concrete tank with large openings

[Structures33]

Hey all - I'm designing a concrete vault for some water lines(it will be empty except for the lines running through). The tentative dimensions will be 6' wide x 20' long and approx. 12' high. The top of the tank will be flush with the ground with some hatches for access. I'm planning to use the PCA publication for Rectangular Concrete Tanks but I'm concerned about the large water line openings. There will be a 42" dia. pipe running through each of the short ends... possibly the other direction as well but don't know yet. What is the general consensus on designing for these openings? There aren't any provisions in the PCA book about openings. I wouldn't be concerned if they were small openings but 42" seems large.
Thanks,
JAH

 

[twinnell]

The roof slab will span the short direction, obviously, and their won't be much lateral load on the end walls because of the pipe openings. Above and below the openings, design the concrete to span horizontally to the long walls for both lateral loads and vertical self weight load. Below the pipe you will need to design the wall to support the pipe, but that shouldn't be a problem. Use the PCA manual but design the long walls as a plate rather than a box. Fixed bottom, hinged top. Unless the tank will be backfilled prior to the top slab being placed. Then you can design it as fixed base free top, or have the contractor shore the walls prior to backfilling.

Be sure to check buoyancy, don't want this thing floating out of the ground. Also, if this is near the coast you need to consider sand and water above the tank. We had to design one for 10 feet of saturated sand on top of the top slab. Top slab ended up being 24" thick due to the span.

 

[civilperson]

Make your access hatch traffic rated or use precast top with lifting lugs. Any rebar interrupted by openings can be added around perimeter at 45 degree angles, (one 45 degree bar replaces one vertical and one horizontal bar stopped short by hole.)

 

[twinnell]

At openings, we always added the bars that were terminated to each side of the opening. Then we also put 45 degree bars at each corner of the opening (if round opening, place 45 degree bars as if square opening). Say 4 vertical and horizontal bars cut due to opening...put 2 bars on each side and 2 bars above and 2 bars below. Then put the 45 degree bars at each corner.

 

[MarcbSE]

Agree with twinnell on the cut bars. ACI specifies that you have to replace the cut bars with additional bars typically placed adjacent to the opening. I would add these bars for strength and then additional bars at 45 degrees to help control cracks.

 

[miecz]

For additional info, see thread507-170996.

 

[civilperson]

Slab openings and wall penetrations in 6' x 12' vault are not the same. More steel is OK but not required by ACI, the pipe wall is reinforced and forces at right angle to the end wall are not gravity as in slabs with holes.

 

[HvZ]

Without the openings I would design the end wall as a plate fixed at the base and the sides using the PCA table or those in Engineering Monograph 27.

For small openings at the base you can cut and add reinforcement as standard. However,you have to be carefull if the openings is near the base are large in relation to the wall. If the width of openinings is a large percentage of the lenght of the wall, then you are cutting of the support provided by the base and then I would design the wall as spanning one way between the side support with the loading taken at the center of the pipes.

I do not khow of any guidance anyware. You just have to make your own assessment.

As a note. With tanks below or above ground, i always design the wall as free at the top. If you allow for a pinned or fixed connection to the roof, chances are good the the contractor will want to backfill or hydro test the structure before adding the roof and you will be ask to check and possibly redesign the walls.

Cheers

HvZ

 

[miecz]

I believe the governing code here is ACI 350. Chapter 13 of ACI 350 applies to tank walls(13.1.1). Article 13.4.2 requires "An amount of reinforcement equivalent to that interrupted by an opening shall be added on the sides of the opening."

For a tank with those dimensions, the forces in the end walls will be heavily influenced by the long side walls. I would expect high bending moments about the vertical axis for the entire six foot width of the wall.

 

[civilperson]

Interesting application of Code Requirements for Environmental Engineering Concrete Structures to an end wall penetration of a vault. The code requirements are meant to produce essentially liquid-tight, minimal leakage performance under normal service conditions. The forces on the end wall may be calculated and resisted with a designed amount of reinforcement or by prescriptive replacement amounts of steel as openings in slabs. As many as six bars,(assuming six inch spacing), could be interrupted in each layer and direction. Thus 12 additional bars are required at each side and top and bottom of penetration, (plus diagonal bars for crack control). This amount of steel is hard to detail and still have clearance between bars. An option to this steel amount is special analysis,(13.4.1).