huangyhg

how to control moments
hi,
i am designing a workshop building which his 24m wide x 66m long x 18m high. it has got a crane at 11m level having a cap of 30t.
i have braced frame in long dir and moment frame in transv dir. now i am getting huge moment in trans dir due to wind. although i have considered 50% cap of crane with wind. is there any other fact which should be applied while considering wind & crane load together. and any other way to control thismoment except providing 1500mm long base plate whic huge anchor bolt size.
thanks in advance for your efforts in understanding my problem.
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shbh,
what country are you in? what code are you using?
if you model the bases as fully fixed then you are always going to get a large base moment.
i tend to model portal frame bases as partially fixed under wind load and pinned under strength state as this tends to get a better result.
you could put a column under the crane beam instead of a corbel?
csd72,
the building is in chile, and asce-7 is the code for loading.
but how u can assume fixed in strength case and pinned in wind case. it has to be either pinned ir fixed irrespective to the lrfd or asd load cases.
i have checked this model with pinned base (rafters have moment connection with columns) but deflection are too high to be acceptable.
this is a steel building and i have w30x116 below crane girder , above which this column reduces to w16x100. thus crane girder is approx 300mm eccentric to the column w30x116. this i have included by applying additional moment where ever i am applying crane load on the column.
i think csd72 is saying that using that method just gives a little closer answer to reality than "fixed" or "pinned" since neither one can be truly realized.
we do this at my firm with type ii connections with wind. we design the beam as pinned-pinned for gravity and design the beam and connection as fixed fixed for wind.
use a truss to span the short direction with rigid moment connections at both top and bottom of truss to deep column or double column laced together on one large baseplate.
the concept with type ii wind connections is a completely different situation then that with column base plates. with type ii connections, you are simply allowing the gravity load to fabricate the top and bottom angles to the correct end rotation for a simple span beam. therefore, there is no gravity end moment.
shbh:
let me give a hint here. i hope it helps.
moment is a function of stiffness.
shbh,
i can see the confusion if you are using asd under asce7 as the strength design and serviceability design are taken as the same.
if you use lrfd then the two are different loadings (ones factored the other isnt) and you can therefore justify that a connection is strong (and stiff) enough to act like a fixed connection under service loads but will yield and act more like a pinned connection at factored loads.
basically it should remain undamaged at service loads but it just has to stand up at strength loads.
to find out how much strength you need for partial fixity:
1. make up an additional load case with pinned supports and a 10kn.m noment applied at each support (in the same direction).
2. measure what the deflection is from this.
3. by principal of superposition: required partial fixity = 10kn.m x(actual deflection - allowable defl.)/(deflection from 10kn.m moments)
4. limit the fixity moment to about 50 or 60 kn.m or the fully fixed condition, whichever is the lesser.
5. design the base plate and bolts as fully fixed under this moment. put the bolts inside the flanges, but as far apart as possible to give the service load moment capacity.
re
csd72
i think this makes sense. but is it necessary to keep the bolts inside the flange, as i am having bracings at this location (bracings are near to the outer flange of the column).
swearingen: i have modelled full structure. and have plan bracings at the roof level.
yes you can put the bolts outside the flange. make sure tha base plate can take the bending under service loads (i wouldnt put any stiffeners as this would make it too stiff).

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