huangyhg

simply supported vs. 2 span continuous beam for crane beam
i was looking for opinions regarding the support conditions for an underhung crane beam. simply supported single span vs. continuous 2 span. (20 ft vs. 40 ft)
this would be with respect to alignment issues and unbraced length.
thanks
with the crane running under the rail, you can provide support for the compression flange for a simply sup arrangement and i'd have thought that all economy is lost when you have to choose a flange robust enough to cope with a compression flange length the kind that you will end up with in a continuous arrangment.
alten
the longer unbraced length reduces your allowable stress. in this case the selected beam for the simply supported condition is also adequate for the continuous beam; with a reduced allowable stress.
there was an aisc article that recommended the simply supported condition without discussion regarding the recommendation.
alignment is also a concern; although in either case i believe the cmaa guidelines are more strict.
not sure about the safety regulations, but i would imagine that a double span would require at least 2 construction cranes for installation, and the single span would only require 1.
this may more than offset any savings.
take a look at aisc design guide 7. they discuss simple span vs. continuous.
mrengineer
for those of us not living in the us could you post the salient points of the discussion?
thanks
here's the listed advantages of simple spans:
a. easier to design.
b. unaffected by differential settlements.
c. more easily replaced if damaged.
d. more easily reinforced.
here's the advantages of continuous beams:
a. reduced deflections.
b. reduced end rotations and movements
c. lighter shapes.
but it also includes the provision that fatigue may be much more significant with continuous steel runway beams in that they have more "parts" that are in tension.
thanks jae
from what i've seen simple spans are certainly a lot more common.
i always design/detail the runway beams for top running cranes as simple span. underhung cranes are another animal. to quote aisc design guide 7, "the splice should allow for a smooth running crane as the wheels transfer from one beam to the next". they show a typical splice detail for underhung runway beams, with field welded web splice plates (bolted for erection only, remove bolts after welding to avoid interference with crane wheels), and full pen weld at bottom flange. the only thing i take exception with is the idea of having only the bottom flange full pen welded. my thought is that once you have introduced some degree of continuity by welding the bottom flange, you may end up with distress in the web connection. therefore i call for weld at both top and bottom flanges. i design as simple span, using appropriate equations for lateral torsional buckling. have had no problems whatsoever to my knowledge.

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