Wind Loading?

[SteelPE]

I have a project that is in a 120mph hurricane prone wind zone and has a ton of glass (IBC 2003). From talking with the architect (who looked at me like I had three heads) they do not plan on using impact resistant glass. This basically means that the building must be designed as partially enclosed.

Due to fire considerations, the building needs an expansion joint placed between two sections of the structure. The design of this joint is so that if one section of the building burns and falls down, it doesn’t bring the other section with it. One of the sections of building basically contains all of the windows making the other section of the building basically enclosed. One side of this expansion joint is going to be constructed out of 12” CMU (enclosed portion of building) the other side is going to be a fire rated stud wall (open portion of building).

Now, when it comes to the design of the stud fire rated wall, this wall is going to have to be designed for the C&C section of the wind code. Making the other masonry wall to be designed for normal interior wall pressures? Is this correct?

For the MWFRS loads, I would basically have to look at each section of building under windward or leeward pressures, not both of them together as if it was one complete structure?

 

[a2mfk]

Is this in Florida? Wind-borne debris region?

 

[ToadJones]

Don't forget about those pesky wind-torsion cases.

 

[SteelPE]

a2mfk,

No, it is not in Florida, but it is on the East Coast in a 120 mph zone with the code specifically referencing wind borne debris.

 

[steellion]

The wind "sees" the building as one structure so you don't have to apply windward/leeward pressures at the BEJ but you do need to apply internal pressures at the wall. I think it's a good idea to design each side of the fire wall locally for C&C pressures. You can conservatively treat them as two separate buildings as well.

Regarding the windows, I'm on the East Coast and I think you're going to need impact-resistant glazing. It's probably a requirement and the architect does not have the right to just ignore it. It's worth following up on.

 

[Ron]

steellion...generally you have the option, in a windborne debris area, of using impact glass or designing the building as partially enclosed. Often less expensive to design as partially enclosed rather than use impact glass.

SteelPE...If the section of the building that will be considered partially enclosed is bounded by the stud wall, it will be subject to internal pressure, and since the stud wall is to be considered a penetrable wall (doors, I presume), the internal pressure of one becomes the external pressure of the other wall, assuming the expansion joint is capable of withstanding the internal pressure.

If my understanding of your sections is backward, then the stud wall would be C&C with no shading by the other section....or essentially two separate buildings, assuming the roof is not structurally contiguous across the joint. Is the roof being supported by a separate beam over the stud wall or is it being supported by the masonry wall on both sides?

 

[SteelPE]

steellion,

The code states that the windows are to be impact resistant otherwise the windows shall be assumed to be openings. So from what I can tell, there is an option that can be selected by the client. Whether there is an architectural code requirement, I do not know.

I agree that each side of the walls should be designed for C&C pressures.... however, in this instance my pressures are around 40psf for a masonry wall and 45psf for a metal stud wall (which is extremely high for a metal stud wall from what I can tell).

 

[steellion]

Whether or not it's listed as a requirement in the code, I think it's foolish to neglect impact-resistant glazing. While designing the building as partially enclosed will protect occupants in a life-safety situation, water intrusion can result in a total building loss if it is beyond repair, not to mention the potential damage of millions of dollars of assets inside the building.

I know it's not your call, steelpe, so if it's not required in the code and not used, nothing you can really do about it. I just hope the owner knows what he's buying and signs off on it.

 

[SteelPE]

steellion

I have never been involved with a project that was withing a wind borne debris region. I have had a few that have come close and I believe the driving factor is the cost of the glazing. In this instance, I believe it is much much higher.

FYI, this building has a wall of glass 100' wide x 40' tall (less some columns and mullions). Not a bright idea in a hurricane prone region but I guess that is why I'm an engineer and not an architect.

 

[cetiger]

Stellion makes an excellent call. I would make sure the owner is made aware of the repercussions of not using impact resistant glass. I might also suggest something in writing as well. Atleast for your protection your professional opinion will be in writing, on the record should anything happen down the road. A high-rise condo project I designed a few years back in is a current law suit with the HOA/Developer against the Contractor...apparently in this instance the GC opted for using a cheaper window wall system than specified or approved by the AOR, thus a lot of residents have been complaining of water intrusion and damages.
In regards to C&C loads...they are applicable to components and cladding, which in my opinion should be applied to the smaller pieces of the structural system. In your case, the glass, fasteners and most likely the studs (with any attachments to steel framing) would be applicable. Depending on the stud and glass spans, I would think you would need some intermediate steel framing. At 120MPH, you'll have some heavy wind pressures to resist and traditional glass curtain wall may not be feasible. You may want to check with the glass system supplier. While architects may LOVE large glass spans with small rather concealed (often aluminum) framing, resisting high winds with larger spans can be difficult to achieve and often very expensive. A lot of times some kind of exposed secondary steel framing is utilized to help support the glass and studs. For the CMU walls (assuming load-bearing), I would think it would be acceptable to design for the MWFRS wind loads. If secondary steel framing is used to support the glass curtain wall, I would think MWFRS wind loads would be acceptable use for these members as well. Obviously, it would be conservative to use the C&C wind loads when designing all members.

