Manhole large hole in small slab?

[Redacted]

Hi there, I need to design a few traffic rated manholes to pull some fibre cables (they cannot be pre-cast). They will be considered submerged and conservatively designed with the water table at the top and cast underground. The manhole structures are quite small 6'x6'x4' (lxwxh); the client wants to have one of these large manhole covers used. See below, the proposed manhole cover in light blue and the manhole wall in light brown.

I am currently thinking of the quickest and easiest way of analysing and designing the cover slab with these large holes (by hand).

I am thinking of setting the slab thickness as 8" and either designing this using the method of strips with strong bands surrounding the holes/on free edges or just designing it as a 2' wide beam that is 8" tall. My main concern would be the torsion as the beam/slab would essentially be supported on 3 edges(if considering the first schematic), with the manhole cover frame concentrating the forces on the free edge (or am I overthinking this?).

Is there a better way of analysing/designing this that I am not thinking of?

 

[BAretired]

One of us is blue-green color blind. Maybe it's me!

Will the cover be steel plate, steel grating or something else? How thick is the cover? Can it be open on top or must it be waterproof? Will road traffic include heavy wheel loads?

You might want to use more than 8" thickness and armor the edges with steel angles to support the cover, well anchored into the concrete. Design the beams to span clear, but provide dowels into the wall to resist torsion. Saving a few bars is not an appropriate goal for a project like this.

BA

 

[dhengr]

Redacted:
Who cares about the exact size as long as it meets some min. dimension requirements to get some equipment in and out of the pit? It seems to me that it would be easier, in both cases, to consider the top slab spanning the full width, and built as three distinct pieces. The top & bot. slab sections span the full width of the pit, supported on three sides by the pit walls, and are not generally easily removed, maybe waterproofed, etc. Their inner edges have a formed lip (shiplap style?) to mate with the middle slab section, which is the removeable cover (manhole cover). This cover slab spans the full width too, a slight span increase, but with good end bearing conditions, and good side mating conditions. This approach should make the design and analysis much easier. It should also make the forming and the rebar much easier to deal with.

 

[Redacted]

@BAretired perhaps that is me haha, I guess it is an aqua green. The client wants a traffic rated square steel manhole cover. They will order this in. I would say the traffic loads for these structures will be quite light as they are not on a main highway subject to frequent traffic.

@Dhengr That's true and I think we are on the same page tbh. The manhole will just be used for subsea cables as they transition onto land. I was planning on just proposing the first ~6'6x2'8" manhole cover one where the cover can bear on the walls on the short edge, whilst the long edge would bear on the adjacent slabs.

I'm not sure if I did the analysis the right way but I just designed the side slabs that the manhole cover is resting on as a cantilever. Is this the correct way to go about this? I also included some strong bands on the free edge to stiffen it a bit (although these prob weren't really necessary?).

See the detail I am thinking of below (does this seem reasonable?):

The only real constraint is that the manhole needs to be traffic rated steel and with the cover able to be lifted off manually. However, this isn't really an issue, I found a supplier for this.

For the design, I considered :

Deadloads ---> 1' of soil on top of the slab (5.5kPa), slab self weight (4.9kPa),
Live loads ---> Traffic load of 5kPa and a 9kN concentrated load (jacking a car or something) on the free edge.

The system ends up needing T12@10" c/c for everything (footing, wall and top slab). In the case of the wall and footing, this was pretty much just the minimum steel required by the code.

Is torsion reinforcement/checks required in situations like this? Or could I just take the slab cantilever bending moment and apply it to the bottom of the wall and design the wall/footing for the additional moment?

 

[Redacted]

Hi all, I never really got this question resolved.

I don't normally need to deal with torsion directly in my concrete designs, so I guess I never really got a firm grasp of the concept.

I essentially have a slab 8'x2' (not sure whether to consider this a beam instead of slab?); that is supported on 3 edges as shown above.

The slab is tied into the walls as a fixed connection. I'm essentially designing the slab as a cantilever in the short span and will provide enough reinforcement to make it stiff enough to minimize deflection.

From carrying out a short desk study, I see that torsion reinforcement is normally applied to the corners of two-way slabs, where the two edges meet. I'm assuming this is to prevent the edges from turning up. However, isn't this what the cantilever rebar would be doing as well?

Not sure if I should be considering torsion and providing minimum torsion reinforcement + the required/minimum tension reinforcement?

If the torsional reinforcement is still required, would that change if I can reduce the cantilever deflections to a negligible amount? If there are negligible deflections, how can torsion occur?

 

[bones206]

One way I approached this is to design the long-direction strips of the slab as beams - one on each side of the opening. Then design the short-direction strips as a one way slab spanning between the wall and beam.

I haven’t given torsion much thought, although I have seen some large concrete chambers with big openings where they actually detailed the long strip “beams” as actual beams with stirrups, flush within the slab thickness.