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Old 13th June 2012, 07:59 AM   #1
Martin Clifton
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Default Cant'r beam deflection

Tony,

I need your help on the following please

just checking a simple cant'r beam with split dead and live loads and have found that the mid span and cant'r end deflections are confusing.

The max deflections should be for span - dead and live load on span and dead load only on cant'r

Max deflections for cant'r should be - dead load only on span and dead and live load on cant'r.

i have made a few checks and it appears that whilst the deflections are tabulated in terms of say 7.81 x 10^8 /E I , the total deflections for the above load cases appear to be incorrect.

Please can you advise/check.

Many thanks Martin
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Old 13th June 2012, 08:24 AM   #2
Tony Bryer
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Default Re: Cant'r beam deflection

Is this a steel or timber beam?
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Old 13th June 2012, 10:18 AM   #3
Martin Clifton
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Default Re: Cant'r beam deflection

Tony,

Sorry, but i think i have answered my own query - if this is a steel beam then there are limits on the live load deflection - if a timber beam, then the max deflections for both span and cant'r are correct.

Live load deflections are considered separately with steel beams ??

Sorry for any confusion - not really much of a steel person - only with timber.

Many thanks for your time as always - i have used superbeam for many years with complete satisfaction.

Kind regards , Martin
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Old 20th June 2012, 04:15 AM   #4
Tony Bryer
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Default Re: Cant'r beam deflection

Yes, for steel beams the code limit has always been for live load deflection only, the assumption being that the dead load deflection can be taken account of when encasing the beam. As is explained in the manual, there are a good few cases where total deflection needs to be considered. Having said this, deflection is rarely an issue with steel beams except, perhaps, where you have chosen to use a UC as a beam for height reasons.

With timber beams deflection is more complicated in that applying a load to a beam gives a immediate deflection which then increases over time due to creep. BS5268 allows for this by restricting total deflection to 0.003L. Eurocode 3 takes account of creep by applying a factor to instantaneous deflections which is a function of the load type (dead, residential etc), service class and load duration, setting a general long term deflection limit of L/250 for beams.

I've just done a quick EC3 check on a typical domestic floor joist, 3m span, 25% dead load, 75% live. Instantaneous deflection is 8.63mm (just within the BS5268 0.003L=9mm), final deflection 11.09mm (L/250=12mm). So much the same answer arrived at in different ways.

Finally, with beams extended as cantilevers you can get some apparently odd results because creep may, depending on the loading pattern, act to reduce deflection rather than increase it.
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Old 24th September 2012, 12:42 PM   #5
asharca
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Default Re: Cant'r beam deflection

I agree with Martin Clifton and you just checking a simple cant’r beam with split dead.
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