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Hira Malik


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Attached below is screen shot of a canopy model. the cantilever portion is 24ft glas with supporting beams and 20psf live load. I am not sure how to check the design for a glass slab. How to check shear? What would be the limiting value of shear stress? Like for RC slabs it is 60psf (1.1*SQRT(fc'). What else do i need to check? Like in RC slabs for the moment we provide reinforcement, but how to deal with glass?


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jazakAllah. Yes we have demanded the specs from manufacturer. Do i need to change/edit stiffness modifiers for glass? why and how? I have never played with these in any structure before, but now i have been asked to do so ad i have no idea how.

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Modifiers are related to cracking and loss of stiffness before reaching ultimate limit state (ULS) which happens in concrete. You wouldn't provide stiffness modifiers for steel or glass as there is no cracking before and change in stiffness. Cracking of glass is failure.


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For the steel canopies with the glass on the top you have to consider few things for the design. As others mentioned you the strength of the glass can be and should be provided by the manufacturers according to your spans and wind load requirements. There is a small software for the glass thickness check saflex which is available for free on net check that to get approximate glass thickness you can put in your initial assumptions for the glass like tampered double glazed with air space  with assumed size etc . This glass thickness can be used for the dead weight calculation to be used in calcs. 

Now for modelling you can model the frame and assign the area shell element with weightless properties. And put your dead loads for glass as well as the framing of the glass with will be provided on the steel to hold the glass panels as oneway loadings on the area elements. Put your live loads mainly for the cleaning purpose on the main steel members as point loads take impact load and apply as pressure load point load on the area element as the glass is tested against the impact of any thing falling on it from a height check the codes there is a value I don't remember now but you can check its steel ball impact test. Also apply the canopy wind loadings you can calculate that from ASCE-7-10 or the check internet for the Australian code it has a special clause for canopy wind laodings. 

Once these loadings are done make you appropriate load combinations and complete your analysis. No need to change any modifiers for steel structure. 

You can check your shear and axial values all along the beams to calculate the connections required for holding the glasses. There area special pressure plates and tobbler coneectors normally supplied by the glass suppler and they are enough for holding the things but still you can verify the small screws provided by the details. 

Please excuse as the dicription is soo long but I hope this will help you. 

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On 2016-12-16 at 8:59 PM, Yasir Saleem said:

Sir Umer sorry for replying so late as I just saw this today and didn't realize that it's a year old.. 

Thanks for your informative reply @Yasir Saleem. You are more than welcome to contribute to topics irrespective of their posted dates.

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