One area of changes in the New UFC 4-010-01, 9 February 2012, is in the design requirements for exterior windows and skylights. The updates are pretty exciting, from this blast engineer’s perspective, because they explicitly address some of the recurrent questions from design teams and owners, and clarify items that seemed to have differing interpretations, depending on who was doing the interpreting. Well done PDC!
Historically, exterior windows and their supports, have been one of the primary areas where application of the UFC 4-010-01 has added costs to projects, so any changes to window requirements should be of the utmost interests to design and construction teams, developers, and building owners.
Here are some of the changes that affect exterior windows:
- Standoff Distances for Window Design: The updated UFC now requires that all exterior windows and skylights be designed for explosive weights I and II located at the actual standoff provided. A caveat to this is that explosive weight II is waived for window design if the standoff exceeds 200 feet. This is a big change from the previous version of the UFC, which required that windows be designed for the conventional construction standoff distances, even if the project had larger standoffs. So….buildings with larger standoff distances could benefit from significant load reductions – which should hopefully translate to cost savings – under the new UFC.
- Analytic Approach: One of the really useful updates to the document is reference to a new PDC document – PDC-TR 10-02 Blast Resistant Design Methodology for Windows Systems Designed Statically and Dynamically, 19 April 2012 – which provides detailed guidance on the static and dynamic analysis approaches for window systems. Including dynamic response limits for frames and mullions, appropriate probability of failure for different glass types, design examples, and the introduction of a new DoD software for designing glazed systems (SBEDS-W). This will help provide consistency in design across DoD projects. Note that Static Analysis is only allowed for Very Low and Low levels of protection, as long as the threat size and standoff are covered by Figure 1 in ASTM 2248.
- Minimum Glass Lay-up Requirements: The only minimum glass requirement is that the interlayer thickness must be greater than 0.030″. There are no longer minimum thickness requirements for the glass itself.
- Frames and Connections: When analyzed statically, window frame and connection designs are based on the glazing resistance and are not reliant on the applied load. This may be useful for window vendors who are interested in being able provide off-the-shelf systems. However, while this may be convenient to provide off-the-shelf solutions, design teams can still introduce significant savings by using dynamic analysis in projects, especially projects with high wind and other protection requirements beyond blast.
- Frames: Window and skylight frames can be designed statically for a load two (2) times the glazing resistance using LRFD with strength reduction factor equal to 1.0.
- Connections: Connections between the window system and the surrounding wall are to be designed using LRFD with their applicable code reduction factor to meet the ASTM F2248 with a design load of a) Two (2) times the glazing resistance if the peak dynamic pressure is larger than half (1/2) the glazing resistance, OR b) One (1) time the glazing resistance if the peak dynamic pressure is less than half (1/2) the glazing resistance.
- Window Supporting Elements: The window supporting elements can be designed statically when the Conventional Construction Standoff Distance for the specific wall type is exceeded. Otherwise, the dynamic approach is required. The PDC-TR 10-02 Section 4-2.9 also limits the applicability of the static approach to punched openings. Remember, this new version of the document has really shaken up the Conventional Construction standoff distances, and project teams will need to pay close attention to the application of this concept to their specific buildings.
Hopefully this helps in understanding how the updated UFC 4-010-01 will impact your projects. If you would like additional information or guidance, give us a call at (646) 649-3169, or send us an email at info@StoneSecurityEngineering.com.