Security has become an important element in much design. AGG recently spoke with Luther C. Blair, vice president, lead designer, and Stephen F. Ours, vice president, senior project manager, of Wisnewski Blair & Associates Ltd., Alexandria, Va. about today’s need to combine design and security particularly in government or government-occupied buildings.

AGG: Can you explain how you work to combine design and security?

Blair: Essentially we work with our blast and structural engineers, Weidlinger Associates. It is important to understand the basic principles of what you are protecting the building from. With every project, the government identifies a target for its threat and the level of protection needed. The resulting blast pressure that loads the windows is transferred from the glass to the window frame into the skin of the building. These loads are absorbed by the structure to which the building skin is attached. 

The glazing is typically made up of a laminated inner lite, the typical air space and normal outer lite. To keep the glazing in the frame, it usually is attached with structural silicone. For most of our projects, we still manage to use aluminum frames, although they are deeper and with thicker walls. For higher blast loads, these frames will be reinforced with steel. The higher the level of protection, the beefier the connections are at the windows and at the attachments of the skin to the structure. 

On most of our buildings, we use some form of panelized punch windows to address the blast requirements most cost-effectively. On all of our projects, the windows are factory glazed unitized systems and come to the site glass-in-frame to be installed. They may not have the snap mullions in place on the outer end, but the glazing is set so it is a matter of lifting it in place and anchoring it into the exterior wall system (which most of the time is precast concrete façade). The anchors into the precast are not your typical bolts; they’re heavier, sometimes 12 inches on center around the entire perimeter frame or a big heavy plate with 40 bolts over a 2-foot section. 

The challenge to the design team is to make it look like a normal building while accommodating these much beefier connections. On most of our projects, the profiles of the window frame are still 2.5 or 3 inches, which looks like a normal system. It’s just that they are heavier and deeper frame sections and the connection into the exterior wall is heavy duty. 

AGG: Would you say that you’re using less glass, more, or about the same?

Blair: Most government buildings want to be LEED certified. Our goal is LEED silver on most buildings, which means that daylight penetration is a credit you want to get. Again, we are using a punched window and we are trying to make it as big as we possibly can so that we get maximum light penetration. 

Ours: The criteria in a lot of the design programs that we get are no more than 40 percent glazing per structural bay but we’re pretty much at that number.

Blair: If you do less, the building looks like a pill box. Our clients want the building to look like normal class A office buildings which have a lot of glass. Most government buildings have a requirement to dictate a pretty large floor-to-floor height because of either raised floor or allowing an extra zone in the plenum for communications. You’re really talking higher than normal floor-to-floor heights. And at 40 percent of the exterior bay, you’re getting the same size windows as on a normal spec office building. Not in a ribbon window configuration, however, because it becomes extremely difficult to get the window forces anchored in such a way that they can transfer the forces back to the structure.

Ours: When you ask if we are using the same amount of glass, in terms of exposed square footage on the outside the answer is yes, but in terms of literally pounds of glass, we are using a lot more because it is much thicker. In blast configurations, for the outside lite we’ll still use ¼-inch tempered and that’s where we’ll try to get our low-E coating or any coloration. Because the glass is so thick, we’ll often use clear to compensate for the color that occurs because of the thickness of the glass. The inner lite does all the work. We still have a ½-inch air space so it’s an insulating glass assembly. The only place we somewhat deviate from that, and we still try to incorporate an air space, is when we get to some of the forced entry glass and the ballistics glass that has a lot of polycarbonate in it because it becomes extremely thick and heavy. 

AGG: Because of these special glazings, is it safe to assume that the cost of the glazing is higher?

Blair: Yes it is. A lot higher.

AGG: Could you give an estimate percentage-wise of how much more the glazing is than if it didn’t have to be so secure?

Blair: We used to say that windows were $65 to $75 a square foot, and now we’re over $100 a square foot, so it’s 30 to 40 percent higher. There’s always pressure in the design to use less glass because of the cost. The equivalent of a precast wall section to meet the same blast criteria, while it is thicker and heavier, the windows are two times the cost of the wall system, unless you’re using some exotic wall system. We’ve done areas where we’ve not used precast concrete and you get into heavy steel plates or metal panel and the cost becomes astronomical. With any system other than precast concrete, the costs are pretty equal to glass. AGG: Can you give any specific example of where you’ve used the steel plate versus concrete?

Ours: Where you’re going to skin the building in something other than a concrete finish. In one case we used flat seamed lead-coated copper panels and in other cases we’ve used Alucobond, or something like that. But often even in the case of Alucobond, it is over concrete.

AGG: Taking all these factors into consideration—the security needs, the costs—how would you characterize the design challenges, because as you’ve said they want the building to look like a normal building and yet there are all these other factors? When you go about designing a building, how do you reconcile all this?

