Required Section Modulus 1

[jbatencio]

This is a very green question, but I'm going in circles here.

It's necessary to "beef up" a column supporting a cantilevered jib crane. The jib crane is capable of rotating a full 180 degrees, and the column supporting it deflects badly in the weak axis. I know I need to increase the section modulus in the weak axis in order to prevent this deflection. My question is -how can I calculate the required section modulus needed to do this?

Am I way off here?

Thanks for the help!

 

[SlideRuleEra]

Since the problem is only related to the minor axis deflection, I assume deflection about the major axis is acceptable. If so, perhaps you can increase minor axis section modulus to equal major axis section modulus and have a more or less symmetrical column. This way you avoid having to make assumptions about what section modulus might, or might not be acceptable and go with what is working now.

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[rb1957]

i agree ... it doesn't design it to any requirement, but it sizes it to something that seems to be working. the question being that whilst the strong axis seems to be good for day-to-day working loads, is it acceptable to the required loads ?

a 2nd question, i think it's unlikely but could the load/moment be higher about the weak axis ?

Quando Omni Flunkus Moritati

 

[Pleebro]

Hi;

If you have deflection limit, you can work it backwards. Max deflection of cantilever beam (UDL) is wl^4/8EI. once you get the moment of inertia I, you can get S by dividing it by the distance from the NA to the furthest point of the shape.

If you are talking about how to find calculate the section modulus of irregular shape, I suggest to use AutoCAD.

I hope this helps

 

[BAretired]

The effect you are trying to fix is deflection, so the property you should be considering is moment of inertia, not section modulus.

Deflection at the end of the jib crane is a result of rotation of the column and deflection of the jib. Measure the deflection of the jib under a trial load (not necessarily the maximum crane rating). Calculate the cantilever deflection of the jib beam and subtract from the measured amount. That will be deflection due to column rotation, δ_measured . Decide how much deflection you consider acceptable.

Column rotation is directly proportional to load P and moment of inertia of column, I_c, so you must increase I_c (existing) by a factor (P_max/P_trial)*(δ_measured)/δ_acceptable .


BA

 

[structSU10]

Though this doesnt answer your question specifically, if you need guidance on jib cranes attached to columns, AISC has a paper from AISC journal called 'Design Concepts for jib Cranes' by Fisher, James M.; Thomas, Steven J. which was written in 2002. It could go into some issues you didnt address. There are some extra loads to consider, and certainly column deformation, along with resulting P-delta effects if it is also a building column, are a big issue.

 

[paddingtongreen]

I endorse BA,s answer.
Alarm bells went off in my head when the first thing I noticed was the reference to section modulus instead of moment of inertia.

Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin