The 2010 Aluminum Code has new criteria for considering the unbraced length of open sections.  The unbraced length for a vertical mullion. Is usually considered to be the distance between horizontal mullions.  However, design wind load charts put out by many manufactures of storefront systems. Are often based on the assumption that the mullion has full lateral buckling support and an unbraced length of zero.  How can this be?

I believe that the manufacturers are considering lateral bracing from the glass and the mechanical gaskets.  However, after review of many industry specs it is clear that mechanical gaskets should not be considered as a means of lateral bracing for open shaped storefront mullions.  Therefore, the charts error on the side of being too liberal.  When it comes to the calculations. They can’t match up and the mullions usually don’t perform as well as the charts indicate.

Unfortunately, it’s in the interest of the manufactures to keep the charts the same because they are competing against one another. For framing systems with the highest performance standards.

Glaziers should keep this in mind when selecting open shaped vertical mullions and stay well under the curve projections that are indicated.    If glaziers use the charts, as is. Reinforcing structural glazing or heavier mullions will likely be needed.

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Authors:

Stewart Jeske. P.E.

Matt Quinlivan, E.I.T.

 Find out more about Storefront Design from JEI Structural Engineering.

 

LinkedIn Group Comments

Group: Building envelope designers

Discussion: Be Careful With Manufacture Storefront Design Charts

 I totally agree with you. Experience shows that manufacturers of facade systems offer the product and show its characteristics, but do not take into account many factors, leaving a wide field of work for designers. Designing the mullion must take into account at least the following factors:

One

1. Wind load is applied in accordance with the applicable Code in this area. Must take into account fluctuating component and an increased load in the corner area of the facade. If you use heavy glass, it is necessary to consider the possible loads caused by vibration of the system.

Two

2. Scheme for the calculation of stresses must be carefully designed. Often the greatest stresses occur at the point of attachment, and at this point the profile is often weakened by mounting bolts. Hence, to calculate the stress characteristics of the mullion should be recalculated. It is necessary to check the bending moment in the mullion connection point.

It is a mistake to assume that the whole profile has the same characteristics. The mullion connection point is often weaker and should be checked separately.
Considering separately each case we can speak of deflections, stresses, and overall system performance.

Posted by Façade Engineer in August, 2011

 

Group: Aluminum Curtianwall, Storefront Extrusions

Discussion: Be Careful With Manufacture Storefront Design Charts

 Normally, the local architectural office (hue approves the building) has to give you the official wind loud charts in the area. From my experience, as building envelope designer, I will recommend to take a 1.5 safety factor in your calculations for the mullion and traverse deflection.

Posted by Technical Director at Vimetco Extrusion in August, 2011

 

Group: Facade (curtain wall) engineering group

Discussion: Be Careful With Manufacture Storefront Design Charts

I couldn’t agree more.  Specifically, these charts don’t take into account whether there are any intermediate horizontal that are essential to bracing of the vertical mullion to develop the spans as noted in those charts. Also some of the open channel mullions won’t be able to achieve the minimum allowable stresses used in those charts due to the local buckling issues.

Posted by a Structural Engineer, P.E. in August, 2011

 

All codes include factors of safety because of the uncertainty about the loading, materials, manufacturing and installation. I can see that you propose to take into account that the glass in the glazing pocket provides bracing for the mullion and as a result to increase the allowable stresses. However, i wonder if after many years of use, the gaskets that are holding the glass and creating a lateral support for the mullion will as effective as when testing a brand new window in the lab.

 

Also, the most of the projects specs specify that any contribution of the glass to the bracing of a million shall be ignored. Further, as you noted when loading front set storefront with negative wind
pressure, the glass is in the tension Zone and thus does not provide any bracing. The truth is that the most critical conditions are at the corners of the building with negative (suction) loads.

As far as you point

As far as you point about using formulas with Ly/Ry vs the formula with torsional constant J. We are using the formula with J for tubular mullions as they give more realistic results. For the open channel sections, the J is so small that it wouldn’t have made any difference. Lastly, we are tasked with a structural design of a very thin walled aluminum sections to span ever increasing heights.

As the architects want to see more glass and less metal. The boundary conditions. Especially at the head are less and less restrained for torsion with horizontals simply snapped in and not physically attached to the mullions. With compensation channels used to restrain a 12 ft tall mullion from fallout in weak axis bending of the comp channel. If the mullion buckles due to the lack of horizontals bracing it. A large glass pane may fall out creating a danger to the left of occupants. I’d say that I don’t see how the requirements of the code are “too conservative”.

