huangyhg

arched joist design
i've got a situation where i am designing an arched front porch roof for a home. the arched porch roof joists will be cut out from rectangular lvl material. the radius is fairly large for the span; a 12' radius for a 6' wide span.
i've looked through many structural books and only find equations for curved glu-lams and they are for fairly long spans w/ large radii. the eqns i've seen assume the grain is perpindicular to the load as in a glu lam, but w/ an lvl, the grain/layers are parallel to the load.
doesn't the bottom of the arched joist change from tension to compresion depending where you are at on the joist (tension at the top and compression towards the ends)? what is the best way to analyze and size these joists?
thanks in advance,
a gothic style arch... have you considered using 3/4" or 1/2" d.fir strips laminated, not using lvl segments? depending on the loads, you may want 6-10 laminations glued using a quality waterproof adhesive.
dik
douglas-fir with 3/4" laminations can be laid up with a 12'-4" radous. as dik indicated by going to smaller laminations you may be able to achieve a lower radius. with souther pine 5/8" lamination you should be able to get down to about a 9' radius.
i would first analyze the arch following the methods given in the aitc manual or by using a computer program. then i would contact the lvl manufacturer for advice about using the lvl material.
the arch will experience axial load, bending moment and shear.
how that effects the lvl i am not sure. probably what you will find is that if you cut a curve into an lvl, that it will viod the warranty.
however you might find the lvl works great. with glulam arches a major concern is radial tension or compression. wood has vert low perpendicular to the grain allowable tension values.
rarswc...
where did you get information on radii for various species? i've designed many 'archrib' type frames and have set up spreadsheets for radial stresses and combined stresses and use risa for analysis. also set up spreadsheets for the various snow and wind loading conditions...
dik
just to clarify: are you suggesting actually curving/bending the 3/4" d. fir strips, so i would be creating my own glulam? or, are you suggesting cutting the radius out of 3/4" material and gluing them together. it seems to me the second option is just creating my own lvl, is it not?
thanks?
depending on your end conditions maybe your joist can be the classical compression only element.
what size lvl are you considering - how tall?? say the height at the bearing is only 7'' but the middle is still 11 7/8.
just analyze as a 1 3/4 x 7'' lvl and be conservative - bring it in at 75% or 80%. check shear at ends
making your own glulam.
dik

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