Blair: It’s difficult. You don’t have all the same choices. There are certain things that you know you have to do. As I said, I know that I’m going to do some form of punched opening window building with a precast concrete façade. Then it’s a matter of taking advantage of that as a medium and articulating that. We generally get a minimum thickness for the concrete facade and where we need design, then we just make it thicker. It’s just the same challenges as designing any building, coming up with a unique look. You have limited choices, however, a limited palette to work with. 

AGG: How would you characterize the glazing products available to you?

Blair: One of the biggest issues is that the government requires proof that it works. So we can only use systems that are field-tested. In the case of our last project, we actually took a precast panel and a window from the job and shipped it to New Mexico to be blown up to prove it really worked. Real test data is critical. If you are going to use something that hasn’t been tested, then the manufacturer has to be willing to foot the costs or somehow get in the costs and have the timing in the schedule to build a full-scale mock-up, coordinate with the laboratory and have it blown up or attacked to prove that it meets the criteria. Having certifiable, stamped and sealed test data is absolutely essential.

Ours: A lot of times, the program requirements for different pieces will be somewhat in conflict with each other. You might have a forced entry requirement and on the other hand you’re trying to get LEED points so you need a certain R-value or shading coefficients. And they won’t necessarily be compatible with each other. Part of our task during the design process, and it’s sort of a technical one, is to work with the client and tenant to figure out what compromises are acceptable to them and then get a glazing system that is the best fit of what is available. There will be a certain standard level of forced entry resistance that you’ve got to provide. This means there are seven or eight glazing assemblies that have tested to that, and that’s what we tend to install in the building. Usually, we use a performance specification. Instead of saying we want a specific product, we say ‘here are the blast pressures, the forced entry resistance requirements, the ballistic resistance requirements’ and then it’s up to the glazing contractor and its suppliers to come up with the glazing assembly that meets all this criteria. It falls back on us to double check their documentation of the results.

Blair: Even if it’s calculated to meet it, there still have to be a real test. 

AGG: Glazing suppliers make the point that the products needed to meet security requirements are actually available as opposed to being custom-made. Is this what you’ve found?

Ours: There are framing systems available for the windows, for example, which various suppliers already have, that have already been tested that will meet certain performance requirements. There are window assemblies available that will meet the forced entry and ballistics requirements. But in blast, because there are different pressures to which the building may be exposed, those tend to be an engineered response. The outer lite is ¼ inch. It does nothing. It forms one side of the air space, but it’s expendable in an event. It breaks and becomes debris. The inner lite does all the work. Therefore, depending on what forces it has to resist, it’s almost inevitably two lites laminated to a very heavy interlayer and how thick the lites are and how heavy the interlayer is and what the glass is, whether it’s fully tempered or heat-strengthened—all this is a response to what the design pressures, the blast pressures are on the building. So those tend not to be pre-engineered because you’re talking about expensive glass and they use a finish that will meet those specific requirements. 

AGG: Are there any other special challenges that we haven’t touched on that you think are important?

Ours: You’re using this heavy material and heavy framing systems and one of the technical challenges is you’ve met the blast and force resistance and now you have to design a door that meets ADA requirements. Which means that you’ve got this monster door that may weigh 1,700 pounds, but you have to design it so that someone in a wheelchair can open it and get in and out of the building without the door closing on them and crushing them. That’s always a challenge. There are always technical solutions to this—door openers, control systems, things like that. Also, because these doors require thick components, they are thicker. If you start the design, and you leave an opening that is wide enough for a standard egress door, you may find that when you get the details on your bullet-proof, forced-entry and blast-resistance door, lo and behold, when it is open it doesn’t give you the egress width you need. We tend to oversize some of those openings because we never know at that point in the design of the project what the final product is going to look like in terms of its thickness and configuration. And we have to allow enough space for these things to work correctly and meet codes in the final assembly. 

Blair: The hardest part is when the window system gets around the entrances and you transition into a glazing storefront system and the doors … We spend more time on the doors and hardware. It’s one of the most time-consuming areas in the whole design of the project. 

AGG: Reconciling all the factors that go into that design?

Blair: You have blast pressures, ballistic pressures, physical forced entry and attack pressures, which leads you to a combination of glass and polycarbonate; polycarbonate is great in stopping bullets but it scratches. If you put glass on the inside of it you have an element that when it’s attacked pieces will spall and become projectiles. I don’t think there has been a good solution in the industry yet for the ballistic-blast situation. There are pluses and minuses to all the alternatives. Ours: Yes, there always seems to be some sort of compromise. It’s a delicate balance of trying to come up with the best compromise from all the different programmatic code and forced entry standards that apply to the building. 

The Firm
Headquartered in Alexandria, Va., Wisnewski Blair & Associates Ltd. (WBA) is an architectural firm of 55 professionals, with particular expertise in providing fully integrated architecture, engineering, interior design, planning, and facilities management in both the public and private sectors. 

Since its formation in 1976, the firm has provided professional services on a wide range of facilities for a broad and diverse client base. In recent years, WBA has become increasingly involved in security design, including anti-terrorism/force protection design, for a variety of facilities and complexes. 

Architects' Guide to Glass & Metal
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