Building Columns Wider At The Top Than The Bottom?

[rulljs]

I've always wondered why when I see a new building under construction (usually a prefabricated type building) the columns are wider at the top than at the base.

Just curious.

Thanks.

 

[youngstructural]

This has everything to do with supplying required strength. If you match your section strength to section demand along the length on a PINNED base frame, you're going to get the heavy at top, small at bottom profile. There are more complications to the selection of a profile (such as deflection and shear), however that's the gist of it.

The buildings you're seeing are likely a kit, pre-fab or mass produced metal shed "standard" design.

Regards,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[Lion06]

youngstructural is right. They are designed as pinned at the base and pinned at the midspan of the beams, so there is a ton of moment getting dumped into the top of the column....... hence the larger section there.

 

[Lion06]

The beams likely taper from large at the column to much more shallow at midspan as well.

 

[rulljs]

Thanks for the quick replies, I was figuring it had to do with the buildings being designed with pinned connections at the base but didn't know if that was the main reason, which from what it sounds like, it is.

StructuralEIT, interesting to know that they are pinned at mid-span of the beam also, I'll have to take a closer look next time I pass one up.

 

[youngstructural]

They aren't necessarily pinned at the apex of the rafters (mid-span of the "beam"). Although that's possible, it's not often done. It was a common assumption in the days before computer analysis.

Under "up to date" analysis, the peak moment at the apex is typically less than the knee moment. That means that you can use a smaller section, or taper your rafters similarly to your columns.

Tapering has more to do with providing required strength independant of your structural assumption.

Cheers,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[msquared48]

What StructuralEIT is descibing is a classic three-hinged arch design. There is nothing new under the sun.

Mike McCann
MMC Engineering

 

[youngstructural]

Sorry Mike; Greatly respect your posts and hope to get your help and advice in future, but a three hinged portal frame is not a three hinges arch. They are related, but not the same.

Cheers,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[hokie66]

May be a generational thing. Technically, YS is correct, because of the shape. But like Mike, I have also called a portal with hinge at ridge a three hinged arch for years, knowing all the time that an arch is curved. The key thing is that both are determinate, and as YS pointed out, that was very important in the days before computer analysis. Not only did we assume the ridge was pinned, in many cases we detailed it that way. There are a lot of old hangars around with trussed columns and rafters and a true pin at the ridge.

 

[msquared48]

Probably playing semantics here guys, but the same principle is involved in the tudor style, three hinge glulam arches used in many churches in the 50's and 60's.

Although useless now in that I have not designed one since leaving school almost 40 years ago, the principle is still the same. These structures are very simple and can be solved graphically if so desired, similar to using a Maxwell diagram to solve determinate trusses.

I guess times change with nomenclature, but some things remain the same. Thanks for the input. I'll update my terminology. Cheers.



Mike McCann
MMC Engineering