RC Beam Missing Shear Reinforcement

[SoFloJoe]

Hi All,

I have an building safety inspection project for a building in South Florida. We are required to do the 40yr and every 10yr structural inspection.

So this building in question is a single story CBS concrete block structure. The original plans are from 1973 and barely legible. But I can read on a section the the tie beam is 8x16 with 4 #5 rebar reinforcement. Which match up with my site measurements.

The issue that I see is at some point someone took out the CMU underneath the tie beams leaving spans of 15ft unsupported RC tie beams. Doing the calcs the size works for the span and tributary area. BUT, we should have shear reinforcement per today's standards. NOTE, that there were no designed areas of the plan that had such a span. The largest opening were for doors at about 6ft.

Right now there are no signs of shear cracks and beams are in relatively good condition. The walls were removed at least 5years ago or more, based on this owners testimony.

Anyone familiar with older construction have any opinion on if shear reinforcement was a requirement? Were they left off the plans as a normal thing but installed anyways?

Has anyone dealt with a scenario like this and offer any insight?

I am thinking of noting on my report that shear reinforcement was unable to be confirmed and therefore recommend an xray on density analysis to confirm if they are there.

Any insight is appreciated.

Thanks in advance!

 

[rowingengineer]

Had a similar situation in Australia, used carbon fibre to strengthen, I figured it needed something as a minimum.

 

[SoFloJoe]

Thanks for the insight. I have never prescribed carbon fiber as a strengthening application. Sounds interesting.

I was thinking more along the lines of adding shear plates along the span or at least at the ends.

 

[Settingsun]

I don't know the US code from the time specifically, but I think it was common that beams didn't need minimum shear reinforcement if they were shallow (<10-12"), wide, or if the shear force was less than half the unreinforced shear capacity. I doubt stirrups would be provided if not shown on the plans, especially for a fully-supported beam.

 

[JoelTXCive]

The carbon fiber strengthening is super-cool.....but not cheap.

 

[MotorCity]

Do you believe shear reinforcement is required merely to satisfy present day code requirements or do you believe it is required to support the loads in the current conditions. If you want it to satisfy todays' code only, I would dive into the IEBC to see if you are really required to bring it up to today's codes. If there are no shear cracks and it seems to be in good condition, shear reinforcement may not be required by today's or yesterday's code

 

[dik]

The code at the time may have been the ACI 1963, edition.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[KootK]

I wouldn't expect to see shear reinforcement in a masonry tie beam of those proportions. Any chance there's block above that could arch over to the ends of the beam such that you could reduce some of the shear demand?

 

[le99]

I consider the removing of blocks away from the cap/infill beam constitutes a "major change" of the wall, as the action alters the function of the structure, so it might well be required to bring up to the latest code requirement. Be aware of the code clauses regarding retrofitting and structural alterations, you might facing some liability issues here.

 

[JedClampett]

In ACI 318-71 minimum shear reinforcing was required. But maybe this fell under an exception. See attached.

 

[EZBuilding]

It sounds as though the beam in question used to be a tie beam at the top of a continuous masonry wall. The tie beams/tie column and infill wall construction was a part of prescriptive design in South Florida for a long time. These concrete beams served no true function in the gravity capacity of the assembly - unless if spanning above a wall opening - and their primary function was in resisting cladding loads and/or connecting the wall to the diaphragm/balance of structure. It was typical to see tie beams with shrinkage and temperature reinforcement and a few ties at 12" at ends and the balance of ties at 48 inches off center.

Masonry design is a mess of prescriptive requirements, residential code, ASD and LRFD. Makes for a lot of confusing and conflicting information and application in practice. Lot's of engineers/architects/plans reviewers believe you need a tie beam capping all masonry walls.

By removing the masonry wall, the beam above should have been reviewed based on the Existing Building Code for it's new function. You should ask you client if the wall removal was done as part of engineered or permitted work.

 

[SoFloJoe]

Thank you all for the input!

@EZBuilding.. my client inherited it this way (of course). There is no documentation or permit for this new function. I took on the role to permit it after the fact as part of the 40yr inspection report.

At this point and with all your great input. I plan to prescribe a x-ray to determine if shear reinforcement can be picked up. Then if not (which is probably the case) I will prescribe a design to either add in angles at the ends, add shear plates along the span or just go with a steel beam underneath. My next step in that situation will be to determine at what point along the beam the shear forces are low enough to not need shear reinforcement. Then I can make my judgement as to what method would be best. Unless anyone has any other thoughts?

 

[EZBuilding]

SoFloJoe,

Steel plates attached to the concrete beam with post-installed anchors is often done for retrofits such as yours. Some of the main benefits of this thought process is in not having to pour any new concrete and not cutting into any head room height.