tapered Column design

[Hasan_123]

I need to design this inverted tapered column with narrow section at the bottom. Since I am a recent graduate, I don't have much experience. Pls guide me to design this. TIA

 

[JoshPlumSE]

There is a design guide that AISC has for Tapered members. This is a pretty good place to start. I've actually only read the 1st edition. It appears there's a newer edition now.

Design Guide 25: Frame Design Using Nonprismatic Members (Second Ed.) | American Institute of Steel Construction

www.aisc.org

In the 1st edition, there are some benchmark problems in Appendix C. There is an example and citation for the "Method of Successive Approximations" for the elastic buckling value of a stepped or tapered column. If that appendix still exists in the 2nd edition (and it should) then this would give a good method for calculating the axial capacity of such a member.

 

[271828]

What material is this?

What are the boundary conditions and loading?

 

[Hasan_123]

What material is this?

What are the boundary conditions and loading?

The whole thing is concrete with a metal cover at the top portion

 

[Lomarandil]

Looks like a pretty standard column, just happens to have a bunch of eccentric dead load on one side. Design as though 9" wide.

Now detailing -- there's a little bit of detailing attention required here, particularly if the column receives any external load. But that's tomorrow's thread.

 

[JoshPlumSE]

I didn't realize the stepped member was a concrete column. Technically, the method of successive approximations is still valid for the calculation of elastic buckling. Though it would likely be a less straightforward incorporation for concrete code checks.

I think for concrete, the most important thing will be how the reinforcement transition works at this stepped area / transition. Honestly, it seems weird that (for concrete) a member would be stepped like this.... I feel like it would be more common to see a tapered depth in that are rather than an abrupt change in depth

 

[BAretired]

There is a hand method developed by Nathan Newmark, which I attach below. See p. 22 and 23 for stepped and tapered columns.

 

[Hasan_123]

I didn't realize the stepped member was a concrete column. Technically, the method of successive approximations is still valid for the calculation of elastic buckling. Though it would likely be a less straightforward incorporation for concrete code checks.

I think for concrete, the most important thing will be how the reinforcement transition works at this stepped area / transition. Honestly, it seems weird that (for concrete) a member would be stepped like this.... I feel like it would be more common to see a tapered depth in that are rather than an abrupt change in depth

it is weird. but architecture!!

 

[cliff234]

A 9" concrete column at the base? Seems too narrow for a concrete column. You are going to have a hard time installing the vertical bars and ties, and maintaining 1.5" of clear cover to the ties. What is the detail at the base? You won't be able to lap splice vertical dowels. Your detail implies that there is a steel plate at the base. Is that correct? How wide is the column? What is the factored load? My suggestion is for you to discuss this further with whoever is supervising you.

 

[271828]

It would help a lot if we knew the magnitude of the axial force, and the moment diagram shape and magnitude.

Lots of "ifs." For example:

If this will support a small area of roof, then the computed column buckling strength will probably be many times larger than the applied load. Coming up with a buckling strength refined estimate would be a waste of time. If the axial load is not small, then you would need that strength estimate.

If there is a significant moment at the bottom, the 9 in. section will control the design by a mile. If you can get that to work, then the rest of the column would likely just be a rebar detailing exercise.

If the moment is zero at the bottom and larger at the top, then...

If the load the loads are from gravity only, vs seismic, then...

If, if, if...

 

[Ron247]

As usual, I get tripped up by terminology. Tapered to me has a constant slope on one or more faces. This appears stepped and not tapered. I see several responses are for something that appears to be based on tapers.

Is your column stepped or truly tapered? Bar placement is totally different for each. Tapered can have a bar that is continuous from top to bottom on both faces where stepped can only have it on a face that is straight (vertical or sloped)

 

[KootK]

The whole thing is concrete with a metal cover at the top portion

Can you tell us more about that metal cover? Is the extra width ad the top of the column just a metal, architectural wrap or something?