Seismic drift - moment of inertia to use for shear walls?

[milkshakelake]

Seismic drift is limited to 0.020 in a building I'm designing with Seismic Design Category B. I'm using 0.35Ig for cracked walls and 0.7Ig for uncracked walls. Can I increase the moment of inertia by 1.4 as noted in ACI 318? I kind of know the answer already, but I'm just double checking.

6.6.3.2 Service load analysis
6.6.3.2.2 It shall be permitted to calculate immediate lateral deflections using a moment of inertia of 1.4 times I defined in 6.6.3.1, or using a more detailed analysis, but the value shall not exceed Ig.

R6.6.3.2.2 Analyses of deflections, vibrations, and building periods are needed at various service (unfactored) load levels (Grossman 1987, 1990) to determine the performance of the structure in service. The moments of inertia of the structural members in the service load analyses should be representative of the degree of cracking at the various service load levels investigated. Unless a more accurate estimate of the degree of cracking at service load level is available, it is satisfactory to use 1.0/0.70 = 1.4 times the moments of inertia provided in 6.6.3.1, not to exceed Ig, for service load analyses.

Being able to take 1.4 increase in Ig would help a lot. Seismic loads are not service loads, so I believe I obviously can't use this. But I'm having such a hard time making drift work for this building, using like 10 ksi concrete and stuff to get it under control.

 

[HTURKAK]

Can I increase the moment of inertia by 1.4 as noted in ACI 318

My interpretation is the same . You can increase the moment of inertia by 1.4 for immediate deflection analysis for service load effects .

I have more experience with Eurocodes and there are two seismic actions ;
- Design seismic action with 10% exceedance probability in 50 years (mean return period: 475 years)
- Serviceability seismic action (for damage limitation) with 10% exceedance probability in 10 years (mean return period: 95 years).

 

[milkshakelake]

You can increase the moment of inertia by 1.4 for immediate deflection analysis for service load effects .

I believe seismic is a factored load, even for checking seismic drift, so I don't think I can take 1.4. ASCE 7 doesn't differentiate between seismic strength and serviceability like Eurocode does.

 

[haynewp]

Seismic drift limits are checking for stability at ultimate considering inelastic effects. I would not use a service load inertia.

 

[milkshakelake]

Seismic drift limits are checking for stability at ultimate considering inelastic effects. I would not use a service load inertia.

I thought so. Thanks for the confirmation!

 

[canwesteng]

You could try and calc the actual moment of inertia instead of the table values. If you're up to 10 ksi concrete, it may result in a decent improvement.

 

[milkshakelake]

You could try and calc the actual moment of inertia instead of the table values.

Lol thanks for that suggestion. I tried it before on a project and it was incredibly tedious, with a lot of difficult hand calculations and trial and error. Maybe I can find a way to automate it one day. I'll keep that in the back pocket if I really can't get this particular design to work. For now, I'm making some headway with outriggers. Now outriggers...that's another discussion I'll probably post here soon.

 

[canwesteng]

Can be a headache, though in your case you can start with how much stiffness you need, add that to the model, and calc the effective I and see if it works out or not, instead of iterating. It either works with some margin or it doesn't, and you know not to take it farther.

 

[slickdeals]

Take a look at ACI 318-14 and newer. They let you take 0.5Ig for everything for factored load analysis.

I have typically never had issues with seismic drifts unless I am in a high seismic area.

How tall is your building? What does your lateral system look like? Mind posting a plan view?

 

[milkshakelake]

@slickdeals I am currently using the 0.5Ig provision to make it work, rather than go through the tedium of cracked and uncracked analysis. I'm not in a high seismic area (usually SDC B, sometimes A) but seismic drift controls a lot of design. Maybe I'm too conservative with it? I don't know, I just make sure it doesn't exceed 0.02 per ASCE 7.

I will be posting about this in another thread, because I have a question about outriggers and overstrength factor. But it's 12 stories. Factored wind base shear is 586 kips because it's Exposure C. I'm checking that number by hand because it seems too high, but that's where I'm at right now. I'm just getting seismic to work, then moving on to wind. Plan view isn't reflective of all the shear walls and stuff I added; the 3D view shows it better.

 

[slickdeals]

Can you add a concrete wall on the corridor side of the stair with link beams running E-W over the doors? That should give you some additional stiffness in the X direction and perhaps help remove the moment frame on the south slab edge? Have you considered include the wall + return on the trash room in addition?

Maybe there are good reasons why the above was not considered.

 

[milkshakelake]

I didn't run a link beam either over the elevator opening or the corridor side of stairs because I don't believe I have enough tension development length. It will connect to about a 12" wall. If the returns were longer, it would be possible. For the trash room, I considered this, but MEP is vehemently against it because a lot of pipes will run through there, making the shear wall with a big opening and rendering it ineffective.

 

[W460x68]

Unfortunately I don't know of any improvements over your current method other than doing a full cracked vs. uncracked analysis on each wall.

Would making that 15'-5" section along the South exterior edge a shear wall give you enough stiffness to solve your problem?

 

[milkshakelake]

@W460x68 I would love to, but that is discontinuous at the base due to a storefront. I didn't want to get into a vertical irregularity or weak story situation.

 

[bookowski]

Not related to your question, but be cautious putting a wall in front of the elevator as you show. They'll want it left open at the base for cab install (or a leave out that you have to pour later, yuck). In your sdc wind drift should control in Y.

 

[milkshakelake]

@bookowski I haven't come across that situation about wall in front of elevator, but I'll keep an eye out for it. Thanks for the tip. Will see where it's going when I speak to the contractor.

Regarding wind drift, yeah, I got seismic under control but wind drift is giving me a lot of problems, with a factored base shear of about 800 kips with a crazy gust effect factor. I might have to do the arduous cracking calculations now.

 

[haynewp]

Isn't the wind drift where you will be able to use the 1.4 factor with the service level wind loads? I remember it still being tough to get to work at h/400 or whatever you are keeping for these taller buildings.

 

[milkshakelake]

@haynewp Yes, I'm using the 1.4 factor for wind serviceability. Still hard to make it work, but that's life.