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WisDOT Bridge Manual Chapter 36 Box Culverts

J uly 2010 36-1

Table of Contents

36.1 General ...............................................................................................................................336.1.1 Bridge or Culvert .........................................................................................................336.1.2 Box Culvert Size Restrictions ......................................................................................4

36.1.3 Stage Construction for Box Culverts ...........................................................................4

36.2 Dead Loads and Earth Pressure .........................................................................................536.3 Live Loads ...........................................................................................................................6

36.3.1 Concentrated Live Loads ............................................................................................636.3.2 Distributed Live Loads .................................................................................................736.3.3 Live Load Soil Pressures.............................................................................................736.3.4 Impact .........................................................................................................................836.3.5 Location for Maximum Moment ...................................................................................8

36.4 Design Information ..............................................................................................................936.5 Detailing of Reinforcing Steel ............................................................................................11

36.5.1 Corner Steel ..............................................................................................................1136.5.2 Positive Moment Slab Steel ......................................................................................1236.5.3 Negative Moment Steel over Interior Walls ...............................................................1236.5.4 Exterior Wall Positive Moment Steel .........................................................................1336.5.5 Interior Wall Moment Steel ........................................................................................1436.5.6 Distribution Reinforcement ........................................................................................1436.5.7 Temperature Reinforcement .....................................................................................15

36.6 Box Culvert Aprons ...........................................................................................................1736.6.1 Type A .......................................................................................................................1736.6.2 Type B, C, D ..............................................................................................................1836.6.3 Type E .......................................................................................................................2036.6.4 Wingwall Design ........................................................................................................20

36.7 Box Culvert Camber ..........................................................................................................2136.7.1 Computation of Settlement ........................................................................................2136.7.2 Configuration of Camber ...........................................................................................2236.7.3 Numerical Example of Settlement Computation ........................................................23

36.8 Box Culvert Structural Excavation and Structure Backfill .................................................2436.9 Box Culvert Headers .........................................................................................................2536.10 Construction Issues ........................................................................................................27

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36.10.1 Weepholes ..............................................................................................................2736.10.2 Cutoff Walls .............................................................................................................2736.10.3 Nameplate ...............................................................................................................2736.10.4 Plans Policy .............................................................................................................2736.10.5 Rubberized Membrane Waterproofing ....................................................................27

36.11 Precast Box Culverts ......................................................................................................28

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Bridges

Advantages Disadvantages

Less susceptible to cloggingwith drift, ice and debris

Require more structuralmaintenance than culverts

Waterway increases withrising water surface untilwater begins to submergestructure

Piers and abutmentssusceptible to failure scour

Susceptible to ice and frostforming on deck

Culverts

Grade rises and wideningprojects sometimes can beaccommodated byextending culvert ends

Silting in multiple barrelculverts may requireperiodic cleanout

Minimum structural

maintenance

No increase in waterway as

stage rises above soffit

Usually easier and quickerto build than bridges

May clog with drift, debris orice

Table 36.1-1Advantages/Disadvantages of Structure Type

36.1.2 Box Culvert Size Restrictions

A minimum vertical opening of 5 feet is desirable for concrete box culverts for clean outpurposes.

A maximum overall horizontal opening of 36 feet for multi-celled concrete box culverts isconsidered desirable. For larger openings the design assumptions of equal load distributionmay not apply.

36.1.3 Stage Construction for Box Culverts

The inconvenience to the traveling public and the restrictions being placed on channelrelocation by environmental concerns has often led to proposals for stage construction onconcrete box culvert projects.

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36.2 Dead Loads and Earth Pressure

The weight of soil above buried structures is taken as 120 pcf. For the lateral pressure fromthe soil an equivalent fluid unit weight of soil equal to 60 pcf is used.

When computing the maximum positive moment in the top and bottom exterior span, use anequivalent fluid unit weight of soil equal to 30 pcf and for multi-cell culverts when computingthe maximum negative moment over the interior wall in the end cells.

