Cast in Place Culvert 5

[structuralaggie]

Some civil punk came up with this great idea to burry a culvert 60+ feet below finish grade and it was left up to me to design it. No big deal, I got the loads from the geotech and designed it. It looks more like a bomb shelter than a culvert, and would probably work as one too, unless it is the monsoon season. This thing is stout and heavily reinforced, but I have not put any sort of expansion or control joints in it. I really think it is fine, just let it crack. Since this is my first culvert to design, I was just wondering if anyone else has designed one of these things and if control joints are typically used. Thanks.

 

[civilperson]

An arch shape can probably save you 60% of the materials cost. I.E. 10" walls/ceiling vs. 24" top and 12" walls. No control joints are required,(if ends are not restrained). The vertical loads can be safely reduced by soil arching, (compared to a vertical prism of overburden). Turn downs and scour protection at the ends will assure a usable life span.

 

[structuralaggie]

An alternate was proposed for the culvert by a precast arch manufacturer. The example dimensions you gave are close to what I saw. I hope they go that way, but for my case you don't think I need any control joints? The precast system would naturally have some at each segment.

 

[Qshake]

If you read the DOT requirements on such things you'll find that some states construct culverts in 50' sections and others do not. What I typically see in the midwest is that culverts less than 80' are constructed without joints and those that are greater have joints every 45 or 50'.

A continuous pour of 100' feet of concrete is rare, especially when it is 12 to 24" thick as may be the case for the base slab. I know that bridge decks are routinely placed in one fell swoop but they are thinner.

So if you'll wind up with a cold joint, you might as well plan for it and make it a contraction or expansion joint.

Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[cvg]

generally around here we pour in a checkerboard fashion to reduce or eliminate shrinkage cracks. Expansion joints shouldn't be necessary since extreme temperature fluctuation is not to be anticipated below ground. Contraction is eliminated by the checkerboard. For high embankments, we would use waterstops at the joints.

 

[dik]

I'd probably put a CJ at midpoint and have a joint at the inlet and outlet structure. I use the PVC 'flat' type of joint that is tacked to the inside of the form... They don't collapse. What are the x-section dimensions of the culvert? Did the geotekkie consider full height of the fill? Can you put a layer or so of geotextile to promote arching action?

Dik

 

[structuralaggie]

The inside dimesions are 8'x10' and it is about 1000 feet long. The geotech gave me equvialent fluid pressures, so it was up to me to add the depth component.

I agree with the no need for expansion joint, and I doubt it will be a continuous pour. So my construction joint should allow contraction? Or not not even worry about it? I was just thinking of showing a shear key and continiuing the hor. reinforcement throuh the joint. If I should allow for contraction, I would put smooth dowels in place of the hor. reinforcement. So, contraction joint or no contraction joint? Thanks.

 

[cvg]

I vote no contraction joint, unless you are also willing to install waterstops

 

[structuralaggie]

I thought about that too. The last thing I wanted was to do was to allow water into my culvert! I'm thinking no contraction joint too.

 

[dik]

2' slab for a 10' span seems a bit much... I would have thought 1/2 that thickness... Use proper corner bars and not just top dowels. You should query your geotekkie and get a real design load envelope from him. 1000' long should have joints at 50'+/- o/c. Maybe a typo?

Dik

 

[structuralaggie]

It is 1000' long. I know I am going to have construction joints at intervals like that, but my question is if I need to allow for any movement in them.

As for the thickness, I designed it based on the geotech's loads. Without using shear reinforcement, it gets pretty thick. I'm thinkink civil person must have done one of these. He had a good guess for not much info.

 

[Qshake]

My experience is that the 2' is pretty good for the depth and span.

However, I do not agree with the continuous pour, there is no way that will happen especially in the walls. Usually the walls are around 12-18" thick and with all the steel can and height of the forms can require extra attentino to consolidation of the concrete. The pour will move along slowly and there is time limits on the amoutn of time a truck can sit on site. And the worst thing you want is a horizontal cold joint in a wall.

Do everyone a favor and place the joints on 50' centers and use a waterstop (good idea by the way). The laborers and inspectors will thank you.

Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[structuralaggie]

Ok, ok! I know I will have construction joints at least every 50', if not more. I know it won't be continuous.

Your wall thickness estimate is right on. I am glad some of you have come up with the same sizes I did. Some of these guys around here thought I was killing it.

 

[LCruiser]

Talk to Hydroconduit or somebody else that does pipe and check out a 108" pipe alternate.

 

[mitchelon]

Contraction joints are, for the most part, nice looking cracks. This is done on exposed slabs/walls where you want to have a defined and uniform pattern of cracks and not a set random crack. They do not prevent cracking. I think the only control joints you need are construction. Will the top of the culvert be also in the ground?

 

[civilperson]

ConArch can construct the culvert in place using their patented rolling arch forms. No expansion or contraction joints are required. Close spacing of steel, (less than eight inches ), will keep cracking to a non-visible size.

 

[LCruiser]

I would say to use corrosion-resistant or epoxy covered rebar. Basic reinforced concrete box would be fine with that rebar as well though - joints only as required for construction. Constructibility gains would more than offset the extra for the better rebar. I'd still compare with pipe though. It's near the crossover size where box becomes more cost effective.

 

[structuralaggie]

mitchelon - yes, the top of the culver will be well below grade, close to 60'. Thanks for the tip, I am thinking along the same lines you are.

civilperson - a Con-something company may end up doing this as an alternate. Out of curiosity, how can the walls be so much thinner? Shear was controling the thickness, since I didn't use any shear reinforcement. Is shear reinforcement used in these arches?

 

[civilperson]

The strength of the arch shape is similar to pipe in that the wall/soil interface is a major strength component. Without suitable backfill, the loads would crush the pipe or the arch. The arch has the advantage of compressive forces throughout the wall/ceiling. Shear strength of concrete in compression can be 32 times more than square root of f'c,(compared to 2 or 4 or whatever the long arithmetic formula gives). Talk to ConArch in Arizona, top notch engineers on staff for really good design,(an education in reading their calculations).

 

[miecz]

If you don't put control joints every 40-50 ft, you'll get random cracks that will leak until dirt fills the cracks. If you can live with a little leakage, then forget about it.

If not, then make your construction joints into control joints. Cut out half or all of your longitudinal reinforcing at the joint and put in a waterstop. Also, your longitudinal reinforcing between joint should be a minimum of .005 of your concrete area, according to Table 7.12 of ACI 350, Code Requirements for Environmental Concrete Structures. I'd use #7@10" for 24" walls.