SHEARWALL and FRAME INTERACTION 1

[gotlboys]

Dear Engineers,

I have read several references on this particular analysis but what I found to be a bit clear was in ASCE codes which states that frame shares 25% of the total base shear while the shearwall shares the remaining; 75% of the total. Some references tend to focus on stiffness method.
Can anyone please enlighten this issue?

 

[KootK]

You distribute your loads based on stiffness as usual but ensure that the moment frames can take at least 25% of the shear demand. This is a way to ensue some degree of structural redundancy.

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.

 

[JoshPlumSE]

Yes, you base the shear distribution on relative stiffness. Then once you've finished your design you go back to your frames and make sure they take at least 25% of the base shear. They may not have because they're so much more flexible than the shear walls.

Note:
I have seen an engineer merely design for the "total strength" demand and not base it on relative stiffness. But, this is wrong, wrong, wrong!

This engineer had an existing shear wall that was not going to be sufficient for the increased loads due to a retrofit. He then added in moment frames to take the additional load. Not paying any attention to the relative stiffness of the two systems. The reality is that when the earthquake hits, the shear wall was going to take almost all of the load (because it is so much stiffer). Then it's going to fail, and only after it fails are the moment frames going to take significant amount of the shear.

 

[gotlboys]

Thanks so much Kootk and JoshPlum. I appreciate that.

So it is clear that relative stiffness is the method.
Situation:
I designed a three-storey apartment bldg (L-shaped) to fulfill the requirement for graduation or thesis. One of my professors questioned me why I have so big columns with more reinforcements. He accepted the design for approval though He challenged me to try finding a way to solve this issue.
After some consultations I decided to use shear-wall on both sides but with larger width on the weaker side. I have created a spreadsheet to calculate all the column stiffness along each direction. I assumed the shear-wall as a column and have its sizes (w being so large compared its thickness) inputted to the spreadsheet calculation and found out that shear distribution to columns has significantly reduced compared to my previous calculation without shear-wall.

Am I doing the right way in this analysis? I am willing to take corrections.

 

[jayrod12]

Yes, if you provide a shear wall along a line of columns, it will attract the majority (I would likely design it for all) of the load. In an ideal world, you size and place your shearwalls such that the centre of rigidity lines up with centre of load. To reduce the effects of building torsion.

In real life, we're never so lucky.