8.4 New Construction
Progressive Collapse Designs that facilitate or are vulnerable to progressive collapse must be avoided. At a minimum, all new facilities shall be designed for the loss of a column for one floor above grade at the building perimeter without progressive collapse. This design and analysis requirement for progressive collapse is not part of a blast analysis. It is intended to ensure adequate redundant load paths in the structure should damage occur for whatever reason. Designers may apply static and/or dynamic methods of analysis to meet this requirement. Ultimate load capacities may be assumed in the analyses.
In recognition that a larger than design explosive (or other) event may cause a partial collapse of the structure, new facilities with a defined threat shall be designed with a reasonable probability that, if local damage occurs, the structure will not collapse or be damaged to an extent disproportionate to the original cause of the damage.
In the event of an internal explosion in an uncontrolled public ground floor area, the design shall prevent progressive collapse due to the loss of one primary column, or the designer shall show that the proposed design precludes such a loss. That is, if columns are sized, reinforced, or protected so that the threat charge will not cause the column to be critically damaged, then progressive collapse calculations are not required for the internal event. For design purposes, assume there is no additional standoff from the column beyond what is permitted by the design.
Discussion: As an example, if an explosive event causes the local failure of one column and major collapse within one structural bay, a design mitigating progressive collapse would preclude the additional loss of primary structural members beyond this localized damage zone (i.e., the loss of additional columns, main girders, etc.). This does not preclude the additional loss of secondary structural or non-structural elements outside the initial zone of localized damage, provided the loss of such members is acceptable for that performance level and the loss does not precipitate the onset of progressive collapse.
Building Materials. All building materials and types acceptable under the model International Building Code are allowed. However, special consideration should be given to materials which have inherent ductility and which are better able to respond to load reversals (i.e., cast in place reinforced concrete and steel construction). Careful detailing is required for material such as prestressed concrete, pre-cast concrete, and masonry to adequately respond to the design loads. The construction type selected must meet all performance criteria of the specified Level of Protection.
Design for limited load:
- Design exterior walls for the actual pressures and impulses up to a maximum of ___ psi and ___ psi-msec (project-specific information to be provided).
- The designer should also ensure that the walls are capable of withstanding the dynamic reactions from the windows.
- Shear walls that are essential to the lateral and vertical load bearing system, and that also function as exterior walls, shall be considered primary structures. Design exterior shear walls to resist the actual blast loads predicted from the threats specified.
- Where exterior walls are not designed for the full design loads, special consideration shall be given to construction types that reduce the potential for injury (see Building Materials in this section).
Design for full load:
Design the exterior walls to resist the actual pressures and impulses acting on the exterior wall surfaces from the threats defined for the facility (see also discussions in Design for limited load above).
- Security of Swinging Door Assemblies ASTM F 476 Grade ____ (project-specific information to be provided).
- Measurement of Forced Entry Resistance of Horizontal Sliding Door Assemblies ASTM F 842 Grade ___ (project-specific information to be provided).
- A medium protection level (per TM 5-853) for walls would be the equivalent of 4” concrete with #5 reinforcing steel at 6” interval each way or 8” CMU with #4 reinforcing steel at 8 in. interval. TM 5-853 provides other alternatives for low, medium, and high protection.
The multidisciplinary team shall evaluate the performance requirements for all security-glazing materials proposed for the project. The multidisciplinary team shall ensure that normal tools carried by firefighters, such as a pick head axe, halligan tool, or similar device, can readily overcome the subject glazing barriers. If the use of more specialized tools, such as a rabbit tool, a k-tool, circular saws, rams, or similar devices is necessary to break through the glazing barrier or if the glazing itself is hardened that a blast may not blow out the windows, alternative methods or systems must be designed to ensure smoke from the incident is not trapped inside the building. (See section on New Construction, Fire Protection Engineering, Smoke Removal Systems).
The following terms are to be applied and identified for each project-specific risk assessment:
No restriction. No restrictions on the type of glazing.
Limited protection. These windows do not require design for specific blast pressure loads. Rather, the designer is encouraged to use glazing materials and designs that minimize the potential risks.
- Preferred systems include: thermally tempered heat strengthened or annealed glass with a security film installed on the interior surface and attached to the frame; laminated thermally tempered, laminated heat strengthened, or laminated annealed glass; and blast curtains.
