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Shear Reinforcing of a Box Culvert

Shear Reinforcing of a Box Culvert

Shear Reinforcing of a Box Culvert

(OP)
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?

RE: Shear Reinforcing of a Box Culvert

How are you designing it? are you looking at hte AASHTO buried structures section? That shot span, we can get a 8" section to work, but we are arched top, not flat top. How much cover you have?

RE: Shear Reinforcing of a Box Culvert

(OP)
Yes. I am using AASHTO. The roof on the culvert is flat and the cover varies between 6 and 9 feet.

RE: Shear Reinforcing of a Box Culvert

What deisgn software you using? I would expect that a span like that, 10-12" max thickness.

RE: Shear Reinforcing of a Box Culvert

(OP)
I am using Risa-3D.

RE: Shear Reinforcing of a Box Culvert

Your thicknesses seem way too high.

Some things I would check:

The biggest savings in shear will be keeping track of what load actually can cause shear failure. Taking your shear loads at d away from the wall will save you a lot of load for the shear design. Keep in mind that your haunches will likely not control so there's 8 or so inches more you save as well. Do two checks, one for the haunch shear and one for the slab shear.

Also, refer to AASHTO LRFD article 5.14.5.3 if you have more than 2 ft soil cover. This is a simplified shear calculation for buried single cell box culvert slabs cast monolithic with walls. Should give you a small boost in shear strength.

Make sure you're figuring your equivalent strip widths correctly, refer to article 4.6.2.10 for slabs with less than 2 ft cover or distribute through the soil per 3.6.1.2.6.

Calculate the impact factor in article 3.6.2.2 for buried components, should reduce the load a lot.

Note that lane loading does not apply per article 3.6.1.3.3. Also, note that culverts shall be designed for single lane loading per article 12.11.2.1 (unless you have a local requirement otherwise).

For negative slab moments you can take the design moments at the intersection of the walls and the slab at the haunches per article 12.11.4.2.

Fatigue and seismic are not required for buried culverts, article 5.5.3.1.

For other loads check the walls for a construction load case assuming they have only backfilled the walls with no load on the top slab. Otherwise your interior wall reinforcement may be too small. Also, check for unbalanced loads on the culvert (surcharge and full lateral soil load on one side, reduced lateral soil load on the other side with no surcharge).

Maine EIT, Civil/Structural.

RE: Shear Reinforcing of a Box Culvert

Oh, almost forgot the most important thing. Live load can be ignored at depths greater than 8 ft and greater than the span length per article 3.6.1.2.6.

Maine EIT, Civil/Structural.

RE: Shear Reinforcing of a Box Culvert

(OP)
TehMightyPirate,

Thanks for the input. I double checked all those and were properly taken into account. Right now, the load case that is governing the design is when the culvert has 9' of fill and the water table is assumed to be at the top of the ground surface. I'm thinking my calculation of the saturated weight of the soil may be a little off.

RE: Shear Reinforcing of a Box Culvert

Saturated soil is significant, and remember that culverts do fail... You may very well be designing it the way is *should* be designed, but usually isn't. Clients often want you to practice at the state of the profession, and as such to do what most similarly qualified and suitably trained & experienced engineers would do. Frustrating, but true...

Also, depending on the cover to the culvert, you can account for a spreading of the load from wheel type point-load (more or less) to a more distributed load on the culvert surface. Have you used some of this "UDLing" of the load? Does your code prohibit doing so?

RE: Shear Reinforcing of a Box Culvert

CELinOttawa: In AASHTO article 3.6.1.2.6 (mentioned above) you can distribute the wheel loads out from their contact area. But if he checked all the items I mentioned he should be doing this.

Sanchez: Make sure you aren't just adding the water directly to the soil dead load. Saturated soil weights should only be about 10% higher than dry soil weights. Also, if this really is a culvert you wouldn't usually overtop it and, if you did, you would have equal and opposite water pressures inside and out as it's an open structure.

Maine EIT, Civil/Structural.

RE: Shear Reinforcing of a Box Culvert

Are you modeling the RISA file with something to resemble the soil? We use CANDE software, and it will definitely help reduce your shear load.

RE: Shear Reinforcing of a Box Culvert

(OP)
I am modeling it similar to TxDOT's "Level 2: RISA 2D With Springs" example. They place 10 compression springs equally spaced along the bottom slab. I've played around with the spring constants and they don't seem to make a significant difference.

RE: Shear Reinforcing of a Box Culvert

I think you need to look into something along the sides. The frame will be supported by the soil, thus reducing the load on the frame, reducing the moment.

RE: Shear Reinforcing of a Box Culvert

What ztengguy said, make sure you model the lateral soil loads in combination with the vertical soil loads as box culverts get a lot of help from the confining action of the soil on the frame.

Putting springs on the bottom isn't a bad idea, that will model the bottom slab soil reaction a lot better. I personally just use a pin/roller at the corners and apply opposite upward soil pressures evenly distributed on the bottom slab. Either works.

Maine EIT, Civil/Structural.

RE: Shear Reinforcing of a Box Culvert

Also remember though your Phi factor will change as you become more compression controlled also. thats the latest in AASHTO, if LRFD

RE: Shear Reinforcing of a Box Culvert

I've never seen concrete box culverts be compression controlled as the flexural stresses are usually too high to have compression control. Doesn't mean it couldn't happen, though. Deep culvert with a small height and span for example.

Maine EIT, Civil/Structural.

RE: Shear Reinforcing of a Box Culvert

(OP)
So, after double checking all the loads and conditions and every possible input field, I cannot find anything that is wrong with my design. Having said that, I've decided to call for shear reinforcing to bring the thicknesses of the walls and slab down a little.

I always enjoy reading what others have to say about various situations.

Thank you all for your input!

RE: Shear Reinforcing of a Box Culvert

What was your final thickness?

RE: Shear Reinforcing of a Box Culvert

(OP)
I came up with a final thicknesses of 12".

RE: Shear Reinforcing of a Box Culvert

Wow, makes you wonder why everyone that specs a box culvert isnt outbid by 3 sided arches.

RE: Shear Reinforcing of a Box Culvert

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.

RE: Shear Reinforcing of a Box Culvert

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.

RE: Shear Reinforcing of a Box Culvert

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.

RE: Shear Reinforcing of a Box Culvert

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.

RE: Shear Reinforcing of a Box Culvert

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.

RE: Shear Reinforcing of a Box Culvert

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.

RE: Shear Reinforcing of a Box Culvert

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.

RE: Shear Reinforcing of a Box Culvert

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.

RE: Shear Reinforcing of a Box Culvert

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.

RE: Shear Reinforcing of a Box Culvert

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

RE: Shear Reinforcing of a Box Culvert

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

Maine EIT, Civil/Structural.

RE: Shear Reinforcing of a Box Culvert

(OP)
My inputs are as follows:(At 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.

RE: Shear Reinforcing of a Box Culvert

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.

RE: Shear Reinforcing of a Box Culvert

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

RE: Shear Reinforcing of a Box Culvert

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?





RE: Shear Reinforcing of a Box Culvert

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.

RE: Shear Reinforcing of a Box Culvert

(OP)
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.


RE: Shear Reinforcing of a Box Culvert

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.

RE: Shear Reinforcing of a Box Culvert

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

RE: Shear Reinforcing of a Box Culvert

Hmmmmmmm. I'm kinda at a loss then, the only thing left is either your factored load combinations or your shear strength. Are you getting the same shear strength ztengguy and I were getting?

Maine EIT, Civil/Structural.

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