Braced Frame Hand Analysis
Braced Frame Hand Analysis
(OP)
Hi all,
I'm doing hand calculations for a one storey braced building. I need to check the deflection of the building. To do so I've done what I thought was a simple hand check (attached).
However, I did a very quick check with STAAD just to make sure my hand calc was accurate but the deflection of the frame in STAAD comes out as exactly twice the deflection that I have calculated. I'm sure there's something very simple wrong with my hand calc but I can't see it myself. I don't want to use the results of STAAD without understanding them.
I would appreciate if one of you could have a quick look and let me know if you can see the issue with my hand calc.
Also, I see that the units for area are incorrect - supposed to be cm2!
Thank you very much
I'm doing hand calculations for a one storey braced building. I need to check the deflection of the building. To do so I've done what I thought was a simple hand check (attached).
However, I did a very quick check with STAAD just to make sure my hand calc was accurate but the deflection of the frame in STAAD comes out as exactly twice the deflection that I have calculated. I'm sure there's something very simple wrong with my hand calc but I can't see it myself. I don't want to use the results of STAAD without understanding them.
I would appreciate if one of you could have a quick look and let me know if you can see the issue with my hand calc.
Also, I see that the units for area are incorrect - supposed to be cm2!
Thank you very much






RE: Braced Frame Hand Analysis
RE: Braced Frame Hand Analysis
I'm just here to say that I'm happy to see someone who doesn't just plug numbers into their simulator of choice and move on to the next. Checking your work and understanding why, closing the loop so to speak, seems to be an instinct that becomes ever more rare as the years pass.
RE: Braced Frame Hand Analysis
RE: Braced Frame Hand Analysis
RE: Braced Frame Hand Analysis
I plugged the numbers into the top formula and I got the correct answer (the STAAD answer)!
Has anyone got a derivation for the formula by any chance/the reference for the book that the formula comes from? How is the Lbrace^3/Lhoriz^2 arrived at?
RE: Braced Frame Hand Analysis
RE: Braced Frame Hand Analysis
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Braced Frame Hand Analysis
RE: Braced Frame Hand Analysis
Applying the virtual work principle, the horizontal displacement would be obtained by equating work done by a unit virtual force on the final displacement of the structure to the internal work done by the internal forces on the member virtual internal displacements:
Applying a virtual horizontal force of 1.0kN at the top node, the horizontal displacement would be the following, assuming a constant area of 40.3cm2:
RE: Braced Frame Hand Analysis
1) Turn off P-Delta.
2) Make sure you are not using any stiffness adjustments per AISC's "Direct Analysis Method".
3) If STAAD allows it, turn off shear deformation.
4) Don't just validate your deflections. Make sure to also validate the force distribution and joint reactions that you have assumed. That might lead you to a difference between your model and your hand calc assumptions.
RE: Braced Frame Hand Analysis
RE: Braced Frame Hand Analysis
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Braced Frame Hand Analysis
RE: Braced Frame Hand Analysis
Sorry. My bad.
RE: Braced Frame Hand Analysis
I now understand where the answer is coming from.
RE: Braced Frame Hand Analysis
1) Calculate the "cantilever" component of deflection at a given floor using the moment of inertia of the chords.
2) Calculate the shear component of deflection a a given floor using the equations cited.
3) Add 1 and 2 to get the total drift.
I think that was a more intuitive way for folks to investigate multi-story frames back in the day before computer modeling became ubiquitous. For low rise frames, the cantilever (column) contribution to drift is often negligible, as evidenced here.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.