- Acceptable systems include thermally tempered glass; and thermally tempered, heat strengthened or annealed glass with film installed on the interior surface (edge to edge, wet glazed, or daylight installations are acceptable).
- Unacceptable systems include untreated monolithic annealed or heat strengthened glass; and wire glass.
The minimum thickness of film that should be considered is 4 mil. In a blast environment, glazing can induce loads three or more times that of conventional loads onto the frames. This must be considered with the application of anti-shatter security film.
The designer should design the window frames so that they do not fail prior to the glazing under lateral load. Likewise, the anchorage should be stronger than the window frame, and the supporting wall should be stronger than the anchorage.
Table 8-1 Glazing Protection Levels Based on Fragment Impact Locations
|Description of Window Glazing Response|
|1||Safe||None||Glazing does not break. No visible damage to glazing or frame.|
|2||Very High||None||Glazing cracks but is retained by the frame. Dusting or very small fragments near sill or on floor acceptable.|
|3a||High||Very Low||Glazing cracks. Fragments enter space and land on floor no further than 3.3 ft. from the window.|
|3b||High||Low||Glazing cracks. Fragments enter space and land on floor no further than 10 ft. from the window.|
|4||Medium||Medium||Glazing cracks. Fragments enter space and land on floor and impact a vertical witness panel at a distance of no more than 10 ft.from the window at a height no greater than 2 ft. above the floor.|
|5||Low||High||Glazing cracks and window system fails catastrophically.Fragments enter space impacting a vertical witness panel at a distance of no more than 10 ft. from the window at a height greater than 2 ft. above the floor.|
|* In conditions 2, 3a, 3b, 4 and 5, glazing fragments may be thrown to the outside of the protected space toward the detonation location.|
The design strength of a window frame and associated anchorage is related to the breaking strength of the glazing. Thermally tempered glass is roughly four times as strong as annealed, and heat strengthened glass is roughly twice as strong as annealed.
Design up to specified load. Window systems design (glazing, frames, anchorage to supporting walls, etc.) on the exterior facade should be balanced to mitigate the hazardous effects of flying glazing following an explosive event. The walls, anchorage, and window framing should fully develop the capacity of the glazing material selected.
The designer may use a combination of methods such as government produced and sponsored computer programs (e.g., WINLAC, GLASTOP, SAFEVU, and BLASTOP/WINGUARD) coupled with test data and recognized dynamic structural analysis techniques to show that the glazing either survives the specified threats or the post damage performance of the glazing protects the occupants in accordance with the conditions specified here (Table 8-1). When using such methods, the designer may consider a breakage probability no higher than 750 breaks per 1000 when calculating loads to frames and anchorage.
Table 8-2 Test Structure
Side view of test structure illustrating performance conditions of Table 8-1
Test window should be in the design position or centered on the wall.
While most test data use glazing framed with a deep bite, this may not be amenable to effective glazing performance or installation. It has been demonstrated that new glazing systems with a 3/4-inch minimum bite can be engineered to meet the performance standards of Table 8-2 with the application of structural silicone. However, not much information is available on the long-term performance of glazing attached by structural silicone or with anchored security films.
All glazing hazard reduction products for these protection levels require product-specific test results and engineering analyses performed by qualified independent agents demonstrating the performance of the product under the specified blast loads, and stating that it meets or exceeds the minimum performance required. Performance levels are based on the protection conditions presented in Table 8-2. A Government-provided database indicating the performance of a wide variety of products will be made available to the designer.
- Window Fenestration: The total fenestration openings are not limited; however, a maximum of 40 percent per structural bay is a preferred design goal.
- Window Frames: The frame system should develop the full capacity of the chosen glazing up to 750 breaks per 1000, and provide the required level of protection without failure. This can be shown through design calculations or approved testing methods.
- Anchorage: The anchorage should remain attached to the walls of the facility during an explosive event without failure. Capacity of the anchorage system can be shown through design calculations or approved tests that demonstrate that failure of the proposed anchorage will not occur and that the required performance level is provided.
Glazing alternatives. Glazing alternatives are as follows:
- Preferred systems include: thermally tempered glass with a security film installed on the interior surface and attached to the frame; laminated thermally tempered, laminated heat strengthened, or laminated annealed glass; and blast curtains.
- Acceptable systems include monolithic thermally tempered glass with or without film if the pane is designed to withstand the full design threat (see Condition 1 on Table 8-2).