 

[SteelPE]

cetiger,

I am not worried about the masonry wall, and I believe we will have some sort of steel framing in the exterior 100'x40' glass wall. In fact, we currently have columns embedded in this area at 20' o.c.. As far as horizontal members, we have one at elevation = 14' and will probably end up with one at elevation = 28 leaving our windows to span 20' wide x 14' tall gap.

In regards to designing the masonry wall for C&C pressures. The pressures are not that high due to the fact that the equation use to compute the tributary area = h^2/3 which gives me some relief for loads applied to the masonry wall. In fact, designing the wall with strength provisions also helps with the design of the wall.

 

[a2mfk]

My whole 13 year career has been in FL in design and forensics. I know Ron does forensics here in FL too, and we can both tell you (not that you don't know) that after most hurricanes (Katrina and Andrew aside), the largest amount of economic damage to buildings is water intrusion. Buildings and homes are left standing, but become mold-infested nightmares that have to be completely gutted. Though many codes (FBC does) allow you to design it as open or partially enclosed and structurally you can make it all work, does it make sense? We all agree no.

Now if an owner is a developer building something he will immediately sell, it is tough to convince them otherwise to spend the money up front (I have been involved in your very situation on design projects). However, if this is owner occupied, the disruption of business could be devastating if for six or more months they cannot use the structure and valuable files/computers/inventory is lost. And if people hang their hat on insurance flipping the entire bill, well, good luck with that... Even if they do eventually pay for everything minus your deductible, your business could be ruined.

If you can get the architect or owner's ear, this is the approach I would try. Sure you can make it structurally work, we can design bomb shelters after all. But does it make economic sense to take that risk, some owners may understand that. And like others said, do it in writing and CC as many parties as possible.

The construction industry in the US sometimes is bewildering. Think of all the engineering, QC, and federal regulations of the car industry. I can look up safety ratings of any car online right now. But no such system is in place for buildings and structures, only building codes and the permit process, which the general public believes and trusts in fully and blindly. Building A and B may look alike, but are they?

 

[DaveAtkins]

I think this is one of those cases subject to engineering judgment. If one side of the building collapses in a fire, the other side must continue to stand, but only for a period of time equal to the fire rating of the wall. Therefore, I think it is too conservative to use exterior wind loading on either wall (you won't have a 120 mph wind storm immediately after a fire).

So perhaps using the internal pressure portion of the C & C wind load (or 5 psf, whichever is greater) is a reasonable compromise.

DaveAtkins

 

[cetiger]

Sounds like you're on the right track.

Again, I would suggest talking with a glass/window supplier to understand the limits of the different systems available. From my experience, these appear to be very large panels of glass AND in a high-wind zone. With having a 14' clear span of glass, I would think it would need to be quite thick just to handle servicability deflections to resist any cracking (i.e.Thicker = Heavier = More Money).

 

[Ron]

steellion...there is only a small percentage relationship between water intrusion and structural efficacy of windows in buildings. Granted, a window damaged by windborne debris will not be water resistant in any manner, and I know that's your point.

For high wind areas, a window must be rated for structural capacity at the appropriate wind loading. For water intrusion, it is only required (at the time of manufacturing) to withstand wind-blown rain at 15 percent of the structural load. That is true whether the glass is impact resistant or not. So if you have a window rated for 40 psf wind loading, it is only required to withstand 6 psf wind-blown rain (and approximately the same for air infiltration).

Impact resistance is a completely different proposition. It has nothing to do with the structural requirement of the window, but separately addresses the resistance of the glass to small or large projectiles, depending on the location of the window in the building.

I agree with you that the owner should be made aware of the impact resistance issue; however, when many are made aware of such, they elect to have the engineer approach the design differently because of the "apparent" cost increase of the windows. Perhaps we as engineers should communicate better to the architect that to design the building for a partially enclosed condition will increase the cost of the structural frame as compared to the enclosed condition, but to do so requires additional structural evaluation that won't be paid for.

steelpe...that's a large expanse of glass. You will have to get creative on the reinforcement for the mullions, since the standard extrusions will not likely withstand the loads. I've had to insert channels welded in odd configurations to reinforce mullions on projects of similar wind loading.

 

[Ron]

a2...exactly.