Concrete Bearing Strength...Precast Bearing Connection

[KimWT]

Hi!

I copied two load bearing spandrels from different projects of parking structure.
Longer spandrel does not have baring plate at both ends though short one has.

Apparently, some precast design engineers do not add bearing plates for precast concrete load bearing spandrels.
Is this industry wide design trend?
Doesn't PCI Design Handbook recommend bearing plate should be added under the assumption of the potential crack line?
I have found some engineers do not use bearing plate for precast inverted beams.

I am looking forward to your comment.

Thanks!

Kim

 

[TehMightyEngineer]

By my experience and understanding, the use of steel bearing plates depend on a lot of things but are generally recommended for many bearing connections, but are also not required and acceptable designs can be made without them. I'm personally a fan of bearing plates unless I can easily justify not using them.

All that said, a bearing "pad" of neoprene or teflon is almost always required for precast bearing connections. I rarely if ever do concrete on concrete bearing.

Ian Riley, PE, SE
Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[BridgeSmith]

I would expect the need for a bearing plate to be based mostly on 2 criteria:

1) Is the bearing area large enough so that the load can be transferred to the seat by the concrete, without exceeding the concrete capacity.

2) Can the lateral force to be resisted at the bearing be transferred from the girder to the bearing without the necessity of a steel embedment.

 

[KimWT]

In the typical load bearing spandrel design in the parking structure, steel bearing plate is 6"x10" (asssumed).
Using PCI design handbook formula, phi Cr (0.85f'c)A1...=0.55x(6ksi)x60in^2=200Kips (concrete bearing strength)
Approximately 20Kips per DT stem and spandrel self-weight=53Kips...
Ultimate load=1.2x53/2+5(# of DTs)x20=132Kips

200 (Bearing strength of plain concrete) >> 132 (Ultimate Load)

To consider tensile load from friction or temperature change, 20% of compression=0.2x132=26.4Kips
26.4/0.9fy=0.5in^2 (generally, some U-shape rebars are added for torsion and prestressing strand end detail)

Therefore, steel bearing plate is not required!

If this logic is true, why are so many engineers adding steel bearing plates?

 

[RickyTickyTavi]

@ KimWT

In my opinion, Its mostly because bearing isnt the only possible load case when youre taking about pointloading a finite area like. The steel plate allows for more shear distribution into the stem during thermal or impact loads. You get spalling on the stems and corbels because the plane of action is small (ie, the corners, etc). When you put steel plates, it allows for it to grab more concrete. Pure bearing isnt the entirely the point of those plates.

 

[KootK]

Doesn't PCI Design Handbook recommend bearing plate should be added under the assumption of the potential crack line?

Yes, that's exactly right I think. You do have the option of doing the specified calculations to justify not having the bearing plate though. Your odds of making this work go up as the axial beam force transmitted through the joint goes down.

Pure bearing isnt the entirely the point of those plates.

I agree and would go even further by saying that, in many cases, the bearing plates don't actually improve concrete bearing. Unless you're developing the bearing force deeper into the concrete with vertical DBA's etc, the concrete bearing on the top of the plate is identical to what it would have been with no plate.

You get spalling on the stems and corbels because the plane of action is small (ie, the corners, etc).

I agree that this is important. While bearing plates may provide a measure of armoring for this condition, I believe that the primary way that this is/should be addressed is via the neoprene bearing pads that TME mentioned. And other stuff like that. With such measures in place and properly designed, concrete-concrete bearing should not be problematic.

One way in which I use bearing plates is to address the prestressed equivalent of "anchorage of positive moment reinforcement" for mild beams. Frankly, I'm not sure if it's just me doing this. I consider the action to be quite similar to the PCI "assumed crack" business so it may be redundant of me to be checking both.

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.