huangyhg

continuous steel beams vs. cantilever beams
i'm a young engineer with 2 years experience and am working for an architect whose father is a retired civil engineer who checks my calculations.
my question is: we have a building with precast hollow-core floor slab bearing on a steel beam. there are five approx. equal beam spans. i designed the beams as cantilever beams using aisc cantilevered beam diagrams(lrfd or asd, same diagrams, same factors) to find the moments, reactions, and cant. dimensions. the beam splices then carry only shear.
my bosses dad wants to design the beams as a continuous beam (with cantilevers) over the five spans with moment carrying splices due to the the fact that he patterned the live load to find the maximum controlling moment.
he is a civil engineer who has only designed bridges, never buildings.
i guess my question is do i need to pattern the live load if i use the cantilever beam charts? he and i come up with significantly different beam weights. and is it normal or general practice to use a moment carrying splices in this sort of situation? in my short career i have only seen shear splices in buildings.
any help is appreciated. i really like this forum. thanks! ben
jae (visitor)13 apr 00 0:30
per all building codes in the usa you are required to pattern the live load on a floor. with simple spans, alternating live loads obviously does nothing to to the shears and moments. once you cantilever, whether moment spliced or shear spliced, you have to consider alternating. this includes odd spans loaded, even spans loaded, and three sets of adjacent spans loaded.
if you choose to only shear splice, you are "forcing" a hinge in the continuous beam at a particular point on the span (usually near the uniform load inflection point)and affecting the resulting moments across the spans. if you choose to moment splice, the moments will behave as though the beam is a purely continuous, with no splices, and slightly different moments will result. either way, your type of splice will affect the moments accordingly and result in an appropriate design. the moment spliced system will be a bit stiffer.
in either case, using cantilever tables is inappropriate as these do not normally consider the alternating load effect.
most buildings do not utilize moment splices for continuous gravity beams. i have never used a moment splice in a building. the only moment connections i've used have been for frame systems resisting lateral loads.
another thing to consider is the total length of your individual spliced sections. fabricators order beams in 5 foot increments (i.e. 30', 35', 40' sections, etc.) and so the placement of the splice sometimes is directed by the resulting total length of the beam. if your beam ends up needing 41 feet, the fabricator must order a 45' section, wasting 4 feet of beam. this adds somewhat to cost. in large warehouses, we typically devise a splice layout that attempts to minimize waste.
hope this helps.
your building is probably built by now, but here's my two cents: the splices would definitely not be moment connections.
only use the csntilevered beam system (aisc green book pg 2-311) for roof framing. the beams would run continuous over the tops of the columns, and the live load, which is snow, would be equal on all spans since it can't snow on alternating bays.

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