- Unacceptable systems include untreated monolithic annealed or heat-strengthened glass; and wire glass.
In general, thicker anti-shatter security films provide higher levels of hazard mitigation than thinner films. Testing has shown that a minimum of a 7 mil thick film, or specially manufactured 4 mil thick film, is the minimum to provide hazard mitigation from blast. The minimum film thickness that should be considered is 4 mil.
Not all windows in a public facility can reasonably be designed to resist the full forces expected from the design blast threats. As a minimum, design window systems (glazing, frames, and anchorage) to achieve the specified performance conditions (Table 8-2) for the actual blast pressure and impulse acting on the windows up to a maximum of ___ psi and ___ psi-msec. As a minimum goal, the window systems should be designed so that at least __ percent of the total glazed areas of the facility meet the specified performance conditions when subjected to the defined threats (project-specific information to be provided).
In some cases, it may be beneficial and economically feasible to select a glazing system that demonstrates a higher, safer performance condition.Where tests indicate that one design will perform better at significantly higher loads, that design could be given greater preference.
Where peak pressures from the design explosive threats can be shown to be below 1 psi acting on the face of the building, the designer may use the reduced requirements of Exterior Walls, Limited Protection, in this section.
- Ballistic windows, if required, shall meet the requirements of UL 752 Bullet-Resistant Glazing Level ___ (project-specific information to be provided). Glass clad polycarbonate or laminated polycarbonate are two types of acceptable glazing material.
- Security glazing, if required, shall meet the requirements of ASTM F1233 or UL 972, Burglary Resistant Glazing Material.
This glazing should meet the minimum performance specified in Table 8-2. However, special consideration should be given to frames and anchorages for ballistic resistant windows and security glazing since their inherent resistance to blast may impart large reaction loads to the supporting walls.
- Resistance of Window Assemblies to Forced Entry (excluding glazing) ASTM F 588 Grade___ (project specific information to be provided; see above for glazing).
- Design for eavesdropping and electronic emanations is beyond the scope of the criteria.
Non-Window Openings. Non-window openings such as mechanical vents and exposed plenums should be designed to the level of protection required for the exterior wall. Designs should account for potential in-filling of blast over-pressures through such openings. The design of structural members and all mechanical system mountings and attachments should resist these interior fill pressures.
Interior Windows. Interior glazing should be minimized where a threat exists. The designer should avoid locating critical functions next to high risk areas with glazing, such as lobbies, loading docks, etc.
Parking. The following criteria apply to parking inside a facility where the building superstructure is supported by the parking structure:
- The designer shall protect primary vertical load carrying members by implementing architectural or structural features that provide a minimum 6-inch standoff.
- All columns in the garage area shall be designed for an unbraced length equal to two floors, or three floors where there are two levels of parking.
Selected Design Areas. For lobbies and other areas with specified threats:
- The designer shall implement architectural or structural features that deny contact with exposed primary vertical load members in these areas. A minimum standoff of at least 6 inches from these members is required.
- Primary vertical load carrying members shall be designed to resist the effects of the specified threat see Progressive Collapse in this section).
Loading Docks. The loading dock design should limit damage to adjacent areas and vent explosive force to the exterior of the building. Significant structural damage to the walls and ceiling of the loading dock is acceptable. However, the areas adjacent to the loading dock should not experience severe structural damage or collapse. The floor of the loading dock does not need to be designed for blast resistance if the area below is not occupied and contains no critical utilities.
Mailrooms and Unscreened Retail Spaces. Mailrooms where packages are received and opened for inspection, and unscreened retail spaces (see Architecture and Interior Design, Planning, Retail in the Lobby and Mailroom) shall be designed to mitigate the effects of a blast on primary vertical or lateral bracing members.Where these rooms are located in occupied areas or adjacent to critical utilities, walls, ceilings, and floors, they should be blast and fragment resistant. Significant structural damage to the walls, ceilings, and floors of the mailroom is acceptable. However, the areas adjacent to the mailroom should not experience severe damage or collapse.
Venting. The designer should consider methods to facilitate the venting of explosive forces and gases from the interior spaces to the outside of the structure. Examples of such methods include the use of blow-out panels and window system designs that provide protection from blast pressure applied to the outside but that readily fail and vent if exposed to blast pressure on the inside.