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?
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
RE: Shear Reinforcing of a Box Culvert
RE: Shear Reinforcing of a Box Culvert
RE: Shear Reinforcing of a Box Culvert
RE: Shear Reinforcing of a Box Culvert
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
Maine EIT, Civil/Structural.
RE: Shear Reinforcing of a Box Culvert
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
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
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
RE: Shear Reinforcing of a Box Culvert
RE: Shear Reinforcing of a Box Culvert
RE: Shear Reinforcing of a Box Culvert
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
RE: Shear Reinforcing of a Box Culvert
Maine EIT, Civil/Structural.
RE: Shear Reinforcing of a Box Culvert
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
RE: Shear Reinforcing of a Box Culvert
RE: Shear Reinforcing of a Box Culvert
RE: Shear Reinforcing of a Box Culvert
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
RE: Shear Reinforcing of a Box Culvert
Maine EIT, Civil/Structural.
RE: Shear Reinforcing of a Box Culvert
RE: Shear Reinforcing of a Box Culvert
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
RE: Shear Reinforcing of a Box Culvert
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
Maine EIT, Civil/Structural.
RE: Shear Reinforcing of a Box Culvert
RE: Shear Reinforcing of a Box Culvert
Maine EIT, Civil/Structural.
RE: Shear Reinforcing of a Box Culvert
RE: Shear Reinforcing of a Box Culvert
Maine EIT, Civil/Structural.
RE: Shear Reinforcing of a Box Culvert
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:
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
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:
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
RE: Shear Reinforcing of a Box Culvert
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
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
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
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
My equivalent strip width is 16.58' (+/- 1" from your calculation).
RE: Shear Reinforcing of a Box Culvert
Maine EIT, Civil/Structural.