reinforcement around openings for catch basins

[Guest]

Usually, when precast the holes for the pipes of catch basin, the detail shows the reinforcement (rebar) around the opening. Previously, some details show the extension is 2 feet typical. But when you have a small structure with a fairly big pipe connection, the rebar will extended into the structure wall. Either the contractor need cut the rebar, or bend it. Do you guys have any idea how far the extension (one rebar extend beyond another perpendicular rebar) need to be? Personally I think 2 feet is more than enough, and that rebar is only to prevent the concrete cracking. And trust me, it is not a typo of 2 inches. I already confirmed with other engineer.

How do you guys think?

Thanks.

 

[MonteBiker]

Usually, when precast the holes for the pipes of catch basin, the detail shows the reinforcement (rebar) around the opening. Previously, some details show the extension is 2 feet typical. But when you have a small structure with a fairly big pipe connection, the rebar will extended into the structure wall. Either the contractor need cut the rebar, or bend it. Do you guys have any idea how far the extension (one rebar extend beyond another perpendicular rebar) need to be? Personally I think 2 feet is more than enough, and that rebar is only to prevent the concrete cracking. And trust me, it is not a typo of 2 inches. I already confirmed with other engineer.How do you guys think?

Thanks.

I would say that as long as you satisfy your development length past the intersection of the bars, you would be pretty safe.

 

[MA_PE]

Agree that the rule of thumb is the development length beyond the opening.

How critical that is depends on what you're doing. Depending on the size of the opening relative to the structure wall that reinforcing may or may not be needed structurally. The generic drawing show an undimensioned hole in an undimensioned wall and calls for a #6 bar on each side. The 2 ft sounds reasonable to me. Note that the development length for a No. 6 bar per ACI 818-05 is approximately 40 in.

Your drawing also requires 2 in. cover from the hole but doesn't say anything about placement in the wall. If the wall has an inside cage and an outside cage, where do you place the diagonal bar?

Another rule of thumb is to replace the steel are cut by the opening with supplemental bars around the perimeter.

I hope there are limits on the hole size.

But when you have a small structure with a fairly big pipe connection, the rebar will extended into the structure wal

yes it will and you may need to check whether that opening has removed enough of the structural wall to cause a problem. When we design structures with pipe penetrations we never assume any contributing strength from the pipe. The structure nneds to handle all of the load.

 

[Guest]

That's what i thought. I thought that should be 2 inches, not 2 feet. But some other details also shows 2 feet.

For me, 5 inches should be enough for the cracking prevention. 2 feet is far way more than enough.

I would say that as long as you satisfy your development length past the intersection of the bars, you would be pretty safe.

 

[Guest]

Thank you MonteBiker. I believe for structures like walls, the opening for windows and doors. 2 feet is reasonable. But for storm structures like inlet, catchbasin, the pipe is small like 6 inches, or 12 inches diameter. The 2 feet development (that you guys call it) length is little too big. I saw the rebar for the tops (cap of catchbasin), they maybe more than 4, with shorter development length.

I would say that as long as you satisfy your development length past the intersection of the bars, you would be pretty safe.

 

[MA_PE]

sorry, but the what the heck are you talking about?

5 in. is no where near the development length of a #6 bar. see above it's about 40 in.

You've got to stop just applying this detail to everything, it doesn't work that way.

For a 6 in. hole you likely don't need any diagonal bars, for a 48 in. hole those single #6's with 2 ft extensions may not be enough.

You can reduce the standard development length by the ratio of the required steel area / provided steel area but you have no clue what the demand is.

 

[MonteBiker]

sorry, but the what the heck are you talking about?
5 in. is no where near the development length of a #6 bar. see above it's about 40 in.

You've got to stop just applying this detail to everything, it doesn't work that way.

For a 6 in. hole you likely don't need any diagonal bars, for a 48 in. hole those single #6's with 2 ft extensions may not be enough.

You can reduce the standard development length by the ratio of the required steel area / provided steel area but you have no clue what the demand is.

I am going to have to concur with MA_PE on this one. This was more along the lines of my reasoning before.

 

[Guest]

Thank you for both of you.

The question comes out when we try to use it for any structure and pipe size. And recently, I attended a seminar by American Concrete Institute. They changed the code for '05 to '08. They shows rebar at windows or doors openings at minimum 2 feet before. And they changes the requirement to the development length to satisfy fy tension. Which for me, make it more complicate, even need the calculation for that length. Don't even know how to do it.

Most likely this detail will be continously shown on drawings, and let precast manufacture bend the development rebar if they are too long. Normally, the storm or sanitary pipe are not that big as 48" diameter.

Thank you again.

I am going to have to concur with MA_PE on this one. This was more along the lines of my reasoning before.

 

[MA_PE]

ahaninil:

what side of this industry are you on? The design/specifying side or the presast/manufacturing side. If you're on the design side, then I suggest that you have someone spend a little time actually designing these "standard" or "typical" details. Also, placing one big bar at the center of the wall isn't really helping cracking either. The wall will likley develop cracks from flexure and the reinforcing should be close to the faces.

You should develop different configurations based on the size of the hole and the thickness of the wall. They'd work much better for all concerned and make a lot more sense.

Something like #5 diagonals each side/each face would be much better than a single #6.