Wondering if some one could point me in the right direction as in determining when the cracking as seen in the attached photo will occur. I am not sure on this, but can one use the standard shear stength equation (ACI 11-1) for this type of shearing failure. The bearing area seen in the picture supports a precast floor system and the specified bearing length of 4" was a judgement call for the size of the load. The resulting crack was a 3.5" horizontal by 15" vertical bearing failure at the concrete wall end. Any assistance would be greatly appreciated.
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Crack in Concrete Bearing Wall at Opening
- Thread starter pioneer09
- Start date
msquared48
Structural
Looks to me like an 8" seat was provided in the wall for the beam and the 4" bearing plate was installed to the outside of that available space. This location would engage less of the shear/bearing reinforcing in the wall. If only 4" was needed, the bearing plate should have been installed further to the left - toward the wall - maybe an inch or so from the inside vertical edge of the wall. This would have engaged much more wall reinforcing. As a further note, I would also have used an angle for the bearing seat (rather than just a flat plate) with welded rebar back into the wall, horizontally and vertically from each leg. The vertical leg of the angle with the welded rebar would have constrained the concrete from cracking in this pattern.
The other scenarion is that the support area is too little for the load seen. You would have to run the numbers.
Mike McCann
MMC Engineering
The other scenarion is that the support area is too little for the load seen. You would have to run the numbers.
Mike McCann
MMC Engineering
You asked when the cracking will occur. I believe it has already occurred.
It appears there is a steel HSS column adjacent to the wall. Is it capable of carrying all of the load by itself?
Is it possible to determine how much load goes to the HSS and how much to the concrete wall? Any sudden change in temperature will affect the amount of load carried by the steel because steel will respond to temperature change faster than concrete.
Also, there may be horizontal forces present due to temperature and shrinkage of the floor structure.
If the wall is needed to carry gravity load, it will be necessary to tie the bearing plate and the triangular chunk of concrete back to the wall to engage more wall reinforcement.
BA
It appears there is a steel HSS column adjacent to the wall. Is it capable of carrying all of the load by itself?
Is it possible to determine how much load goes to the HSS and how much to the concrete wall? Any sudden change in temperature will affect the amount of load carried by the steel because steel will respond to temperature change faster than concrete.
Also, there may be horizontal forces present due to temperature and shrinkage of the floor structure.
If the wall is needed to carry gravity load, it will be necessary to tie the bearing plate and the triangular chunk of concrete back to the wall to engage more wall reinforcement.
BA
TXStructural
Structural
Good practice would detail as Mike suggests, with DBAs providing the tie forces. This very likely has these, otherwise I would expect the crack to result in detachment of the embed. There may also have been enough restraint to cause this crack, depending upon how the structure above was configured. I have frequently seen construction where the embed would have been better if it were farther into the recess, but it was marginal as constructed (not truly deficient.)
The crack initiated under shear force as you surmise, with or without restraint of the embed by structure above. The crack stopped moving once the reinforcement was mobilized. If the reinforcement/DBAs were stressed significantly, but within yield, the crack would widen under load and then squeeze back together when unloaded.
I agree that the design of the support, and the size and placement of the embed may have been inadequate for the forces involved. But, if the reinforcement is adequate and there is sufficient shear friction developed by aggregate interlock, this member may still be serviceable. Is there any vertical displacement? And do you have shops for the embed?
The crack initiated under shear force as you surmise, with or without restraint of the embed by structure above. The crack stopped moving once the reinforcement was mobilized. If the reinforcement/DBAs were stressed significantly, but within yield, the crack would widen under load and then squeeze back together when unloaded.
I agree that the design of the support, and the size and placement of the embed may have been inadequate for the forces involved. But, if the reinforcement is adequate and there is sufficient shear friction developed by aggregate interlock, this member may still be serviceable. Is there any vertical displacement? And do you have shops for the embed?
msquared48
Structural
Was the steel tube column put there as a temporary shore due to the cracking?
Mike McCann
MMC Engineering
Mike McCann
MMC Engineering
- Thread starter
- #6
The steel tube column was added after it was noticed that cracking/shear failure occured in the wall.
If one assumes that no strength is provided by reinforcement in design (plain concrete shear strength), is it correct to assume that the shear failure plane will propogate from the center of the bearing plate to the outside of the wall at a 45 degree angle during design? When looking at the crack in the picture, the shear plane goes from the back side of the bearing plate at a approx. 60 degree angle to the outside of the wall.
If one assumes that no strength is provided by reinforcement in design (plain concrete shear strength), is it correct to assume that the shear failure plane will propogate from the center of the bearing plate to the outside of the wall at a 45 degree angle during design? When looking at the crack in the picture, the shear plane goes from the back side of the bearing plate at a approx. 60 degree angle to the outside of the wall.
I don't believe that assumption would be correct. The concrete at the left edge of the base plate in the photo has a compression on one side and zero stress on the other, so a crack would be more likely to propagate from the left edge.
The angle would vary depending on the magnitude of horizontal tension in the floor system.
BA
The angle would vary depending on the magnitude of horizontal tension in the floor system.
BA
msquared48
Structural
This stgeel beam could also serve as a drag strut. The problem may not be bearing in nature at all, or a combination of the two.
If the beam and connection seat were not designed to take drag forces, the beam should have had horizontally eleongated holes for the bolts provided for longitudinal movement/slippage. If they were not, therein lies the problem.
Mike McCann
MMC Engineering
If the beam and connection seat were not designed to take drag forces, the beam should have had horizontally eleongated holes for the bolts provided for longitudinal movement/slippage. If they were not, therein lies the problem.
Mike McCann
MMC Engineering
Along with Mike's thought I would add that if the beam is connected to another concrete wall on the other end it could be acting to restraint wall shrinkage as it cured. Typically concrete walls should have movement joints every 25 feet or so. If this beam were combining two 25 foot concrete walls and had a span of 20 feet or something like that then essentially you have 70 feet of connected concrete wall. If this is the case than you might make the steel column a new permanent support if the foundation will allow it. So there are my two cents.
John Southard, M.S., P.E.
John Southard, M.S., P.E.
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