What to choose spandrel or pier

[mhdhamood]

Dears;
I am appreciating your acknowledgment
Also I would like to thank you for your interests to teach people.
I have a quesion about:
1. I have a shear wall so when I consider the shell element as spandrel and when it behaves like a pier?
2. What is the difference in behaviour between them, I mean that ; does pier element takes more shear forces than spandrel?
3. according to the attached picture shall I consider the part of shear wall above the opeining as a spandrel or pier and why?
4. If not, can I consider it all as pier?
Regards

 

[KootK]

In simplified terms: pier = column; spandrel = beam. I'd call the bit above the opening a spandrel. I don't believe that there is any difference analytically. Both piers and spandrels are just panels constructed of shell elements. The difference lies in the post processing. If you're using a program like ETABS, the suggested reinforcement detailing will differ depending on how you've designated a particular panel within the software.

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.

 

[mhdhamood]

Dear KootK (Structural);
Thanks a lot for your response. I have to add something that analytically there is a defference that for spandrels we have m11=m22=m12=f22=f12=0.35 but for pier m11=m22=m12=f11=f12=0.70 .
My question appear obviuosly if you see the attached picture.
Best regards and thanks again.
Mohammad

 

[ahmedhegazi]

you do not have any spandrels in this sketch, you have wall +Column down and walla above .
if you model Pier (wall) and stress level is reach rupture (cracking) then your modifiers are 0.35

 

[MrHershey]

There's a little bit of difference between the two in how you apply crack factors as OP noted. Also important to recognize that the 0.7 being used for piers above is for *uncracked* walls. If you've got enough moment in them to crack them, then ACI actually recommends using the same 0.35 being used for spandrel.

Another quirk that we've found recently in applying cracked factors, there's not a real great way right now to isolate so your shell crack factors are only applied for moment loads (while you can do this for frame elements). So if you put the 0.35 factor into F11 for a cracked wall pier, you're not just applying the factor to moment stiffness, you're applying it to axial as well. Could run into some load sharing issues between elements modeled as frames (with no axial crack factor) and elements modeled as shells (with the cracked wall crack factor) where your columns are attracting more load than they should because their axial stiffness is unmodified at 1.0 while the walls have been decreased to 0.35. This could also result in walls and their foundations not being designed for enough load.

One way we've combated this is to set the axial stiffness modifier for columns to match the stiffness modifiers for walls in the vicinity. Don't plan on getting any useful deflection information out of that model though.

 

[MrHershey]

Should note that second portion of post above is related to using ETABS. Perhaps other software packages are better at accounting for this.