendix 4: Structural Design for Fire Conditions.
The National Institute of Science and Technology (NIST) has drastically expanded the Gaithersburg, Maryland, Fire Testing Lab to include a live-fire hood capable of testing a multi-story building portion with the aim of obtaining empirical large-scale structural system data to further refine and validate their free computational fluid dynamics fire modeling software. Europe, the United Kingdom, New Zealand, and Japan have all adopted performance-based methods for structural fire, and the US industry has been slow to recognize and allow this building design approach. These developments considering structural fire represent a paradigm shift for structures in a fire situation.
Prescriptive methods are the standard for structures in the US, but structural engineering codes and standards are trending toward performance-based methods. Fire Protection Engineers (FPE) have been using performance-based methods for fire for a couple of decades in the US, and Structural Engineers (SE) are beginning to develop similar methods (to be on par with our international colleagues). However, there are some marked differences in focus between FPE and SE. FPE considers smoke ventilation, egress, fire prevention systems, notification systems, and compartmentalization to restrict fire spread, while SE considers the effect fire has on the structures’ ability to remain stable and support service loadings.
Meuller et al. (2014) illustrated this difference in dramatic fashion by testing a reinforced concrete bearing wall under a single-sided heating condition. Prescriptive fire resistance methods consider the tested bearing wall as having a 2-hour rating, due to its thickness. However, the project found upon testing the wall, a complete failure occurred at approximately 42 minutes.
Building fires are rare events—the annual likelihood that a business-occupied building will experience a fire in any given year is on the order of 0.05% (Xin and Huang 2013). But just because the chances are rare does not mean we should not keep improving and refining our fire designs. We have been using prescriptive methods for building fire resistance for nearly 100 years, and yet we still don’t have a good representation of the effectiveness of these provisions.
We could potentially eliminate a lot of conservatism in our fire-resisting elements and still maintain a similar, or possibly improved, building performance at a lesser cost to building owners and developers. Through performance-based design approaches, we could eliminate prescriptively required fire resisting elements (pending jurisdictional fire official approval) such as a reduction of compartment walls thickness through polypropylene fiber addition to the concrete mix to reduce fire response spalling or by refining structural member fire resistance with intumescent paint. There is also a possibility that we could shift our designs toward structures that are more easily reparable after a fire, thus increasing the resilience and life cycle of our buildings.
If your project has structural fire requirements, or you have any questions about the updated codes, please feel free to contact Nishkian Dean.
Aerik Carlton, is an Engineering Designer with Nishkian Dean a structural engineering consulting firm in Portland, Oregon.
Mueller, K. A., Kurama, Y. C., and Mcginnis, M. J. (2014). “Out-of-Plane Behavior of Two Reinforced Concrete Bearing Walls under Fire: Full-Scale Experimental Investigation.” ACI Structural Journal, 111(5).
Xin, J., and Huang, C. (2013). “Fire risk analysis of residential buildings based on scenario clusters and its application in fire risk management.” Fire Safety Journal, 62, 72–78.
By Rachel Wong, S.E., CAPM®
With the January 1st implementation of 2016 California Building Code (CBC), there is a new Building Code in town. Much of the 2016 CBC is similar to the previous 2013 CBC with respect to Structural Engineering with minor updates scattered throughout. However, one of the more significant updates was in regards to existing buildings. The 2015 International Existing Building Code® (IEBC) was adopted with the 2016 CBC as the latest and greatest guideline for existing building repair and modifications.
Originally drafted in 2003, the IEBC has been in the International Code Council (ICC) family of codes for over a decade, but faced limited adoption due to the presence of IBC/CBC Chapter 34 for existing buildings. Previously, IBC/CBC Chapter 34 was responsible for minimum requirements in existing building modifications, but had limited content for the variety of projects it covered. In 2014, the code committee decided that Chapter 34 should be eliminated in favor of the more fully-depicted IEBC. The IEBC maintains much of the prior CBC information, while expanding and clarifying specific topics. For example, a path for compliance of relocated buildings is provided in IEBC, and was previously considered to be a design “grey area”. Within IEBC Appendix A, a series of subsections are now provided for masonry, wood, concrete, and steel design, which previously were beyond the scope of Chapter 34.
But a lot of familiar requirements are present in IEBC, too. Previous CBC Sections 3402 to 3411 can now be found incorporated into the contents of IEBC Chapter 4, and Section 3412 has been relocated to IEBC Chapter 14. The CBC retrofit/strengthening threshold of 5% gravity/10% seismic modifications to existing elements without requiring strengthening to these elements is still applicable for Level 2 alterations that impact less than 50% of the building area.
The IEBC provides options for either prescriptive compliance of a structure or performance-based compliance, and permits the use of alternate methods as well. One of the seismic retrofit documents that go hand-in hand with these provisions is the relatively new ASCE 41-13 document, which will be featured in our upcoming May article.
Viscous Damper Brace Frames in an existing Steel Building
(Performance-Based Compliance Upgrade)
Each of the Nishkian offices has extensive experience with providing cost-effective solutions in retrofit, alteration and additions to existing structures. Should you need assistance with understanding how the new code will affect your existing building project, do not hesitate to contact one of our offices to receive expert assistance with any questions you may have. We are here to help!