Volume 36, Issue 6, June 2001

Live-Load Deflection

Correcting Problems in Curtainwall Installations

by Craig Carson

Many times we see the use of curtainwall products as large storefront systems for the main lobby of office buildings. These installations usually occur because the architect has opened up the lobby with tall glass areas so the strength of a strong system to resist wind load is required. But, what happens when you are asked to provide this same type of system in a multi-story application? 

In many of the projects I see, architects are concerned with aesthetics of the curtainwall and how it fits with the rest of their design. In addition, most projects are issued for bidding while only half complete. Engineering requirements for the curtainwall are almost never reviewed until after the bids have been accepted, and then only if the low bidder has taken issue with them. 

This creates many areas of concern, including lack of depth in the curtainwall system to resist wind load and the fact that the mullion spacing is so far apart that the horizontals cannot meet the dead-load requirements from the weight of the glass. But, the most common problem I find is the live-load deflection of the building.

Typically, structural engineers design live-load deflection at L/360. This requirement is what the new International Building Code 2000 (and the others that preceded it) demand for load-bearing beams. However, 
this is inadequate for curtainwall installations. 

Let’s take an example of a typical office building built in Denver. Many of the buildings here have 360-inch grids, which means the engineers are allowing 1-inch deflection in the center of the span. How this affects the curtainwall design is that the typical pressure plate curtainwall stack joint is designed for approximately ¼ inch (+/-) of movement for thermal expansion, assuming you provide a splice at every floor line (approximately 14 feet). The allowance for twin span (the vertical curtainwall mullion spanning two floors, 28 feet (+/-) will further increase the allowance for thermal expansion. Returning to the single-span application for our argument, you can see that 1-inch live-load deflection does not go into the ¼-inch tolerance for the splice joint. 

Ignoring this design flaw could result in either disengagement of the glazing material when the floor below the splice joint is fully-loaded to its limit and the joint is opened beyond its design limit. Alternatively, the opposite could happen as the floor above the splice joint becomes fully-loaded. The splice will close up and break the glazing material below as the horizontal will first compress the glass and then the upper vertical will compress further on to the lower vertical mullion potentially damaging both of them. 
Now, if the structural engineer and architect are unwilling or unable to stiffen the beam to eliminate this discrepancy, you must allow for this movement in the splice joint. This will change the design of your splice from a splice in the vertical mullion to a stacking horizontal, similar to the stacking horizontal used in unitized curtainwall. 

In the example I described, my metal supplier provided a design for a stacking horizontal that would accommodate that amount of movement. The design was able to keep the edge bite of the glass as per the Glass Association of North America’s recommendations, but it resulted in a face dimension of the stacking horizontal of 53/8 inches. This was beyond the architect’s acceptable range for his design. The building is being re-engineered to reduce the live-load deflection.

With other types of curtainwalls such as I-beam or unitized ones, the problem still exists. However, since the factory and/or fabricator engineers these systems, this problem is addressed during the bid process or in shop drawings. 

Do not let your company ignore this problem. Address this issue with the architect early and have your aluminum supplier show you how its system allows for this deflection. They are usually more than happy to help you, or to call on the architect to explain the problem to you. 

 


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