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# Drift Limit of Glass

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## Drift Limit of Glass

(OP)
Hi all,

I am trying to ascertain the drift limit of some glazing. I am looking at an old building with a curtain of single glazed windows (700mm x 1000mm tall panels).

I have been looking into ASCE 41-06 (pge 334/335) and have found that the drift limit of the windows is dependent on the geometry of the window. The calculation is based on the principle of the window (rectangular) becomes a parallelogram as a result of storey drift and that the glass to frame contact occurs when the length of the shorter diagonal of the parallelogram is equal to the diagonal of the glass panel itself.

I have the width and height of the glazing panes but I am unsure as to what to take for the clearance between the edges of the glass pane and the frame.

The glass panels I am looking at were installed quite some time ago (building built in 1950s so assume the windows have been replaced since then but do not have accurate information on this). The glass panels are glazed with a sealant but I understand that over time the sealants harden reducing the lateral drive capacity. In a paper presented at the annual Australian Earthquake Engineers Conference from 2009 on Glazed Facade Systems I have found that typical edge clearances between windows panes and frames vary from 6mm to 13mm in general practice.

I would appreciate it if anyone has any knowledge on this subject and could help me out. The drift capacity is largely dependent on this edge distance.

To give the background, this is an old building with high occupancy and we are assessing the structure's performance in a quake. We need to limit the drift of the building to ensure that the class does not fall in on the occupants, therefore the drift of the glass is the governing factor.

Thank you.

### RE: Drift Limit of Glass

The clearance between glass and frame depends on the window product you are using. Some window systems have more clerance than other. I would suggets consulting with a window manufacturer to find out the exact clearances.
Just out of curiosity, what is the inter-story drift you are using for such 50+ year old building?, and how did you calculate it?

### RE: Drift Limit of Glass

(OP)
Hi Alejandrus,

I haven't got the figures in front of me here but it is a timber building and we are allowing for a drift limit in the region of 3.0%.

We are using new Seismic Retrofit Guidelines specific to BC, Canada. Based on this allowable drift we can calculate the probability of the structure exceeding this drift limit.

Thanks for the reply - I used that same rational, turned out that the allowable drift was limited by the glass.

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