Precast slab bearing

[JStructsteel]

Have a precast slab bearing on a steel beam, about 2.25" bearing width. I was going to use Eq 5-54 from PCI design handbook (7th ed) and set A1 = A2 since the slab is bearing on steel, not other way round. Would also use Cr = 1.0.

With that, I am getting 74K for a 12" wide bearing. I always call out full bearing width pads (Full length along the beam).

Just seems high, any suggestions for a different calc? Just un-reinforced concrete?

With phi*2(sq-rt f'c)b*d I get around 9.7k

Even at the lower number, I am still ok. Just curious if I am using the first equation wrong.

 

[r13]

Can you post a diagram for better understanding? Is the bearing depth is 2.25", not the width?

 

[JStructsteel]

Here you go. Really 2" bearing. Bearing will be continuous though down the slabs.

 

[r13]

Now let's get into your load. You stated that you get 74 kips per foot of bearing, the number seems impossible. If I assume you got 74 psi/12", then you have a panel end reaction of 74*12*2=1776 plf along the panel edge, which is still high, but more doable. Please double check your load/support reaction.

 

[JStructsteel]

The 74k is what i was getting for the bearing equation.

I think my capacity is really the un-reinforced concrete capacity, 9.7K/ft, my actual loads are 1.4K/ft DL, 1.15K/ft LL. I am going to put some #3 hair pins at the end of the slab to take that load.

 

[KootK]

Is this hollow core or solid slab?

 

[JStructsteel]

KootK, solid slab, 8" thick

 

[HTURKAK]

The proposed formula is for design bearing strength for bearing on plain concrete . Your case, solid slab is supported on WF with rubber or neoprene pads..

This snap from PCI design handbook 6th ed.

If this is new design, i will suggest to use angle with dowels welded to the angle . You are expected to check the strength for possible vertical and horizontal cracks. There is a worked example at page 4-78.

 

[JLNJ]

If that sketch is to-scale, it looks like the beam flanges will bend, concentrating the reaction toward the beam’s web.

 

[KootK]

KootK, solid slab, 8" thick

Got it, thanks.

1) In my markets, it is most typical to to bear the precast directly on the steel with no bearing pads etc. Once you weld a clip angle connecting the plank embeds to the supporting steel, any future applied loads will bypass the pads anyhow.

2) Direct bearing capacity for these kinds of products is typically very high. Even with hollow core, a stacked plank on CMU building needs to be six to eight stories tall before it even becomes necessary to grout the cores of the plank ends.

3) As you've surmised, plank one way shear capacity may well govern relative to plank bearing capacity.

4) similar to HTURKAK's comments, my main concern for something like this is the end anchorage of the positive moment steel at the support. That, particularly given that:

a) pre-stressing develops slowly and;

b) solid slabs often utilize mild rebar which won't even make it out to the very end of the planks as pre-stressing does.

c) you'll not typically be dealing with a very wide bearing surface.

d) it's dicey to rely on unreinforced concrete capacity in situations like this where in place shrinkage may be meaningful.

5) In a thin solid slab fabricated in North America, I doubt that you'll ever see shear reinforcing or anchored bearing angles at the plank ends as mechanically appealing as those things are. If somebody's experience has been different, I'd love to hear of it. Even using hooked bars is questionable in my opinion. Often the hooks just result in an even greater segment at the end of the plank being, effectively, unreinforced concrete.

6) In North America, we'll typically try to deal with this by getting as much bearing width as we can and attempting to take advantage of partial reinforcing development as one moves away from the support. Frankly, this isn't great. In a thin solid slab fabricated in North America, I doubt that you'll ever see shear reinforcing or anchored bearing angles at the plank ends as mechanically appealing as those things are.

7) In other parts of the world, you'll sometimes see negative bending reinforcement installed in a topping, in the keyways, or in the cores. In my opinion, this is a much better setup as it makes the end anchorage of the bottom steel much less critical.

 

[r13]

So far, your description worries me. I don't think this is an quality/acceptable bearing detail without strengthening/modification.

 

[Agent666]

The bearing load won't be nor does it need to be centered on the studs r13.

 

[r13]

Does not matter, the bearing pad will only feel, and pass the force to the flange, which is my focus concern. BTW, the 2" bearing depth may be a concern for the precast slab. I don't have access to PCI manual, so no comment on it.