huangyhg

wide span portal frame.
been asked to design a 60m wide clear span portal frame (eaves height 7.5m). the maximum ive ever done so far is about 35m clear span. has anyone designed such a wide span portal using standard available sections? if so designed by plastic or elastic analysis? any further comments ideas would be appreciated.

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not a simple task, but can be done. is the roof to be flat, or tappered? what is the use of this building? need more details.
i remains in linear-elastic thinking. others may have better view on plastic design, which could be beneficial for long span structures.
by the way, i afraid the "standard shapes" is not readily available for this job, you may need build-up shapes.
you are going to need big sections and some decent moment splices. it may be worth while tapering the beams or even using castellated section.
plastic design could be beneficial to you, especially if you can develop fixity at the bases. if it is architecturally allowable, i would try to put some knee braces at the top corners. i have found those to be helpful on wide, short frames.
truss the end columns to increase the end moments and reduce deflection.
the building will be used to house fertilizer. welded beams are readily available and these come at about 1200mm deep sections pretty heavy. im thinking of haunching at the knee and at the ridge of course. the bay spacings i will put at 9.0m centres. im more concerned about the knee connection. all comments would be greatly appreciated.
also any good software for designing these particular types of structures?
if you have access to pre-engineered building supplier's website, you may gain some ideas on the most economical tappered-hunched frame for your task.
in general, i vision a outward taper column (interior straight), and a hunch beam with the largest dimension at the beam-column joint, and gradually reduces depth to the middle. if lateral load is not of great concern, could it be a 3 hinged structure? you may want to throw some numbers to find out the advantages and disadvantages vs a rigid frame.
the hunch area would consist stiffeners and huge web panels. pay attention to buckling and warping. you can model the hunch using finite element program to make sure the stresses are confortably within allowable. i am not aware of any commericial program can handle this type of analysis. try to see can you get some advices from pre-engineered manufacturer, they deal with this type of construction in a daily basis, but not to this size.
no problem! i've designed 55m portals with standard welded beam sections.
(building is still standing after 1 year!!)

kslee thanks
what would be the advantage of a 3 hinged portal ? and what would the ridge connection be like as a pin?
nicam thanks !
did you design plastic or limit state design? also did you exceed deflection limits?
by provide a hinge in the mid span, it becomes a static determinate frame, which can be calculated by hand. i have tables to minimize your trouble in determining reactions for varies types of loading conditions, i will upload on monday.

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