Floor Deflection Limits at Exterior Windows 2

[lkjh345]

I have a case where I have the head of window mullions that is set on the foundation wall butting into the underside of some framing that is supported by the roof structure. Therefore, the window will not be moving up or down, but the framing above it will be. The window suppilier will need to aacomadate the movement of the roof structure with his system.

On the drawings the window suppilier was told to design his windows system for 1/2" vertical movement in the structure above

Now that he has the job, he is wining that 1/2" live load deflection is " very large" and that they " typically see 1/4" live load deflections ".

I have never seen, or designed to such a tight criteria. The some of the roof beams are 54 feet long, so the 1/2" live load deflection limit is already L/1,296. This seem very reasonable to me.

I would like to know what kind of live load deflections limits others typical show for window suppiliers to design too, and if you think a 1/2" limit is unreasonably large.

Thank You.

 

[JAE]

1/2" doesn't sound too excessive to me. But perhaps you normally see the window framing as part of a localized wall where the lintels don't see much load and thus...don't deflect all that much. And the beam deflection is taken up between beam and wall framing and not beam-to-window.

 

[boo1]

Consider framing the windows 1/2" oversized leaving the gap at top.

 

[DaveAtkins]

I think it depends on the window head being used. I worked on a project once where deflection had to be limited to 1/8"!
I recommend working with the window supplier to see what can be done. I think the supplier will accomodate 1/2" if you push the issue.
If you have to redesign beams, I think you should get additional fee. What you have done is not out of the ordinary.

DaveAtkins

 

[jike]

You are right on track with what you are requiring! Perhaps the architectural spec wasn't written properly or the architectural details don't show the required slip track at the head of the windows and this might be why the window supplier is resisting.

We would always require a slip track at the head of continuous strip windows and long punched windows! Often the details came back from the window supplier showing no deflection provision and we would always mark it up.

You might want to check the architectural spec and details to make sure they are correct!

 

[lkjh345]

Thanks All.

Sometimes its good to know I am not asking for something unreasonable.

 

[RARWOOD]

If the 1/2" criteria was on the drawings the supplier should not be complaining. Like many suppliers he or she probably bid the job hoping that he or she could get the requirement reduced after the contract was awarded.

The answer to your question is provided by your beam deflection. I don't think it would be uncommon to design to a l/500 deflection over openings.

When there are criteria that are excesive or unreasonable, the time to complain is before the bid is due.

 

[JohnNevets]

I work for a window manufacture and we have accomplished 1/2 or more of live load movement before but it is not as common as 1/4 or 3/8. The other thing to remember they are probably getting pressure to keep a small caulk joint at the head and depending how it is detailed and what caulk is used the joint will likely have to be double the movement (created both from live load, and thermal expansion). So chances are this joint is going to be 1 1/4" + and most architects don't like this so the window guy is stuck in the middle.

 

[lkjh345]

JohnNevets:

I understand the problem with the large caulk joints.

However, for a 54' long beam, I am not sure how practical a 1/4" deflection limit is. This would be very stiff for a beam subject to roof snow and drift loading IMO.

Another engineer in our company recently did a very similiar building, and required the window suppilier to handle a 3/4" live load deflection and he didn't hear a complaint from the suppilier on that job. So part of the problem is that we don't get consistant feedback from the window suppiliers. We assume that if one guy can handle 3/4", then 1/2" isn't an unreasonable critieria.

 

[RARWOOD]

JohnNevets has made several good points. Part of the solution probably relies on good detailing by the architect. It is easy for an architect or engineer to give a criteria to the supplier and make, the supplier try to solve the problem. However many times if the architect or engineer anticipates the problem they can detail to make the suppliers life easier.

My experience with dealing with architects and engineers today is that although many of them are well educated, most of them have never been taught how to properly detail. Part of this of course is "our" fault, as those of us with more experience, need to help educate those in our field with less experience.