In the last Volume of our Industry Briefing, we gave you a heads-up that Project Manager Arturo Montalva had an upcoming article in STRUCTURE Magazine – well here it is! Follow this link to read the article on the use of Open Source Software in structural engineering http://bit.ly/9ICysY.
Very Low Level of Protection Windows vs Low Level of Protection Windows
While it seems that the Very Low Level of Protection (VLLOP) window requirements of the UFC 4-010-01 should result in a less restrictive design than the Low Level of Protection (LLOP) window requirements, this is not the case. The UFC 4-010-01 varies the level of protection by allowing shorter Conventional Construction Standoffs for the VLLOP instead of modifying the design procedure. This approach results in a more robust design for glazed systems for VLLOP than for LLOP (assuming all Conventional Construction standoffs are met).
As an example, let’s consider the case of two facilities which both meet or exceed Conventional Construction standoffs to both the controlled perimeter and to parking and roadways within the controlled perimeter. Facility A is required to meet a Low Level of Protection and Facility B is required to meet a Very Low Level of Protection.
UFC 4-010-01 Conventional Construction standoffs are summarized in the following table.
Based on these standoffs, the following figure and table summarizes the design load values considered in UFC 4-010-01 based on the ASTM F2248.
You can see from the table and the figure that the glazing design loads for VLLOP Facilities are higher than those for LLOP (159psf vs. 66.4psf). This results in a more robust design for mullions, connections and framing for a VLLOP Facility due to the reduced allowable standoff.
Regarding glass selection, prescriptive values of glass thickness and interlayer thickness are provided for Low Level of Protection Facilities, which should only be superseded by constructability considerations. However, glazing in Very Low Level of Protection Facilities must be designed in accordance with ASTM F2248/ ASTM E1300 and therefore may result in a thicker glass system.
Unfortunately, there is no advantage in VLLOP Facilities that provide larger standoff as the design is controlled by the 33 feet (Conventional Construction Standoff). For these cases, a more appropriate and economical approach would be to use dynamic analysis methods as allowed in UFC 4-010-01 section B-3.1.4.
When designing DoD facilities, these differences in window design requirements, using static or dynamic approach, should be kept in mind and the architect may want to keep glazed openings in VLLOP facilities to a minimum.
An explosion is a rapid release of energy in the form of light, heat, sound, and a shock wave. The shock wave travels outward, in all directions, from the source of the explosion and is the primary source of building damage. The duration of the shockwave is fast, measured in milli-seconds rather than seconds (think of a blink of the eye) and the forces imposed on anything it its path (be it a building or a person) are enormous – many times greater than hurricanes.
Many factors contribute to how a building will respond to an explosive event, the most critical are.
• The size of the bomb
• The distance from the bomb to the building
• The type of building construction.
The size of the bomb (also termed Net Explosive Weight) and the distance from the bomb to the building (standoff distance) determine the magnitude of the pressure (force over area) and the duration that the pressure acts on the building element. Pressures decrease as the standoff distance increases and durations tend to increase with greater standoff. Shorter durations and smaller pressures cause less damage than higher pressures and longer durations.
The type of construction is also a significant factor in how much damage a building will experience from an explosion. It is important to remember that the vast majority of buildings were not built with explosive loading in mind. Therefore, just because a building does not respond well to a bomb does not mean that the building was poorly designed. Blast loading is an extremely abnormal event and stresses buildings in ways never anticipated during the original design and construction. Buildings are generally designed to hold up gravity (downward) loads and lateral wind loads. In earthquake regions, they are also designed to withstand forces created by ground movements. Standard buildings are not designed to withstand large, aboveground shock waves of the magnitudes associated with explosions. Very lightweight buildings and buildings built of unreinforced masonry (brick or concrete block units) tend to respond the worst to explosions, while concrete and steel framed buildings built in high seismic zones tend to respond the best.
