Very short concrete edge support

[bugbus]

I have a situation where I need to support some heavy precast concrete trough covers (lids) along a cast in place concrete wall. The covers are 200 mm (8") thick and will support pedestrian loading as well as some vehicle loads. They also need to be removable to inspect the trough below. Due to various constraints, I only have about 50 mm (2") of bearing width between the cover and the wall as shown below. Also, the clear space within the trough cannot be encroached on, so there is no possibility of supporting the covers on a ledge or bracket.

I have a few concerns: (1) bearing failure at the edge of the cover or wall with possibility of a diagonal crack forming; (2) anchorage of the reinforcement; (3) general wear and tear around the edges of the concrete as the lids are occasionally lifted and replaced and shifted around.

My concept is sketched below. I believe the only way of doing this would be to cast in some steel angles that would be anchored into the respective concrete elements with welded reinforcement. But this would require a lot of welding, which might be problematic.

Is there a better way of doing this that doesn't involve so much welding?

 

[bugbus]

Closer detail of the support area

 

[KootK]

I definitely think that you're on the right path with respect to the anchorage of the bottom reinforcement. With regard to the welding, I would propose:

1) Shop weld some rebar couplers to the angle.

2) Field install some threaded starter bars into the couplers.

3) Lap your bottom steel with the starter bars.

This won't actually save any welding but it should lower the cost of the welding and handling substantially.

 

[KootK]

I would also use a longer horizontal angle leg and weld some vertical, headed studs to it for added durability.

 

[Klitor]

Cant you make kerb 30-50mm thinner (only place one layer of reinforcement in the center)?
It is a standard powerplant detail around here.

 

[bugbus]

Thanks KootK

Klitor, yes it is an option, but there also needs to be a handrail post bolted to the middle of the kerb. With the two layers of reinforcement, the bolts would sit nicely between the two layers. With a single layer, there may be a clash. But it is something I will consider.

 

[phamENG]

I don't have any specific ideas, only more problems. With so little space, make sure you consider erection tolerances for those precast walls. If you're an in "every millimeter counts" sort of design...you may need to specify tighter tolerances.

 

[EcoGen]

Yeah 2 inches seems narrow. Your proposed edge angles are a good idea regardless since hatch panels like these that are removed and reinstalled most likely damaging the concrete corners over time. I'm not sure why you would need to weld the edge angles to the steel reinforcing? You could just have headed diagonal studs at 2'-0" on center welded to inside corner of each angle. That would all be shop welded and cast into the concrete corners.

The 2 inch bearing width is still a concern though. I would consider cutting into the wall an inch or two more. Or make sure you're satisfied via calcs that the 2 inch bearing is sufficient.

 

[dik]

Or a 45deg stud at the fillet... 2" bearing should be adequate... I've got a 6 storey building down the street with 6" CIP shearwalls and HC slab with 2" bearing...

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

-Dik

 

[BridgeSmith]

...will support pedestrian loading as well as some vehicle loads.

...there also needs to be a handrail post bolted to the middle of the kerb.

No traffic rail required?

Anyway, for a handrail/pedestrian safety rail, it should be possible to design the kerb for the handrail load with a single layer of reinforcing.

You should be able to anchor the angles with automatically end-welded studs (Nelson studs).



Rod Smith, P.E., The artist formerly known as HotRod10

 

[bugbus]

I'm not sure why you would need to weld the edge angles to the steel reinforcing?

It is not so much to do with the anchorage of the angle itself, but the reinforcement in the slab.

Given such a short bearing length, this would not satisfy the anchorage requirements unless the bars are anchored by welding or some other means.

On the wall part, welded studs should be enough.

I've got a 6 storey building down the street with 6" CIP shearwalls and HC slab with 2" bearing...

May I ask how the flexural reinforcement in the slab is anchored over such a short bearing width?

No traffic rail required?

Anyway, for a handrail/pedestrian safety rail, it should be possible to design the kerb for the handrail load with a single layer of reinforcing.

I should have mentioned that the vehicle loading is for maintenance only. It is a dedicated pedestrian path.



Thanks again

 

[dik]

The floor slab construction is 8" prestressed hollowcore slab. My drafting from 50 years back...Bearing is less than I remembered; it's 1-1/2". HC bearing material is not shown, it used to be 1/8" untempered masonite.

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

-Dik

 

[bones206]

Might want to consider armoring the top of the curb as well, since it will likely get beat up when lowering the cover slabs onto the pit. If you do armor the curb with an angle or channel, that would provide an opportunity to attach the guardrail without anchor bolts, which in turn would make it easier to narrow down the curb section as others suggested.

 

[bugbus]

Good point bones206, thanks

 

[KootK]

May I ask how the flexural reinforcement in the slab is anchored over such a short bearing width?

With 2" or less of bearing, I'm not sure that it is anchored, even for a hollow core slab product. Frankly, I've never been clear on how the precast folks get away with the anchorage condition at the ends of hollow core plank. Pre-stressing strand develops more slowly than mild rebar. As far as I'm concerned, plank is effectively unreinforced and unprestressed concrete 2" in from the end. Agent666 has posted some interesting stuff in the past that leads me to believe that they're paying more attention to this in NZ than we are in North America.

In my opinion, a lot of the details that address this wind up, effectively, creating a condition where there's hogging moment at the support rather than zero moment (simple support). Composite toppings with rebar etc.

It is not so much to do with the anchorage of the angle itself, but the reinforcement in the slab.

I see it the same and am glad to see another engineer taking it seriously.

 

[JStructsteel]

I would put some dowels direct off the horizontal leg, and then turn them down. into the outside face.

 

[dik]

Headed studs at 45deg off the angle fillet.

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

-Dik

 

[dik]

Neither am I, but it's common with HC construction and has been for decades.

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

-Dik

 

[KootK]

Neither am I, but it's common with HC construction and has been for decades.

I know it. I've probably designed a few hundred kilometers worth of plank since 2016. Another thing that I know about plank is that it almost never gets taxed in gravity applications. If it did, there would be cracks all over the place and frightened villagers running for their lives. And, perhaps, the odd catastrophic shear failure.

 

[dik]

Just did a quick search of the prestressed concrete institute's site and could find nothing... not even a space to post a question I wasn't trying to be glib but I suspect the strand tension and the anchorage... likely starting from 1/4" from the end... I suspect it takes a couple of feet to develop the smooth high strength strand. It's an interesting question.

On the apartment down the street there were only two issues that I recall. I checked with the precaster and 1-1/2" bearing was OK (Nothing in writing though). The first one was the use of HC slab for banconies. The engineer from con-Force that I was talking to said HC slabs should not be used for exterior slabs and I asked and received a letter from him on that which I promptly gave a copy to the developer. He ignored the letter. It was his building and liability insurance wasn't essential back then.

The other issue was that after the first two lower storey walls were stripped, the next day he started placing HC slabs. As expected the top of the wall started spalling off and I think this was the first repair detail that I had to prepare. I did the structural and architectural drawings for it (you could do that back then). I asked him that he should hire an architect... my aesthetics are terrible. He declined, and the building is 'ugly'. The mechanical and electrical were shopped out to mech and elect subtrades who provided sealed engineering drawings. I think this was my first big project on my own. I'm not sure, but I think 1975.

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

-Dik