Posted by a Structural Engineer, P.E. in September, 2011

 

The question poses a dilemma for us as a manufacturer. The user, be it glazing sub or architect, wants to know the size of the system given the span, mullion spacing, and wind load of a given situation. These charts allow that. And, as the original question posed, that by no means CANNOT answer all the questions.The horizontals for bracing are only one part of the answer. So is thermal expansion / contraction, so is transition to surrounding substrates, so is the glass being designed for the size / applicable wind load. There are other issues as well, seismic loads (when applicable), building movements, etc. All these can’t be charted, but have to be addressed individually and collated into a working solution with the intended system. When one specific system doesn’t qualify. Then others means must be sought to resolve the issues on a given project’s set of conditions.

But for starters, it’s not a bad place to at least have a little info out there. The appropriateness of the final solution, is in the hands of the end user(s), be it glazing sub or architect who reviews / approves the shop drawings AND structural calculations. Once shouldn’t be done without the other, one would hope.

Posted by Curtain Wall Manager at Technical Glass Products in September, 2011

 

I agree with you that this is a good start to at least “lock in” the system. However, I think it should be noted (in fine print if necessary) that these charts are for mullions that are braced at the minimum spacing of “X” ft. If the horizontal spacing exceeds this value the use is to seek additional engineering assistance.

Posted by a Structural Engineer, P.E. in September, 2011

 

Suggesting that wind load charts developed by the manufacturers list. A maximum horizontal spacing is a great idea and is something that I feel. Should be done in order to eliminate any confusion of glazing contractors. However, this is an idea that may have difficulty catching on. If one manufacturer develops wind load charts indicating a maximum horizontal mullion spacing while all others do not.

Posted by Matthew Quinlivan, EIT JEI Structural Engineering in September, 2011

 

Yes, I agree with you that it may be disadvantageous for a manufacturer to put this limitation on their charts. However, this is also a question of marketing vs. potential liability. If the contractor selects the mull size
based on these charts and the mulls fail, the manufacturer may be found liable.

Posted by a Structural Engineer, P.E. in September, 2011

 

Limitations of manufacturer’s charts definitely need to be spelled out better. Right now manufacturer’s charts are very misleading and often get the glazing contractor into a position where they have ordered material. And the engineering calcs will show that reinforcing and/or added horizontals are needed. And this is very frustrating for all involved. We usually get the comment that, “We’ve been doing it like this for years and have never had a problem”.

It’s likely that many of the systems have never been subject to a design wind event and if it does, it’s many years down the road. And the failure that may be seen is probably in the seal as the mullions slightly buckle and there may be slight permanent deformation.  This is hypothetical, but I have a feeling that its probably the way it goes.

What do you all think?

I would also really appreciate comments from engineers who work for storefront manufactures.

Posted by Stewart Jeske, P.E. JEI Structural Engineering in September, 2011

 

I understand the concern about the liability. But anyone in their right minds who uses those charts and never consults a PE? That’s counter-intuitive, but you’re right, it probably does happen.So the next time our charts are updated. We’ll see if we can add a “take two aspirin and call a doctor… “, no, sorry, add a clause stating “use of these charts is should be done in conjunction with a review by PE for final approval of span, anchorage, and stress of members depicted in the charts. I’d tell the architect they should check with their structural engineers or the manufacturers if there were any questions about any of this.

one thought here. Our primary framing members are rectangles, so that’s a pretty efficient shape when it comes to stress, avoiding buckling issues. Given the limits of the glass manufacturers on the size lites they can produce. Is the absense of bracing horizontals and thereby stress in the main mullion(s) really going to be a factor if using tubes or rectangles and limit the deflection to L/175 or 3/4” / etc?

Our Custom Shapes

I know on our custom shapes, the T’s and the I-beams scare the heck out of me for buckling, and we always get a PE review for them – in design / profile selection -just in case. But on the rectangles.  I haven’t ever seen the stress be a factor. Is this stress issue more for alum or for steel (TGP primarily does steel framing, not alum).

I think most owners would assume something bad’s happened especially if the glass were to evacuate the opening. First thought is, the material’s been damaged, and shouldn’t be reused. I think any glazing sub who walks up on a situation like that would take the prudent route and replace the framing, don’t you?

Posted by Curtain Wall Manager at Technical Glass Products in September, 2011

 

Many, many glazing contractors use the manufacturer’s charts and never consult an engineer. Very seldom are engineering calculations/seal a part of the requirements for submittal in the specifications for storefronts. They are the open section storefront shapes that have a clip in filler plate. When ever we follow the aluminum code requirements for calculations on these “open” shapes most of the time stress limitations control the design and not L/175 or 3/4″. I would agreee with you about the rectangular closed shapes – usully its the deflection limit that controls.  It can’t have a snap in filler plate.

I think a statement like what you suggest would be good to have on the charts. In most cases for the “open” shape storefront mullions. The charts are very misleading. Contractors do end up in trouble over and over again when they use those charts for estimating/ordering and then need calculations to meet the submittal requirements of the specs.

