huangyhg

lintel - supporting 12" masonry
i have a school with a fine arts building. it has an opening for the stage of 45鈥? and i need to design the lintel. the cmu above is a 44ft and i feel i can not do triangle loading due to the need of control joints. also, it is supporting a fly-loft and roof, because i can not do the triangle loading (dl = 4000plf, ll = 1200plf).
i have tried fixing column to help with deflection, but for a lintel of this length i am having very heavy members.
is there any suggestions on a better approach for this long of a span?

fire the architect!! just kidding, sort of.
is there depth for a steel truss? perhaps you could design a reinforced cmu beam?
my immediate thoughts mirror pmr perfectly. i would think you need beam action out of that masonry.
regards,
ys
b.eng (carleton)
working in new zealand, thinking of my snow covered home...
take a hard look at whether you really need the "cookbook" guidlines for control joints. if you can justify eliminating the need for the control joints, it would solve many problems.
i assume you are in the interior, where you lose some of the factors creating the normal requirements, which usually are used for both interior and exterior applications in the interest of simplicity.
you would still have the curing and carcbonation shrinkage of the masonry units (at least 75% occurs before laying) and the mortar shrinkage. you may be able to eliminate the moisture shrinkage and the effects of temperature, which can be major items for an exterior wall, but may not exist for your specific situation.
the lintel itself is interior, but half way up the wall it becomes exterior. while possible, i think i can get a deep masonry lintel work, i am hesitant with such a large span. i have never developed one for that type of span or loading, but i guess there is a first for everything.
i have plenty of room to develope what i need, and was indeed looking to do the truss. however, in general, we try to keep a 3/8" deflection on masonry, but for this long of a span i thing 3/4" to 1" should be adequate. i have been informed that as deflection occurs during loading masons adjust there grout spacing.
thanks for everyones input.
for a span of this length the 3/8" is not practical and you really need to look at more like l/500 to l/600 as a limit.
if you have jointed masonry then the larger deflections will not be a problem as long as your joint widths are sufficient to take it.
the 3/8" limit is more for when you have arching and the masonry above a certain level will stay where it is and the masonry below this will deflect with the lintel. a crack would then develop at the interface between the 2. in the arching case the actual deflection is important and is not related to the span.
in your case, if you use jointed masonry, then all the wall will move as one. it is then the rotation of the panels and the curvature of the beam that is important. both of these are related to span.
have you tried a deep concrete beam or concrete-encased steel beam?
i had a project several years ago with the same scenario but a 50' span. i don't have my calcs anymore but a quick stab at it give me about 2.5 kips/ft and i had an 11' deep truss w/ w8x58 chords. the top chord was near the auditorium roof and was braced by the roof deck. the truss is firred out with light gauge framing and gyp, as is the rest of the opening below the truss.
i work for an a&e firm, we don't design them with the fly loft anymore. i guess there have been advances in rigging that have (thankfully) allowed us to get away from the fly loft.
you should consider control joints 12 or 16 inches back from the support opening and design the wall as a deep beam. with the depth, you should be able to consider triangular loading, but check to see if you that you have sufficient resistance for arching action if you do... also problematic with control joint at 12-16". what do you have to provide lateral support for the top of the wall? can you use an inverted 't' beam with hangers to structure above?
dik
a steel truss with large depth, (l/8), will be cost effective using standard shapes. rest the masonry on the bottom chord.
i am envisioning an auditorium in my mind, and i doubt there is enough masonry on each side of the 45' opening to resist the thrust generated by arch action over the opening. so, i would design for the full load, as you originally proposed. a steel truss, or maybe a steel plate girder, could be used, if a standard wide flange does not work. i doubt a cmu beam will work across this span, but as was mentioned earlier, perhaps a concrete beam (or precast?).
daveatkins

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