Shear Reinforcing of a Box Culvert

[JSanchez88]

Hello,

I have been asked to design a new CIP box culvert that is to replace an existing culvert. The new culvert is 13'(W) x 15'(H) clear inside dimensions. I have already looked at several DOT (TX, KS, and MN) guides along with ASTM C857-12a and applied loads to it accordingly:

1. Vertical Earth Load on the roof
2. 3 and 4 axle HS20-44 Loads traveling across the culvert (applied in separate conditions)
3. (2) trucks side by side traveling parallel to the culvert
4. Lateral earth pressure when the water level is both above and below the culvert
5. Corresponding hydrostatic pressure when water level is above the culvert
6. Surcharge of the wheel loads
7. Interior hydrostatic pressure when the culvert is full


When figuring the slab and wall thicknesses without shear reinforcing, they become excessively large (21" - 28"). When looking at the plans for the existing culvert(which is dated around 1960), their design calls for 12" thicknesses. I've also seen several box culverts being designed to have wall thicknesses between 12-18" and slabs up to 24" - but not up to 28!

I know recent designs are made to where the thicknesses are just large enough to not require any type of shear reinforcing but are not needing to be 21-28".

My question is: Am I taking something into account that I shouldn't? If not, are there any recommendations to yield a more suitable design thickness?

 

[TehMightyEngineer]

Footing pressure.

3 sided arches have almost the same amount of concrete and without the bottom slab tying things together the flexural rebar goes up. You will save money using a 3-sided culvert on low soil cover, long span, low height culverts though.

JSanchez88: Now you've got me really curious. Is there some way you can post your project inputs somewhere so that I can work up a parallel design? I'm really curious what happened to give you such a large required thickness.

Maine EIT, Civil/Structural.

 

[ztengguy]

With good soil, I cannot see the footing being over 3 feet wide for this structure at a couple feet cover, and also the leg/slab thickness being over 8" thick. Those thicknesses would be good from 1' up to about 15' of cover.

 

[TehMightyEngineer]

Right, but these culverts are usually beside streams with poor soil. Combine that with 15 feet of cover and a large span and you now need a 6' wide footing on either side. About the same as your span. But, yes, for a low cover, long span with reasonable soil it usually is better to do 3-sided vs 4-sided.

Maine EIT, Civil/Structural.

 

[ztengguy]

Right, but you are saving 33% concrete on the other 3 sides, plus shipping, crane size, etc. And you still keep greenpeace happy with a natural stream bed.

 

[TehMightyEngineer]

Not necessarily, the concrete ends up being the same because, without the bottom slab, the walls span twice as far doubling the shear and flexure in the corners.

But, yes, often you will save money on shipping and crane size. Locally we are allowed to do 4 sided culverts with buried bottom slabs and erosion barriers that satisfy the natural bottom requirements but some municipalities have still required the 3-sided structures so that the little fishes are perfectly happy.

Overall it entirely depends on the job. I'll just say that I usually see more 4-side culverts than 3-sided for the precaster we do a lot of work for.

Maine EIT, Civil/Structural.

 

[ztengguy]

Maybe the next time you get the chance, look into a 3 sided. if you are over 24' span, more than likely it will be cheaper, if precaster doenst have any pre made.

 

[TehMightyEngineer]

I've done multiple designs where they precaster wanted to know which way worked better. It was extremely dependent on cover, span, wall height, bearing pressure, loads, etc. But, yes, we have found that longer spans usually lend themselves to being cheaper with 3-sided. Deep culverts with short spans and high walls almost always work better as 4-sided. The precaster never has them premade, they vary far too much between jobs.

Some examples:

10' span by 8' height, 7' soil cover. Location precluded crane, 4 sided was cheaper, though. 2,000 PSF gross allowable per geotechnical. Ended up doing a 3-sided sitting on a continuous footing (so a two part 3 sided).

20' span by 7' height, 3' soil cover. 3-sided was cheaper by far. Actually ended up casting part of the walls into the footing so it was almost a slab bridge on frost walls. Helped the precaster a lot though because a full 7'x20' would be hard to ship.

4' by 4', 4' soil cover. 4-sided all the way.

24' by 4.5', 1.5 ft soil cover. No geotechnical report so we designed for 2,500 PSF and had contractor verify no poor soils were found. Installed on crushed stone fill with geotextile. 3-sided was selected as it was cheaper than 4-sided. Footings were 6' wide and the low cover prevented the soil reaction from helping the top slab flexure out. Resulting top slab thickness was 19" (deflection controlled strangely).

Maine EIT, Civil/Structural.

 

[TehMightyEngineer]

By the way, not saying you're wrong ztengguy, just providing what I've seen. You're absolutely right that 3-sided can (and sometimes will) be cheaper than 4-sided.

Maine EIT, Civil/Structural.

 

[ztengguy]

Im sorry, when I talk about 3 sided, I am referring to a Conspan type shape. There is even a arch looking shape now with the legs flared out.

 

[TehMightyEngineer]

Ooooooohh, I thought you might be but figured you weren't. My misunderstanding, yes, arching action will help a great deal. This is what I was referring to:

http://www.concretebuildingsupply.com/content/DSC03762 large.JPG

Maine EIT, Civil/Structural.

 

[ztengguy]

Yea, 3 sided boxes have their place, but very inefficient. Spans less than 16', say ok, over that, forget it.

 

[TehMightyEngineer]

Yeah, I don't know of any local precasters who do the arch shape culverts. Not quite sure why.

Maine EIT, Civil/Structural.

