two way slab middle strip reinforcement

[johndeng]

I am checking a two way slab with 20 bays each way, each bay are 25'x25' with concrete beams 4 sides. Right now it has beams in column strip so it only used less than half of it's capacity. however the middle strip it only has one top bar #5@ 20", which is only less than half of the required capacity of the column strip. Will this work? It shifts all loads to column strip.

I remember I saw some PT slab before, they using bundled tendons in column strip while minimum steel reinforcement in middle strip.

Please advise. Thanks

 

[johndeng]

Need your comments please help.

 

[driftLimiter]

You haven't provided much information. And your essentially asking for us to do the engineering for you. What kind of analysis have you performed so far? Why are you so concerned with whether the middle strip capacity is half the column strip anyway?

What kind of loading does the middle strip have? Thickness of slab?

#5 @ 20" O.C. is beyond the maximum spacing for rebar in ACI.

Sounds like your middle strip is worthless to me.

 

[johndeng]

Thanks for replying!
I did all the calculations for the existing slab, however, I found the column strip reinforcement is overdesigned and the middle strip is super underdesigned. I am asking whether this is legitimate. Will the middle strip really disconnect from the rest of the slab system due to the under reinforcement. The building has been there over 50 years.

 

[JLNJ]

How did you "check" the slab reinforcement? It is perfectly normal for the column strip to carry significantly more load than the middle strip.

 

[johndeng]

Yes column strip takes 70% total moment by saying, and middle strip take 30%.
But the design at column strip can take 140% while the middle strip can take only 15%, will this be fine? This is the existing design.

 

[driftLimiter]

Does the adjusted strength of the middle strip exceed the factored loads? Answering this question is the only way to get what you need.

 

[johndeng]

No, the strength of the middle strip is way less than the loads as I said above.
I repeated myself many times. The column strip is over design, and middle strip is under design, will the strength of column strip compensate for the middle strip. If not, how will the middle strip fail. This is an existing old building.

 

[driftLimiter]

No, the strength of the middle strip is way less than the loads as I said above

Although I can't find where you say that above, I think you have your answer. If the strength is less than demand than its no good.

If its an existing building have you considered using the IEBC or the ASCE 41 to evaluate the capacity?

 

[BAretired]

I am checking a two way slab with 20 bays each way, each bay are 25'x25' with concrete beams 4 sides. Right now it has beams in column strip so it only used less than half of it's capacity. however the middle strip it only has one top bar #5@ 20", which is only less than half of the required capacity of the column strip. Will this work? It shifts all loads to column strip.

#5@20 is more than one top bar. It is 7.5 bars. Are you saying the middle strip has no bottom bars? If so, it has no capacity to speak of.

The slab is 500' by 500', so it should have expansion/contraction joints.

How will the slab fail? Who knows; it could be getting full load for the first time, it could fail by shrinkage or temperature change. Or it may not fail if you have improperly interpreted the drawings.

 

[johndeng]

Hi it's actually the UPS Manhanton building. The top bar #5@20 is because the slab is 16" hollow block with 4" rib, only one top bar @ each rib. The bottom bar is 2#4 @ each rib.
Yes it has expansion joints, probably 8 bays max each section.

 

[BAretired]

That information changes everything. It should have been given in the OP. If the sum of column and middle strip in each direction satisfies statics, it likely will not fail. The distribution of moment between column and middle strips for two way slabs varies amongst codes. In the end, it may be governed by engineering judgment, which is also variable.

 

[johndeng]

@BAretired, just follow up and what you said is correct. The other two engineers dont even understand the question. After we check CRSI and it clearly states the distribution of moment between column and middle strips for two way slab is NOT CRITICAL, as long the total area of reinforcement (based on M0) is provided.

 

[rapt]

To not be critical will depend on a few things, especially ductility of the reinforcing steel.

How ductile was the reinforcing steel used 50 years ago?

And you cannot have 90-100% in the column strip in both directions!

And that also, it only applies to collapse load, deflection and cracking will be affected by the distribution. They can get away with it with PT slabs because of the effects of PT at service on deflection and cracking. You do not get the same benefits with RC.

 

[johndeng]

@rapt, You are right too, this is an old building, guessing 50ksi steel, not sure, here is the plan attached.

 

[lexpatrie]

What's the age of the building, though. Older rebar tended to be 40 ksi at least for a while, then it moved to 60 ksi. I don't have a timeframe for that but somebody could look into it and update the thread with their findings.

When I deal with older buildings I look for property information, it's not always the right building, but it gives one an idea. Google Earth and Historic Aerials can also be of some limited use, but they are fairly modern. Going farther back, some of the local historical societies will have information on buildings, nothing spectacular, but when it was built can be helpful.

John, please use the "Image" link so we can see things without downloading.

Those drawings are signed 1948. I'd be surprised if there was 60 ksi steel in there. I don't think I've heard of 50 ksi rebar, maybe you meant 60 not 50.

You may get some use from the FAQ on concrete, I just added a link to the 1957 CRSI that's posted on archive.org there. That's after the fact but it's better than nothing.

 

[johndeng]

[URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1713371326/tips/STR_lhjrhw.pdf[/url]
Let me try the image link first