Need a book? Engineering books recommendations...
RE: Lateral Stability of a Box Beam ?[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: <seaint(--nospam--at)seaint.org>
- Subject: RE: Lateral Stability of a Box Beam ?
- From: "Conrad Harrison" <sch.tectonic(--nospam--at)bigpond.com>
- Date: Mon, 19 Jan 2009 12:48:24 +1030
Thanks Paul, What you say makes sense. But I have to think about it, something seems odd. Coldformed C/Z are braced by bridging, normally a channel section fastened to the web above and below the neutral axis. The bridging provides both lateral and torsional restraint: that is Ly, and Lz are reduced. The two beams when they kick laterally: could both kick towards each other, away from each other or in the same direction. If the beams attempt to twist they place the bridging in bending. Ignoring the complexities of the stress gradient from compression to tension which causes the twisting. Then simply have elements in compression (F=M/d), which will buckle about their weak axis. Which suggests either a battened column (steel structures code) or spaced column (timber structures code). A quick look at timber structures code (AS1720) indicates a modified slenderness, or modified effective length for the spaced column. Also what is the difference between a floor joist crossing the members, or a floor deck? Lateral displacement of one beam will cause displacement of the others crossed by the joist or deck. So if we only have two beams do we ignore the restraint provided by the joist? Or the chords of a truss, effective length between webs. For parallel chord truss, top chord effectively bridged against bottom chord. The whole truss can buckle out of plane but the chords are braced in one direction. For a bridge two trusses braced against each other. The CSI steel designers manual has an article on U-shaped bridges, and the effectiveness of the horizontal framing (bottom) to provide restraint to top chord of vertical trusses. So it just seems like it needs closer inspection. As for a box section. The boxed section has higher torsional resistance, and generally higher resistance: no change to effective lengths with that analogy so benefit probably not as great. The plywood would be forming the flanges, which is unusual the plywood normally forms the web. And again in simple terms force in flange is F=M/d: flange needs appropriate area and appropriate spacing between fasteners to reduce buckling of compression flange between fasteners. Checking fasteners for shear flow will probably provide for such. See Gere and Timoshenko for shear flow and examples for timber boxed sections. Also timber structures code (AS1702) gives formula for fastening of built up sections. I guess NDS doesn't. In any case boxing up the beams, some what makes the beams redundant, since they become the webs of a single section, resisting shear, and the plywood the flange taking the normal stresses. The built up beam now having to carry twice the load: the overall benefit is probably small. Refer ASCE guide to bracing cold-formed steel structures which contains George Winters original research paper and his use of corrugated cardboard to provide lateral restraint. Steel structures codes seem to have rules requiring the restraining force to be between 2% and 2.5% of the flange force. This is a simplification to avoid calculating the stiffness, the ASCE guide has theory and examples calculating stiffness. Not really about two tied beams. The bridging reduces the half-wavelength of the buckle in the thin plate. The theory mainly assumes the buckle is sinusoidal in nature, a half sine wave is the length of the buckle. Two plates bridged together have smaller buckles between each bridging member, and therefore overall potentially less kick sideways. Needs closer inspection. Regards Conrad Harrison B.Tech (mfg & mech), MIIE, gradTIEAust mailto:sch.tectonic(--nospam--at)bigpond.com Adelaide South Australia -----Original Message----- From: Paul Ransom [mailto:ad026(--nospam--at)hwcn.org] Sent: Sunday, 18 January 2009 06:54 To: seaint(--nospam--at)seaint.org Subject: Re: Lateral Stability of a Box Beam ? If the 2 beams are identical with identical loading, there is no benefit to simple ties. Unless there is some shear resistance mechanism between the brace points, the unbraced length for lateral buckling is not reduced and the ladder simply behaves like 2 tied beams (e.g. Iy1 + Iy2) and they just displace in unison. Regards Paul -- Paul Ransom, P.Eng. ph 905 639-9628 fax 905 639-3866 ad026(--nospam--at)hwcn.org ******* ****** ******* ******** ******* ******* ******* *** * Read list FAQ at: http://www.seaint.org/list_FAQ.asp * * This email was sent to you via Structural Engineers * Association of Southern California (SEAOSC) server. To * subscribe (no fee) or UnSubscribe, please go to: * * http://www.seaint.org/sealist1.asp * * Questions to seaint-ad(--nospam--at)seaint.org. Remember, any email you * send to the list is public domain and may be re-posted * without your permission. Make sure you visit our web * site at: http://www.seaint.org ******* ****** ****** ****** ******* ****** ****** ********
- Re: Lateral Stability of a Box Beam ?
- From: Paul Ransom
- Re: Lateral Stability of a Box Beam ?
- Prev by Subject: Re: Lateral Stability of a Box Beam ?
- Next by Subject: Re: Lateral Stability of a Box Beam ?
- Previous by thread: Re: Lateral Stability of a Box Beam ?
- Next by thread: Re: Lateral Stability of a Box Beam ?