Last Question

I’m not sure where you were going with your last question. In my previous comment. I certainly was not suggesting that a glazing contractor would or should reuse something that had suffered damage. I was pointing out that most of the storefronts that are constructed. Probably don’t see the wind load that they were designed. For until many years down the road. And if there is any damage because mullions have not been appropriately designed by an engineer (accounting for the stress issue associated with open shapes). Then the damage is probably very slight permanent deformation and the seals will probably stop working. At that point the owner probably forgets who is responsible for the poor performance of the storefront and just calls a glazing contractor to have it replaced.

Posted by Stewart Jeske, P.E. JEI Structural Engineering in September, 2011

 

I think 2 aspirins will do… Albeit may not help a leaky windowJYou are right. The closed tubular members are pretty efficient in terms of lateral torsional buckling. My main concern with buckling is for open channel rectangular shapes that are used in screw spline storefront systems. However, there are case where long span curtain walls with 10 ft tall glass panes will also have considerable allowable stress reduction.

The horizontals spacing doesn’t have an effect on the deflection of mullions but on the allowable stress. In other words, the section may fail in lateral torsional buckling before it yields. That issue applies to all materials and has to do with the shape’s geometrical properties and lateral constraints such as horizontals. The concern is that not only that the member fails structurally. But also that the system loses its water and air tightness and that the glass may fall out (which is a major concern). Any post buckled material should definitely removed and replaced.

Posted by a Structural Engineer, P.E. in September, 2011

 

I agree with you that this is a good start to at least “lock in” the system. However. I think it should be noted (in fine print if necessary). That these charts are for mullions that are braced at the minimum spacing of “X” ft. If the horizontal spacing exceeds this value the use is to seek additional engineering assistance. Let’s face it, unless the specifications and/or design documents call for engineered storefront solution. These charts will be used as the basis of design. And no further engineering will be provided.

Posted by a Structural Engineer, P.E. in September, 2011

 

We’ve discussed this in our office many times.   However, is this something that the manufacturers would actually do???   I would have to say no.  As a result of the competiveness of the industry. Putting limitations on the wind load charts would make their product seem inferior to other manufacturers. Whose wind load charts have a higher capacity due to the fact they assume a continuous laterally braced frame.

Posted by Matthew Quinlivan, EIT JEI Structural Engineering in September, 2011

 

Storefront Manufacture Comments

 

These charts do not consider lateral buckling as was explained earlier. And as noted in the notes in our Detail Manual.  In reality, the glass lateral stiffness does provide some bracing when using our standard side blocks.  There has always been the question of how much bracing does the glass provide. Some consultants will not allow the glass to provide any bracing and some will consider the glass to provide full bracing.

Our testing has shown that there can be some lateral deflection but at these lower loads. You will probably not get permanent lateral buckling. But it would be hard to convince some consultants unless we did a test of the exact unit in question.  I would say if this consultant is going to be on the job we probably do not want to fight his recommendation.  If there were no consultant. I would have no problem using this heavy weight mullion in this application.

Emailed by a Storefront Manufacture

I received the information above from one storefront manufacture regarding our issues with lateral buckling in open shaped storefront members.  They basically say that their wind load charts do not take into account any lateral buckling due to the fact that the lateral stiffness of the glass provides some bracing for the vertical member. We do not necessarily agree with this in that, under loading and over time. It cannot be assumed that the mechanical gaskets holding the glass in place will not weaken.

If these gaskets weaken over time. The glass can be free to move slightly. And cannot be used for lateral bracing.  This is one reason why project specifications often do not allow for mechanical gaskets and glass to provide lateral support.  Especially when we are dealing with large spans having few horizontal mullions.

Posted by Matthew Quinlivan, EIT JEI Structural Engineering in September, 2011

 

I reviewed the module for lateral buckling using the shapes that you requested.  None of the shapes work by the calculations.  Additionally, the shear-block mullion does not work structurally either.Your choices would be to add horizontals which would decrease the un-braced length. For lateral buckling or change to a tubular system that will meet the wind load requirements.

Simply stated, Aluminum manufactures would be shooting at a moving target if they tried to publish lateral buckling charts.  The allowable stress of the mullion changes based on the length of the mullion and the dimension between horizontals.  When projects require the need for need lateral buckling design. We priced into the project PE calculations and designed the project accordingly.

Storefront Manufacture – Engineering Department

 

We do have a lateral buckling problem with our storefront shapes. The additional comments that I received are.  It is possible that a continuous silicone seal along the glass would supply enough support. And if the architect and consultant accept that solution. We should not have a concern as long as the water path along the edge of the glass is not blocked into the sub-sill.  The other option of additional horizontals would also work.

To answer the question on why the mullions without horizontals will go longer spans than the mullions. With horizontals is due to the fact that the trapezoidal loading applies less load to the mullion when there are no horizontals.  Since lateral buckling is not considered in the charts the mullion without horizontals will go higher.

Storefront Manufacture – Engineering Department

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