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example calculation for reduced depth timber needed
hi. i have a contractor who wishes to reduce the depth of the rafters over a stairwell as he hasn't left sufficient headroom. the stairwell is 900mm wide and there are 2 rafters equally spaced which are 46 x 220 tr26. the span on plan is 4018mm and he wishes to reduce the depth to 150mm, 1168mm from the end. shear and bending are fine, i'm struggling to know how to check the deflection given the now non-uniform depth. any ideas?
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model it with a computer and let the machine do the work. to ensure that you are seeing a reasonable result i would further hand calculate considering the point where the beam begins to taper as the support point for two back to back cantilevers. split the beam at that point and treat each separately under the known load. you are then calculating the deflection of a tapered cantilever, and you should be able to simply split the difference of the two results, in ratio to their lengths. can't say i've ever seen a design procedure for this, however it feels quite rational and were it to agree to within 20% of the fem, i'd use it.
very interesting question; please post to let us know what you do!
good luck,
ys
b.eng (carleton)
working in new zealand, thinking of my snow covered home...

2009-09-09, 08:46 AM	#1
huangyhg 超级版主注册日期: 04-03 帖子: 18592 精华: 36 现金: 249466 标准币资产: 1080358888 标准币	example calculation for reduced depth timber needed example calculation for reduced depth timber needed hi. i have a contractor who wishes to reduce the depth of the rafters over a stairwell as he hasn't left sufficient headroom. the stairwell is 900mm wide and there are 2 rafters equally spaced which are 46 x 220 tr26. the span on plan is 4018mm and he wishes to reduce the depth to 150mm, 1168mm from the end. shear and bending are fine, i'm struggling to know how to check the deflection given the now non-uniform depth. any ideas? check out our whitepaper library. model it with a computer and let the machine do the work. to ensure that you are seeing a reasonable result i would further hand calculate considering the point where the beam begins to taper as the support point for two back to back cantilevers. split the beam at that point and treat each separately under the known load. you are then calculating the deflection of a tapered cantilever, and you should be able to simply split the difference of the two results, in ratio to their lengths. can't say i've ever seen a design procedure for this, however it feels quite rational and were it to agree to within 20% of the fem, i'd use it. very interesting question; please post to let us know what you do! good luck, ys b.eng (carleton) working in new zealand, thinking of my snow covered home... __________________ 借用达朗贝尔的名言：前进吧，你会得到信心! [url="http://www.dimcax.com"]几何尺寸与公差标准[/url]