Valve Vault Top Slab w/ Multiple Access Hatch Openings

[EastEng1012]

Hey All,

I am designing a cast-in-place rectangular valve vault structure. Owner requested CIP instead of precast. There are several structures with different dimensions (5'x10', 5' x 15', 10' x 20'). They are 6-feet deep. They will have a non-traffic live load (300 psf). We originally designed it as a one-way slab (10" thick) with one aluminum access hatch to enter the vault.

Now, the staff is asking for multiple hatches in the cover slab for the longer structures. The reasoning being they want to avoid entering a confined space as much as often. So with multiple hatches they can visually check valves and even reach down and operate them from the top slab if need be. They want off the shelf products (no custom hatches). So sizes of opening are based on Halliday product catalog.

Our mechanical engineer sent a sketch of the proposed openings (see attached figures) and some of them concern me with how the load will be distributed around the openings. To simplify, I looked at it as beam segments in each direction around the opening. Then distributed the uniform live load. I am trying to avoid FEM. I am also concerned with the sections that would essentially cantilever from the wall to the edge of the opening. I assume I will need beams configured to support the top slab with the multiple openings. However, I feel like i have seen top slabs with multiple openings like this before.

Any thoughts on how you would approach this? Or am I over complicating this?

Thanks!

 

[HTURKAK]

In past i used steel profile for a similar case.The perforated slab can be supported on I profiles in the range of 8 in WF (SAY HE 200 ) can be used below the CIP concrete slab..

 

[EastEng1012]

Did you use a steel deck or just the wide flange supporting the underside of the slab?

 

[r13]

It may help, if you have roof plan with hatch openings shown.

 

[EastEng1012]

Sorry I thought I attached the sketch in my original post (see attachment)

 

[r13]

I thought those are floor plans. Anyway, here are my suggestions, assuming uniform slab thickness. Adjustment can be made to better arrangement, and to add a few edge closures.

 

[EastEng1012]

Retired13 - yea sorry, those are floor plans, the hatches were highlighted in that plan view to show the key equipment the openings would cover. The roof plan has not been updated yet to include the new hatch openings.

Thanks for suggested layout of beams. That was generally what i was thinking was necessary. Couple of questions though:
1. By embed beam, do you mean including a beam within the slab (the same thickness) but including shear reinforcement along the beam strip and additional
flexural rebar as necessary?

2. How generally would you assume the distributed load from the hatch would be distributed to concrete? Equal load to each side?

 

[r13]

1) Yes, design the narrow bends as beams with flexural and shear reinforcement. For shear reinforcement, you may use u bar combined with a tie (with end hooks) on top. You might need to increase the slab thickness to alleviate congestion though.
2) The load on the hatch cover is transferred to its supporting frame using tributary area method (triangle and trapezoidal shapes). The transferred load can either be line load around the hatch (for frame with continuous edge embedment), or concentrate load at the isolate/individual edge support embedment. Both load types will introduce torsion on the concrete beams. For the latter case, The concrete strength to secure the embedment in place should be checked.

 

[HTURKAK]

Did you use a steel deck or just the wide flange supporting the underside of the slab?

The wide flange supporting the underside of the CIP R.C. slab..

In your case , you may provide the steel beams supported on the recesses on the walls ..

 

[bridgebuster]

In NYC, we regularly use W8 for utility vault roof slabs when we need to provide framing for hatches. Rather than casting the slab on top of the beams, the W8's, are buried in the roof slab, which is set below finished grade. The slab will crack along the beam flanges but we waterproof the roof and install a blacktop or concrete topping.

There's advantages to both methods of roof construction. If the beams are in the slab it eliminates having a low point in the vault and installing the hatch is easier. Putting the slab on the beams eliminates the waterproofing and the topping.