Slab column moments - Shearwall building

[slickdeals]

Greetings,
I have a few questions to which I hope you guys will be able to provide a rationale.

In the serviceability design of concrete buildings, I use the following rationale.
1st run - Start with 1.0 Ig for all columns and shear walls. I use 0.5 Ig for the slabs (P/T). The reason I use 0.5 Ig is because I am assuming that my slabs crack and that I want more forces to be distributed to the walls and not the columns. I run my analysis and looked at my stresses in the walls. If I see that the walls are cracked (based on 7.5 * sqrt fc' and P/A+M/s and using D+0.5L+W) then I assign a Ig of 0.7 to the walls and re run my analysis. For the 2nd run, I have Ig = 0.7 for walls (if cracked), Ig = 1.0 for columns and Ig = 0.5 for slabs. This is what I use to determine my drift at service level. I use L/500 for a 50 year wind even though I know many others use a 10 year wind. The reason is because the architects typically love to put holes in our walls and I want to have some fluff in it. I should mention here that I find my walls taking about 85% of the service wind moments and the frame accounting for the rest. Is this a number commonly seen by other engineers?

For the strength design, I start with columns Ig = 0.7, walls = 0.7 and slabs = 0.35. I run my analysis and determine if my wall is cracked under factored load combo of 0.9D + 1.6W. If it is, then I rerun my cracked walls with 0.35 Ig. That gives me the final design forces for the walls and columns.

The problem/question I have is when I design the slab column joint. Since I use Ram Concept to design my slabs, I cannot export results into it. As a result the slab gets designed only for gravity and no lateral forces. There is typically no bottom reinforcement in my slabs at the columns. However, I know that there will be some wind moments in my slab-column joint because the wall is not taking 100% of the wind moment. How do I account for this? ETABS does not give me any slab column joint moment unless I use SAFE.

What do you engineers typically do for a PT Slab/shear wall building. I have heard engineers pin all the columns and design the walls for 100% moment. This can result in not using 15-25% of frame action resulting in bigger shear wall sizes.

Your input is very much appreciated.

Thanks
Anantha

 

[hokie66]

You worry too much. Design the shear wall to take the shear, and if the columns want to take a bit, so be it. Sounds like you may be spending too much time analysing and not enough time designing. Don't take offence, just the opinion of an old hand who has come down that route.

 

[ash060]

Check your ETABS results for the columns and find out how much is actually going into them. If the column moments are small do not worry about the slab moments because they will be small as well. If the column moments are large than take a look at the joint and estimate what the slab moments should be using equilbrium at the joint. You really only have to worry if you get a moment reversal. This would most likely happen in the 0.9D + 1.6W case. If the slab is post-tensioned the tendons will be working against you and there is the greater possibility of a moment reversal. If you have very large moments this may effect your punching shear as well but it is unlikely because the shears should be much smaller in the wind controlling case.

If you have a good wall layout the frame moments should not be to bad, but there will be cases when bottom bars will be required to transfer the moment into the column. I would recommend you check out ACI's publication on monolithic slab-column connections it has some very good explainations and a few suggested details.

I have had to use SAFE before to get a better handle on the slab moments due to wind. There really is no easy way when using Concept and ETABS as far as wind moments are concerned. You have to copy the data from SAFE and check the moments from Concept and add or subtract the two as needed just make sure your design strips match.

 

[slickdeals]

" If the slab is post-tensioned the tendons will be working against you and there is the greater possibility of a moment reversal "

You bring up a very important point. How do you account for secondary moments due to post tensioning when you do your lateral analysis. Is there a way to take care of it or is it generally not accounted for?

"If the column moments are large than take a look at the joint and estimate what the slab moments should be using equilbrium at the joint."
I am able to look at the column moments, but then do I take my sum of moments and distribute that equally to either side of the column? would you please explain what you do in these cases?

Thanks
Anantha

 

[ash060]

I usually use the secondary moments from Ram Concept when accounting for the post-tensioning. On one side the post-tensioning is subtracting from the wind and gravity moments but this case will not usually control over the straight gravity case. In the other condition I add the secondary moments from Ram Concept to the wind moments using the design strip auditor.

In the case of the joint equilbrium I just use an estimate for the moment. I do a ratio of the spans with the shorter span taking more moment because it will be stiffer. This is just an appoximation, but I am comfortable doing it. I don't really know for sure, but I have never been able to get a better answer from anyone else (but I have not asked a lot of other engineers either). When the geometry is very complicated (very irregular column layouts) I think I am just taking a shot in the dark. In that case I would recommend using SAFE but of course the averaged slab moments you get out of the program depend a great deal on the design strip you are using.

The truth is I do this very rarely because it just takes to long and the moments in the slab vary floor to floor, and checking every column on every floor is just crazy.

When doing the shear wall buildings in ETABS I usually will pin the columns and count on just the walls. After I get a wall layout that works for deflection, I will fix the columns and the design the wall using the lower moments due to the columns being fixed. This will usually result in a lower amount of steel in the walls. And because there is so much wall the column moments are small enough that I don't have to worry about designing them for wind.

I wish I could be of more help, if you come across something good involving this matter please post.

 

[slickdeals]

"When doing the shear wall buildings in ETABS I usually will pin the columns and count on just the walls"
Do you release the axial forces in the column transferring everything to the wall or just the moments at the top and bottom of the column?

"I will fix the columns and the design the wall using the lower moments due to the columns being fixed"
This can potentially mean a 15-30% reduction depending on the number of columns in your building. You are conservatively sized for deflection, but you are relying on the frame for strength. What L/? do you use and do you check a 10 year or 50 year wind?

Do you provide any bottom reinforcing on the columns (a lot of research talks about 2-3 bottom bars across the column cage) or do you just provide stud rails for shear on P/T buildings? And when you talk about small moments in the columns, what order of magnitude are you talking about (anything less than 20-30 k-ft or higher?)

 

[ash060]

Just release the moments. You can't release the axial DOF from the model or your gravity resisting system will be incorrect. The walls will take to much load.

The axial loads that the columns take do to wind are very small when compared to the gravity load. The amount of load the frame will take is usually going to be no greater than 20% because most of the load will go to the shearwalls being they are the much stiffer element (assuming a rigid diaphragm).

I will usually use L/500 with the 50-year wind load. This is converative, but not too much so. The result after the frame is added in maybe as high as L/800, but it makes the calculations much, much easier.

I will not put bottom bars around the column in PT slabs because unless they are required. In places were I am very concerned about punching shear I may place "hanger" bars as a belt and suspenders deal. I will use Studrails for punching shear reinforcing.

I like the wind moments to be no greater than half of the gravity moments, that seems good enough to be very confident that there will be no moment reversal.