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ACI Shear Strength Requirements 1

SteelPE

SteelPE

Structural
Mar 9, 2006
2,766
I have a project that is currently under construction that is a single-story metal building. The building is somewhat large and has a base shear of Vu = 49.7 kips. We designed the foundation utilizing moment resistant foundations as we have had issues lately with clients ripping out tie rods and hairpins. We specified 4 ksi concrete on the design documents to resist this load.... things were spectacular.


Until ..... We are now running into concrete that is not hitting the 28-day compressive strength specified on the drawings. The testing agency is holding some breaks for 56 days, but the contractor is looking to start steel erection early next week. At this point, we are not exactly sure why we are having a series of breaks below the necessary compressive strength, but we were asked to go back and reevaluate the concrete strength at a lower capacity in order to help the construction schedule.


When evaluating the concrete at a lower strength we are now running into issues with the shear strength of the piers. We are calculating their capacity to be phi Vc = 88 kips. Since the capacity of the pier is not 2x the demand, we now have to provide a minimum amount of shear reinforcing in the pier. We do have stirrups that we provided in the pier, but they are not quite enough to meet the minimum shear requirements.


Just wondering what others would do in this instance. I am inclined to say it’s ok as are 1.77x vs 2x the shear requirement.
 
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SteelPE said:
I am inclined to say it’s ok as are 1.77x vs 2x the shear requirement.
Click to expand...

I would not take that approach.

Firstly, it is quite simply not code compliant. Maybe you are comfortable with that and maybe you are not.

Secondly, it sounds as though your design is seismically governed. Having non-code compliant, brittle shear response in a vertical element is not a great look.
 
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Just as a follow up to this.

At the point I wrote this inquiry, we had not received the concrete breaks for these column piers. After some conversations with a few other engineers, we decided that if needed, we could repair the piers, if necessary, without much interruption to the construction process. In this instance, the piers are most stressed under DL+SL load combination (because it is a clear span metal building).... and since we know the direction of the load (out and away from the building) we could do of of the following:

1) Install a tie rod across the building to the column on the other side of the building (which has an equal and opposite load under the DL+SL load combination)
2) Install some sort of concrete buttress on the outside of the building (below grade).... and design this buttress to take the lateral thrusts from the base of the column and distribute these loads to the footing.

We figure that 2 is the best option if needed

Late on Friday we received the 7 day breaks for the piers in question..... and things look better (with the concrete attaining 90% of its strength in the first 7 days).... so we may need to do nothing.
 
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For the good of all of us, please charge the owner for this remedial investigation . . . not to mention remedial design.
Not your fault. If I call for one tie too many to try to head off a Contractor problem like this, I get hammered.
 
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I had set this up with the architect prior to the analysis when the breaks started coming in low. I gave them a few options on could be done (rip out the effected areas...... easiest for me, analyze the foundation for the lower concrete strength..... easiest on the contractor). I told them it would take about a day to figure out if there was a problem and that they would be invoiced for this additional time.
 
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SteelPE - you may already know this but here is an FAQ I wrote a while back - based on an earlier ACI 318 but I would think it's still pretty accurate. Many engineers overreact at times with low initial breaks. The ACI acceptance criteria is pretty clear and fair I think:



https://www.eng-tips.com/forums/507/faqs/1575
 
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JAE,

We are basically following these provisions (which is good to know). We have evaluated the structure for lower strength concrete and are OK with the design with a few exceptions at specific locations.... which we have told the GC and owner about. Unfortunately we will not know if those areas are sufficient until the end of this month when the 28 day breaks come in. In the meantime, the contractor starts steel erection this week.
 
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You may get some relief on the shear reduction factor from ACI 562.
 
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