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Cracked CIP Shear Wall Shear Stiffness

Cracked CIP Shear Wall Shear Stiffness

Cracked CIP Shear Wall Shear Stiffness

(OP)
For the purpose of shear wall modelling, how should one calculate wall shear stiffness, accounting for cracking? I've found some guidance for high seismic areas (1/10 to 1/20 elastic value) but nothing for areas of low to moderate seismicity. We're all modelling our shear walls in snazzy software packages these days. What are we doing in this regard? Bonus points for anybody who can cite a reference of some kind.

Thanks,

KootK

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Cracked CIP Shear Wall Shear Stiffness

In canadian code (CSA A23.3-04) we use axial and bending stiffness modification factor = ±0.7 for an axial load of 10% * F'c * Ag.
No shear stiffness modification is applied.

RE: Cracked CIP Shear Wall Shear Stiffness

(OP)
Thanks for the response Pico. I'm a Canuck too. What's your justification for no shear stiffness modification? Based on the information provided for seismic situations, shear stiffness is a good deal more sensitive to cracking than bending stiffness. I could be out to lunch on this but, it seems to me that if you're going to allow your walls to crack flexurally, one ought to assume that they're cracked for shear as well (flexural-shear cracks and such).

This little blurb on CSI's website would seem to imply that some folks are modifying shear stiffness: Link

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Cracked CIP Shear Wall Shear Stiffness

You refer to : «There is no recommendation about reducing the GAshear. You should, however, note that some of our users use modifiers for f12 also, where they expect deterioration of shear stiffness and want to be realistic in their modeling.»

Cracking is already accounted in bending bihavour.

I my opinion, If you reduce your lateral stiffness even more, you increase your period, and decrease the acceleration response from design uniform spectra.

So, if it is not prescribed by code, I won't reduce my design seismic force with a shear modifier. To be safe, i keep by shear stiffness uncracked to get higher design force.

In my opinion, non-linear shear amplification due to higher mode effect (not covered by canadian code yet) is even more important !

RE: Cracked CIP Shear Wall Shear Stiffness

(OP)
Pico,

Loss of flexural stiffness due to flexural cracking is included in bending behaviour but loss of shear stiffness is not. They may represent the same physical cracks but they are separate and distinct inputs when it comes to modelling. While I agree with you about conservatism in the period calculation, whether or not ignoring cracked shear stiffness reduction depends on the parameter of interest. In a low to mid-rise building, I would expect that ignoring cracked shear stiffness reduction would lead to underestimating drift and stability issues and perhaps messing up the distribution of forces to the bracing elements.

KootK

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Cracked CIP Shear Wall Shear Stiffness

Even if it is called a SHEARwall, don't forget that displacement is mainly due to wall bending, so by code, shear deformation can be sort of neglected. Not so long time ago, 12EI/L³ was used instead of membrane elements !

Anyways, deflection sould not be a problem. If you are near the deflexion limit of 2.5%, You will have a lot more problem wih capacity design (rotational capacity !) !

In my case, I still only modify axial and bending as prescribed by code.

RE: Cracked CIP Shear Wall Shear Stiffness

(OP)
Thanks for the discussion Pico. Certainly, I agree with you for anything tallish.

For the curious, I've actually managed to find an answer of sorts on my own. MacGregor's concrete book references a PCA document that addresses shear stiffness by way of a modified flexural moment of inertia (I_mod). The form of the equation makes it pretty clear that it's a rearranging of the usual combined stiffness equation (k_eff = 1/(1/k_shear + 1/k_fexural)). Equating the two and solving suggests that the factor on shear stiffness ought to be 3/8. In the absence of more authoritative guidance, I'm going to roll forward with that.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

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