Cracking in multispan rc beams

[Quasar87]

Hello,
I’m in the process of assessing a continuous cast-in-place RC beam in a structure. The beam is designed originally as a T-beam and carries typical loads in a parking garage. In one span (8-m-long shown in the sketch) there is a rc wall that transfers all the load from the upper stories and is quite significant. The issue: severly cracked at the bottom right under the wall but with the crack widths just under the limit under permanenent loads only (no cars yet).
I’ve been asked to take a look as to why has cracked and this are the facts:
- beam originally designed to have 6 bars dia. 25 mm at the bottom and 20 bars at the top at the support.
- due to an error on the drawings the beam gets reinforced with 10 bars at the bottom and only 12 at support.

Now although the beam is reinforced extra at the bottom than original design and under-reinforced at the support it cracks significantly in the field.

I run my own analysis and in order to limit the cracks to the code I would need 9 bars in the field and and a lot more (25 bars) at the support.

My question:
Assuming no overloading has taken place why does it crack so much although there are more bars than my analysis shows I would need at the bottom

. Could there be any connection with the fact that a lot of bars are missing at the support (although no cracks has been noticed around support).

 

[rapt]

I still do not understand why you think the support moment is so high compared to the span moment.

For a fixed ended beam with a point load at mid span, support and span moments are equal.

With a continuous beam like this one, the support moments would be less, especially with a shorter span at one side.

So span moment and reinforcement should be more than support moment and reinforcement, due to the point load.

 

[Quasar87]

@rapt
You are corect, however, there are differences between bottom and top of the floor when it comes to limitations of crack widths. You would need more reinforcement to keep the crack widths to 0.2 mm rather than 0.4 mm(at the bottom)

 

[HTURKAK]

- UT and RT stands for ultrasound test and radiographic test used for detection of reinf.
- I performed a simple calculation for ULS , the reinf . = 10 T 25 mm Bottom + 12 T 25 mm Top seems OK.
- I guess the cracks for the lapping of all the bottom reinf. at midspan .( my guess is based on past similar experience)
- If there is no lap at midspan , the early loading could be another logical reason. Do you have the data for form stripping , prop removal dates? How old the concrete was during prop removal ?


He is like a man building a house, who dug deep and laid the foundation on the rock. And when the flood arose, the stream beat vehemently against that house, and could not shake it, for it was founded on the rock..

Luke 6:48

 

[Quasar87]

@HTURKAK

Thank you. I used a GPR for the reinforcement location.
I don’t have those info yet but I send the questions to the client. I also heard rumors about some heavy mobile crane during execution time. Need to dig deeper and find out if that’s the case.

 

[rapt]

Quarsar87,

Not 3 times more.

 

[dik]

The cracks look to be too large and too close together for shrinkage restraint. Can you post an elevation that shows dimensions and reinforcing? for columns as well as beams? and loads? With cracking shown and no load, off the top, it looks like a job for the big concrete erasor.

Don't lap bottom bars at midspan.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[sdz]

"The wall has no signs of cracking."
If I'm reading the drawing correctly the wall runs at right angles to the beam, so no surprise.

Ooo eee ooo ah ah ting tang walla walla bing bang
Ooo eee ooo ah ah ting tang walla walla bing bang

 

[haynewp]

Did you happen to measure the deflection along the length of that beam versus the adjacent beam that are not cracked?