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Interpretation of bottom rebar tag: "#4@10"(B)x33'0" -STAGGER"
2

Interpretation of bottom rebar tag: "#4@10"(B)x33'0" -STAGGER"

Interpretation of bottom rebar tag: "#4@10"(B)x33'0" -STAGGER"

(OP)
I have a project where we are about to commence an investigation of an UNbonded PT roof slab (corrosion damage, failure of tendons, etc.) constructed in late 1970's - designed by T.Y. Lin's office - and the original structural drawings show the following bottom rebar tag:



Do you interpret the above to mean at MIDSPAN region:

OPTION (A): There is 2#4 every 10" (As=0.40 in2/10") so equivalent to #4@5";

OR:

OPTION (B): There is 1#4 every 10" (As=0.20 in2/10").

We will be checking definitively when the field work commences, just doing some prelim checking to see where the slabs are, capacity wise. Damn, those old designs stretched the limits - 40' endspan with 10" slab ==> L/D=48!

RE: Interpretation of bottom rebar tag: "#4@10"(B)x33'0" -STAGGER"

(B)

RE: Interpretation of bottom rebar tag: "#4@10"(B)x33'0" -STAGGER"

Also (B)

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.

RE: Interpretation of bottom rebar tag: "#4@10"(B)x33'0" -STAGGER"

(B)

Not even sure how A would make sense... Stagger i guess...

RE: Interpretation of bottom rebar tag: "#4@10"(B)x33'0" -STAGGER"

(B), but because the 'bubble' includes both, there is a remote possibility that (A) was intended. Get a rebar locator or a good stud finder that works with concrete and confirm at mid span.

Dik

RE: Interpretation of bottom rebar tag: "#4@10"(B)x33'0" -STAGGER"

I'm going to respectfully disagree. To me it appears to be a two span condition with the longer span on the right. The two staggered 33' bars (staggered left and right in longer span) would result with 2 bars at midspan.

gjc

RE: Interpretation of bottom rebar tag: "#4@10"(B)x33'0" -STAGGER"

I'm with mtu, I think.

This is not exactly how I would use the term "staggered" in rebar layout. I would expect staggered to mean that two adjacent conditions are not similar (i.e. the staggering of laps so that they don't line up).

It looks to me like you would have 2#4s 33' long and 2#4s 11' long, with these four bars set every 10" c/c. There would be two bars in the +or- 20' long section where the 33'-0" long bars overlap.

The only thing which is slightly curious is the TWO 11'-0" bars. Why not one longer bar? (Would be about a 20 footer).

RE: Interpretation of bottom rebar tag: "#4@10"(B)x33'0" -STAGGER"

My vote is (B), #4@10 at midspan based on drawings I have seen that use similar convention. Other clues:

1. P/A in the slab relatively high at 328 psi--high enough that the design likely did not require #4@5 at midspan.
2. Top steel at interior support is #4@8 where moment demand is greater than at midspan.

RE: Interpretation of bottom rebar tag: "#4@10"(B)x33'0" -STAGGER"

(OP)
Thank you all for your input.

I was OPTION (B) (without hesitation) until I reviewed the calcs of another engineer who did some checking of the slab capacities a few years ago, including assumed % loss of section to rebar due to corrosion and he/she interpretted the bottom rebar as OPTION (A).

We shall be GPR scanning the slabs and undertaking invasive probing soon, so I will get definitive info on what was built.

DEKER - your 'other clues' are well thought out and logical - thank you. Somewhat interesting is that one section of the slab near a ramp termination (40' single span, 10" thick) has 72.6 k/ft (600 psi of P/A) with same bottom rebar - needless to say the slabs have significant restraint and flexural cracking - and no waterproofing membrane was ever applied.

I appreciate all your responses.

RE: Interpretation of bottom rebar tag: "#4@10"(B)x33'0" -STAGGER"

If the spans ratios are 3:1, a point load at mid span in the longer span will yield a positive mid-span moment about 30% greater than the interior negative moment.

For uniform loads, the negative moment exceeds the positive moment, even with load just on the longer span.

If the design was controlled by a large point load, such as a truck, then you might want more reinforcement in the bottom of that longer span.

RE: Interpretation of bottom rebar tag: "#4@10"(B)x33'0" -STAGGER"

I change my vote to (A) for the same reasons expressed by MTU. I'm afraid that I failed to study this carefully enough the first time around. Thought the lasso was a darn column.

Confidence = 91.9%

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.

RE: Interpretation of bottom rebar tag: "#4@10"(B)x33'0" -STAGGER"

(OP)
Thanks JLNJ:

The parking structure was originally designed for LL of 50 psf. We are checking for LL = 40 psf and applicable concentrated loads.

I am also checking the original design of min. bonded rebar considering the old UBC progressive collapse clause requirement of "DL+0.25LL" for one-way unbonded systems.

The clearance height is only 5'10" from lower levels - so fortunately 'big trucks' cannot traverse the roof level.

RE: Interpretation of bottom rebar tag: "#4@10"(B)x33'0" -STAGGER"

Damn T.Y.Lin. He may have been brilliant, but he has caused a lot of problems.

