Member Strength and Stability for Bending and Compression Forces 1

[JohnnnyBoy]

I'm a new graduate and having a little difficulty understanding frames under bending and compression forces.

I'm currently working on a 31' tall building, in several section there is a second floor although there are many section where there would be no intermediate bracing. The building is a steel building on piles. When i'm designing a member for compressive forces I can use the Cr for flexural buckling under axial compression. On the other hand for bending I could consider the column to act like a beam and calculate my moment resistance that way. This method would also include calculation for lateral torsional bucking because I no longer have bracing from floor joists. Now using equation 13.8.2 it talks about members resisting both bending moments and axial compression, and the equation is as follows.:

Cf/Cr+0.85*U1x*Mfx/Mrx+B*U1y*Mfy/Mry <= 1.0

So would I calculate both the way I described above and make sure the column follows the above equation and consider the beam adequate?

I'm from Canada and using the cisc Handbook of Steel construction.

Furthermore I then models the structure using Etabs, and most of the columns were outputing W18x733 sections, which is grossly too strong for this type of structure and still failing the section. I looked into it a bit and found its using a K value for the columns as something around 24. Now from my understanding for a structure like this the base is a pinned connection and the top is free for moving but moment free and would therefore have a K-value of 2.

If someone could point me in the right direction to solve this it would be greatly appreciated.

 

[JohnnnyBoy]

I should have mentioned the tributary area on this member is 24'x20' with roof loading of DL=25psf and LL=31psf. Lateral Load would be wind load for a section of 31' High by 24' Long. I also understand when your taking large structure like this into account that bracing does take a certain amount of the load as well as transfers into several other columns, not specifically all taken by this single column. How do I properly calculate this?

Much Thanks

 

[jayrod12]

how is the wall framed between the columns, how many columns along the lines?

Post a sketch of your framing plan.

A lot of error in code check when modelling are due to unbraced length problems, and release condition problems. You need to ensure your model transfers moment across joints when you intend it, and don't transfer moments when you don't.

Do you have some sort of roof diaphragm? Or bracing? Your column would typically be designed as pinned-pinned on a standard steel building.

 

[JohnnnyBoy]

I've attached a quick framing plan and detail for one of the columns. There is no bracing between the columns as there are many section of full windows therefore the unbraced length would be 31'-0". My understanding was that in steel buildings it is best to have a moment connection for the top of steel. Please correct me if I should be always designing for pin-pin connections.

Much Thanks

 

[jayrod12]

Every situation is different, but in general I personally design as pinned top and bottom if at all possible. Standard beam connection details (single or double angles, extended end plates) are typical and only transfer shear and a nominal amount of moment. A full moment connection between beam and column are significantly more expensive usually.

Your sketch has no bracing in the north-south direction, is that how it's intended? If so, then you will need some form of moment connection along the east and west walls between beams and columns, but a 31 foot tall moment frame will likely never work with reasonable column sizes, especially in the weak direction.

What kind of components and cladding wind load do you have? around 20psf I'm going to guess but I may be wrong.

 

[JohnnnyBoy]

For this case I'm using 25psf for wind loading. I can add a section of bracing on the North-South Direction in the middle sections, although they can only extend to 7'-0" up from the bottom. As well one set of bracing along a section of a wall doesn't seem to me to be able to withstand the total wind force that's produced on the building.

Also would my first statement be correct on how to calculate and ensure that the columns have the required strength they need, or am I missing something in the design method.

 

[jayrod12]

No your first statement seems correct, And if you're able to adequately brace the structure then I would be designing the columns as pinned-pinned.

For the bracing in the middle run, do you mean it has to start at 7 ft from the floor and go up to the roof? or only from floor to 7 feet and then must be open above that?

Either way, I would likely be doing that (it creates a psuedo moment frame above(below) the bracing where it frames into the column. A full 31' moment frame won't work reasonably. That could be where your crazy numbers are coming from.

For a non-corner, exterior column I get a Pf = 37.5 kips and an Mf= 72 kft. An HSS 8x8x1/2" or 10x10x3/8" would work, these are not unreasonable sizes, an equivalent W column should be reasonable as well, in the range of W8x48 or W10x45.

 

[JohnnnyBoy]

Ya I meant from 7'-0" from the ground to top of column.

Thanks a lot for your help!!