Hogging region stiffness on composite frame analysis

[Italo01]

Hello,

I'll design a two storey building that must be unbraced on both directions (architrctural reasons).Because of this restriction, i'll place beam with strong axis on both directions on the perimeter and provide moment connections. Since its a low-rise building, i don't think that wind will be a great problem but i'm trying to be very careful with the analysis.

I looked a very nice method called Wind moment method by SCI(UK) but my building does not satisfy some assumptions, so i'm using a software called Strap.

My problem is the following:
I contacted the software company and the said that at the stage of frame analysis, the software does not consider the stiffness difference between the sagging and hogging regions of the composite beam, considering a uniform stiffness throughout the beam. At the design stage, the software considers this difference to calculate the beam displacements, but using the bending moments obtained from the previous simplified frame analysis.

Since for wind load the moments on the beams will depend on the stiffness(increasing for a increase in stiffness), if a consider the sagging stiffness for the whole beam, i will obtain smaller sway deflections, greater bending moments on the beams and smaller on the column bases.

In order to accurately predict the stiffness, i would have to break the beams on the zero moment points and change stiffness from the hogging moment manually, but this will br a iteractive process, since when i change the stiffness the bending moments will change, moving the zero moment points. I expected the software to do this iteractions and got a bit frustrated with their answer. They said that nobody does this, as if i'm being irrational.

Am i overreacting?

Would be appopriate to use the uniform stiffess for the beam, reducing a little bit my allowable sway deflection and counting on the plastic LBT of plastic analysis to just accept the bending moments obtained?

Thank you in advance.

 

[human909]

Wind moment method by SCI(UK)

An interesting approach...

But it seems you are wringing your hands over nothing given that in the Wimd moment method assumes that the frame is statically determinant so the stiffness doesn't matter. Its right there in the SCI publication.

Regarding you software problem. Find a software that suits your needs or use hand methods. The wind moment method seems to be made for hand calcs.

 

[Italo01]

Human909, the wind moment method is not adequate for this situation because of these necessary conditions that are not satisfied:

- The building must be braced on the other direction(In this case the building is unbraced in both directions);

- The bottom story height is less tham 4,50m;

-The largest bay width/bottom storey height is greater than 2,67;

With respect to the software, are you aware of any siftware that would do the analysis in this way. The technical support said that nobody does that, which for me is a weak response, but maybe there's a reason why nobody does.

Would be appopriate to use the uniform stiffess for the beam, reducing a little bit my allowable sway deflection and counting on the plastic LBT of plastic analysis to just accept the bending moments obtained?

With respect to this, what do you think?
I was clear?

 

[human909]

Ok i misread the bit where you chose not to use it.

 

[Settingsun]

Am I overreacting?

Is your goal to finish the design, or to have calculations with no loose ends? Eg for an external reviewer.

I don't think there would be too many iterations if you really wanted to, just need to identify a few critical cases and run separate models. I'm assuming the building isn't large given you mentioned moment frames on the perimeter. But I'm not familiar with your software so don't know how much manual effort is involved in the design stage. I gather the software does the member design (?) so just a few hours iterating the models and comparing the results to get worst case for each member and connection (? again)

For the lower bound theorem, larger connection moments are probably a good thing. That's a point of lower ductility that could compromise the LBT. If the second order effects are fairly small, I'd live with the under-prediction but, if second order isn't small, I'd stifeen it up.

Or, do it in two models: sagging stiffness in one; and hogging in the other. This bounds the problem at the expense of some conservatism. But you may find the difference is acceptably small. Or you may decide to pick the middle ground.

 

[Italo01]

Is your goal to finish the design, or to have calculations with no loose ends? Eg for an external reviewer.

To finish the design.

Yeah, i think i'm gonna do the interactions to get the accurate results.

Thank you.