 

[JSanchez88]

My inputs are as followsAt 6ft below surface/at 9ft below surface)

1. Vertical Earth Load on the roof = 720/1080 psf
2. 3 and 4 axle HS20-44 Loads traveling across the culvert (applied in separate conditions) = 175/84 psf (4 axle)
3. (2) trucks side by side traveling parallel to the culvert = 350/252 psf
4. Lateral earth pressure when the water level is both above and below the culvert:

Above water table = 360-1320/540-1500.
​

Below water table = 227-634/260-720
​

5. Corresponding hydrostatic pressure when water level is above the culvert = 375-1373/562-1560
6. Surcharge of the wheel loads = 160 psf applied to top 2' when culvert is 6' below grade (does not apply when culvert is 9' below)
7. Interior hydrostatic pressure when the culvert is full = 0-936 psf
8. Horizontal pressure of backfill(nothing on top yet) = 0-1920 psf
9. Construction load = (2) 6.6k point loads traveling across the section 13.5 feet apart

I want to note that the reason my thicknesses were so large was because I was trying to get a design that would not require any shear reinforcing. So, after determining that shear reinforcing was going to be required, I refined the thicknesses.

 

[TehMightyEngineer]

My calculated inputs for the 9 ft soil cover case. 13' span x 15' height:

1. Vertical Earth Load on the roof = 1296 psf (significant difference)
2. 3 and 4 axle HS20-44 Loads traveling across the culvert (applied in separate conditions) = 195 psf over 9.833 ft length for 3 axle HL-93, 152.5 psf per axle area (9.833 ft) for the tandem axle load
3. (2) trucks side by side traveling parallel to the culvert = not checked (single lane loading per AASHTO article 12.11.2.1)
4. Lateral earth pressure:

Above water table = 540-1580 psf (almost the same)​

Below water table = 259-758 psf (almost the same)​

5. Hydrostatic pressure when water level is above the culvert = 468-1550 psf (almost the same, assumed water table 1.5 ft below grade)
6. Surcharge of the wheel loads = 136 psf applied to upper 4 ft (live loads are disregarded 8 ft OR span length below grade)
7. Interior hydrostatic pressure when the culvert is full = 0-936 psf (same)
8. Horizontal pressure of backfill (nothing on top yet) = same as lateral pressure above (interesting, why do you have a higher soil load? Is it because you are assuming it's uncompacted?)
9. Construction load = assumed not to control

Overall mine were +/- the same as yours so it doesn't appear your inputs are off. My equivalent strip width was 16.67 ft per axle.

I assumed a 12" slab and walls. My factored shear load at d from the face of the wall was 16,124 lb/ft (though the assumed 8" haunch). My factored shear resistance of the haunch + slab was 19,672 lb/ft (the slab was actually higher strength using the shear strength for culvert slab shear equation of AASHTO article 5.14.5.3 (23,864 lb/ft).

I came up with a rough rebar size of #6 bars at 5" o.c. top and bottom (parallel to span) with longitudinal rebar as #4 at 16" o.c. (perp to span).

TL;DR: Seems like your loads are fine but something with how you're calculating the shear capacity isn't right.

Maine EIT, Civil/Structural.

 

[JedClampett]

I smell a conversion from capacity per inch to load per foot mistake.

 

[ztengguy]

Well, this proves a reason to get away from flat tops, spec a 3 sided arch, and let the mfg design it.

TehMighty, Are you using LRFD? He might be using LFD shear equations...at that cover it should be 3 sqrt F'c b*d....So Vc = 3*70.71*12*9ish= 23K right? Not sure what equation he used.

Why the additional surcharge from the truck? The load is calculated, why add surcharge when its really part of the load already?

 

[TehMightyEngineer]

Jed: Or something along those lines. Perhaps JSanchez isn't calculating the equivalent strip correctly? The axle loads get destributed over an area perpendicular to the span and you could lose a lot of capacity if this is calculated differently than I'm doing. Please see AASHTO article 3.6.1.2.5, 3.6.1.2.6, and 4.6.2.10.

ztengguy: Yes, AASHTO 2012 LRFD. I'm using the shear equation for box culvert top slabs per article 5.14.5.3: Vc = [0.0676*SQRT(f'c) + 4.6*[As/(b*de)]*[(Vu*de)/Mu]]*(b*de) = 23,864 lb/ft (Almost exactly the same as yours.) I was using 3,000 psi IIRC for cast-in-place.

The surcharge is lateral soil surcharge assuming a loaded vehicle adjacent to the culvert. See AASHTO article 3.11.6.4.

If it requires a stamp I imagine most precasters would farm out the engineering anyway. JSanchez said it was cast-in-place though so maybe there's a reason they can't do precast.

Maine EIT, Civil/Structural.

 

[JSanchez88]

TehMightyPirate,

1. How did you obtain such a larger value for the vertical earth pressure? What value are you using for the soil density?
2. It doesn't feel right to neglect such significant construction loads.
3. My value for item 8 was incorrect.

 

[TehMightyEngineer]

Jsanchez:

1. I calculated the soil load using 120 PCF per AASHTO article 12.11.2.2. I assumed an embankment installation so I have Fe = 1 + 0.2*(H/Bc) = 1.12 (< or = 1.15 for compacted fill) W_E = Fe * 120 PCF * H = 1210 PSF (my original number was slightly off).

2. No, I simply did it for speed on my end. Can't spend all night on this.

3. Gotcha.

What was your equivalent strip width for the axle loads?

Maine EIT, Civil/Structural.

 

[JSanchez88]

That makes sense.
My equivalent strip width is 16.58' (+/- 1" from your calculation).