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Re: L in deflection criteria
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- Subject: Re: L in deflection criteria
- From: "Bill Sherman" <SHERMANWC(--nospam--at)cdm.com>
- Date: 17 Jul 98 19:58:24 -0400
Roger Turk wrote: >..."L" should be the distance between points of inflection. >With regard to a cantilever..."L" equal to twice the cantilever length would >be a conservative assumption ... I do not agree with this. I think that "L" should simply be the length of the member relative to its support points, i.e. for a cantilever, the length from support point to the free end of the member; or for non-cantilevered members, the length between support points. The concern is maximum total deflection relative to the support point where "zero" relative deflection is assumed - why should the type of beam curvature matter? But that raises another question: what if you have a secondary framing member supported by a primary framing member? The total deflection relative to column supports would be the sum of the member deflections (but I would only check each individual span per the code). >The requirements of the UBC (Table 16-D, 1997 UBC) have, for non-cambered >construction, been in error for as many years as I have been an engineer... >In essence, the loading for the deflection limitation for L/360 should be not >less than LL+K(DL), where K is the percent of initial DL deflection that >represents the creep that would occur after the brittle finish is applied. >In non-cambered construction, the loading for the L/240 limitation should be >not less than LL+(1+K)DL. You raise a good point here - the code formulas do appear to be in error.
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