RE: Interpretation of bottom rebar tag: "#4@10"(B)x33'0" -STAGGER"

Quote (Ingenuity)

I am also checking the original design of min. bonded rebar considering the old UBC progressive collapse clause requirement of "DL+0.25LL" for one-way unbonded systems.

Initially I thought that #4@10 might be too light to meet the collapse prevention requirements of the UBC, but you mentioned that the structure was built in the late 1970's. The DL+0.25LL requirement didn't enter the UBC until 1976, so it's possible that it wasn't considered in the original design.

RE: Interpretation of bottom rebar tag: "#4@10"(B)x33'0" -STAGGER"

This is an excellent lesson in the clarity of notes. I vote for (A) by process of elimination.

Option B, referring only to #4 @10" at midspan, would imply that the spacing of positive reinforcing near the ends of the span is 20".

RE: Interpretation of bottom rebar tag: "#4@10"(B)x33'0" -STAGGER"

kipfoot,
The problem with that reasoning is that you are thinking about what the reinforcement should be, rather than what is actually there. There is a huge problem with many of these early post-tensioned slab designs, in that restraint induced direct tension cracking was not even considered. Anyway, Ingenuity will find out with his testing, and hopefully will let us know who guessed right.

RE: Interpretation of bottom rebar tag: "#4@10"(B)x33'0" -STAGGER"

(OP)

Quote (hokie66)

Anyway, Ingenuity will find out with his testing, and hopefully will let us know who guessed right.

I shall indeed.

RE: Interpretation of bottom rebar tag: "#4@10"(B)x33'0" -STAGGER"

kipfoot,

ACI318 does not require the minimum bottom reinforcement in a PT flat slab to extend full length of the span! So 20" is better than the code requires.

Not that I agree with it.

RE: Interpretation of bottom rebar tag: "#4@10"(B)x33'0" -STAGGER"

rapt and hokie66, you're both right. I wouldn't feel right with a 20" spacing and I'm projecting my own sensibility on the design.

RE: Interpretation of bottom rebar tag: "#4@10"(B)x33'0" -STAGGER"

... and I wouldn't normally use 5" spacing for #4 bars, either... I've often seen early post-tensioning work with supplementary regular reinforcing having large spaces. We'll have to wait for the NDT...

Dik

RE: Interpretation of bottom rebar tag: "#4@10"(B)x33'0" -STAGGER"

(OP)
7/31/2017 ==> FOLLOW-UP:

I did some GPR scanning to the soffit of the slab today and can 99.9% confirm that OPTION (B) is what was constructed.

Photo of scan results below. The scanning was done 'inverted' so the bottom rebar is shown on the top of the screen shot. Scan location was each side of where the 33' long rebar stops and staggers - so about 7' from end. 10" and 20" spacing conforms the OPTION (B).



We will be 100% definitive when we start invasive probing mid-August.

It may be a challenging project with a few corroded and failed tendons clearly evident:





And it is never good to have PT anchorage zones with an non-encapsulated system below planter boxes that have CMU block that looks like this:



EDIT: The two photos above of the PT tendons are soffit photos @ midspan.

RE: Interpretation of bottom rebar tag: "#4@10"(B)x33'0" -STAGGER"

Really interesting stuff. I hope you follow up again when you get farther along. Looks like a good challenge.

Those planter boxes... yikes.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries
https://www.facebook.com/AmericanConcrete/

RE: Interpretation of bottom rebar tag: "#4@10"(B)x33'0" -STAGGER"

I don't know if it's too late, but, Vector has been working on an epoxy injection system for accommodating corroded strand.

Dik

RE: Interpretation of bottom rebar tag: "#4@10"(B)x33'0" -STAGGER"

(OP)

Quote (TME)

I hope you follow up again when you get farther along.

Will do.

Quote (dik)

Vector has been working on an epoxy injection system for accommodating corroded strand.

I know Dave Whitmore and Chris Ball from Vector and they have a gas-purge system too to dry-out tendons with water. There is (was?) another Canadian co. headed by Jack Morrow from Calgary that patented a polyurethane injection system a few years back, and was somewhat popular in Colorado. Problem with both PU and especially epoxy injection is that it makes it extremely difficult (often impossible for epoxy) to remove the tendon in the future should repairs be required.

For this structure, I suspect that the near 40 years of water damage will have resulted in significant pitting (with loss of section) and broken wires that strand removal and replacement is probable.

RE: Interpretation of bottom rebar tag: "#4@10"(B)x33'0" -STAGGER"

(OP)
8/15/2017 ==> FOLLOW-UP:

We commenced field work to this project today and have 100% confirmation that 'as-built' was indeed OPTION (B): 1#4 every 10"

RE: Interpretation of bottom rebar tag: "#4@10"(B)x33'0" -STAGGER"

The concrete is cleanly chipped out, and those particular rebars look pretty good. (Can't tell about the others of coourse).

Can you add more concrete below the damaged areas to recover strength?
Can the damaged tendons be pulled out, pulling replacement new tendons back into the old holes? Obviously, the old holes would need cleaning and drying.

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