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# Composite Steel Encased Column

## Composite Steel Encased Column

(OP)
1) How to calculate encased steel column modulus of elasticity?

2) How to model this in ETABS and what modifiers to be applied to check axia shortening of this section?

3) What is the limit for gravity only sway of the building? H/1000?

### RE: Composite Steel Encased Column

(OP)

Dear all, Please see the attached image. This formula is suggested to calculate EI of composite section. In my case I have

concrete section of 600x1700mm
4 corner dia 25mm bars assumed
W shape of 1000mm depth and 400mm flange widths with 50mm thickness for flange & web

I calculated the EI eff = 4.86644E+15 N.mmÂ²

Now I want to model in ETABS only the concrete section (original 600x1700)
but with increased EI...For this I can either increase I modifier in ETABS or define a new material for this specific section with E value

I just want to check the deflections (elastic). I dont need ETABS to design this way...

Now my question is: If I want to change only E in material property of this concrete section...

I will be divide EI eff (calculated above)

by I (moment of inertia of concrete section to be modelled in ETABS..and that is

600x1700mm conc column)

So I will get E value which will be used in ETABS material properties...
As you can see in excel sheet snapshot that new value of E is about 50% (0.5) of Ec of concrete...

I mean my Ec was equal to 39323 MPa and my new value for E effective = 19810 MPa only.

Does this makes sense? Is the concept valid? I heard that you need to put E = 2 times the Ec to model a composite column in ETABS..but in my case I have only 0.5 factor...

Thanks
By the way:

Is, As etc are parameters for Steel Section

Ic, Ac etc are parameters for Net Concrete Section

Ig, Ag etc are parameters for Gross Concrete Section

Icr, Acr etc are parameters for Steel Rebar

Ec = modulus of elasticity for concrete = 4700 under the root f prime c
Es = modulus of elasticity for steel = 200 000 MPa

### RE: Composite Steel Encased Column

This software might help you. You can create a section of an encased steel member and have its properties calculated by choosing Analyze->Inertia Data->Analyze. A base material for the calculation (for instance steel) can be set in the calculation parameters using Analysis->Calculation Parameters menu item.

Regards.

John

### RE: Composite Steel Encased Column

H/1000 is the dead load drift limit that I use. You need to be careful with ETABS or any other finite element software because generally the meshing will be too coarse to get proper/correct load distribution to your columns and you should run a staged analysis to account for some of the dead load drift getting built out.

If the building does want to lean under dead loads because of sloping columns, transfer slabs or uneven stress distribution to the vertical elements than you will need to make sure that you properly account for additional deflection that occurs from creep.

### RE: Composite Steel Encased Column

Hi ranawaseem,
you can calculate the stiffness of the composite section in the same way you calculate the stiffness of a reinforced section, the only difference is that in the reinforced section you don't take into account the moment of inertia of every bar because it doesn't give an appreciable contribution to the total stiffness, whereas with a stell section it does.
If you do not remember how to do it, just tell me and I will check if I have any document about it, otherwise I will write how to do it.
Cheers

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