Earth pressures or loads on culverts may be computed as the weight of earth directly abovethe structure. A load factor of 1.3 is used for vertical earth pressure and dead loads, and 1.69(1.3 x 1.3) is used for lateral earth pressure. Vertical earth pressure is also multiplied by asoil structure interaction factor, Fe as stated in AASHTO 17.6.4.2.1. Embankmentinstallations are always assumed.

Figure 36.2-1Vertical and Lateral Earth Pressures

Figure 36.2-1 shows the factored earth load pressures acting on a box culvert. The earthpressure from the dead load of the concrete is distributed equally over the bottom of the box.

When designing the bottom slab of a culvert do not forget that the weight of the concrete inthe bottom slab acts in an opposite direction than the bottom soil pressure and thus reducesthe design moments and shears. See AASHTO 17.6.4 for values of Fe shown in figure.

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A haunch is provided only when the slab depth required at the interior wall is more than 2inches greater than that required for the remainder of the span. Minimum haunch depth is 3inches and minimum length is 6 inches. Haunch depth is increased in 3 inch increments andhaunch length is increased in 6 inch increments. The haunch length is not to exceed 7 timesthe depth.

Fatigue requirements in bar steel are satisfied by limiting the truck service load stress toAASHTO fatigue limits using working stress analysis. In addition to the truck live load, anequivalent fluid unit weight of soil equal to 30 pcf or 1/2 this value is added to the outside ofthe box. Vertical earth pressure and concrete dead load are not considered in the fatigueanalysis. The fatigue stress limit for concrete usually does not govern. The AASHTOrequirements for distribution of flexural reinforcement for crack control is not required.Cracking has not been a problem for culverts designed without considering it. Reinforcedconcrete design is based on the strength design method and AASHTO Specifications.Material strengths are:

Concrete - f'c = 3.5 ksi

Bar Steel - fy = 60 ksi

The slab thickness required is determined by moment or shear, whichever governs.Thickness based on moment is determined from using 37.5% of the reinforcement ratioproducing balanced conditions.

The shear in the top and bottom slabs is assumed to occur at a distance "d" from the face ofthe walls. The value for "d" equals the distance from the centroid of the reinforcing steel tothe face of the concrete in compression. When a haunch is used, shear must also bechecked at the end of the haunch.

For multi-cell culverts make interior and exterior walls of equal thickness.

For culverts under high fills use a separate design for the ends if the reduced section may beused for at least two panel pours per end of culvert. Maximum length of panel pour is 40 feet.

Barrel lengths are based on the roadway sections and wing lengths are based on a 2 1/2:1slope of fill from the top of box to apron.

Dimensions on drawings are given to the nearest inch only.

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36.5 Detailing of Reinforcing Steel

To calculate the required bar steel area and cutoff points a maximum positive and negativemoment envelope is computed. Bar cutoff points may also be determined by an approximatemethod based on the points of dead load contraflexure. It is assumed that the required bar

lengths in the top slab are longer than those in the bottom slab. Therefore, cutoff points arecomputed for the top slab and are also used in the bottom slab.

36.5.1 Corner Steel

Figure 36.5-1Layout of Corner Steel

The area of steel required is the maximum computed from using the top and bottom cornermoments and the thickness of the slab or wall, whichever controls. Identical bars are thenused in the top and bottom. Top and bottom negative steel is cut in the walls and detailed intwo lengths when a savings of over 1 foot in a single bar length can be obtained. Cornersteel is always lapped at the center of the wall. If two bar lengths are used, only alternatebars are lapped.

Distance "L" is computed from the maximum negative moment envelope for the top slab.

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36.5.2 Positive Moment Slab Steel

Figure 36.5-2Layout of Positive Moment Steel

The area of steel required is determined by the maximum positive moments in each span.Top and bottom steel may be of different size and spacing. Detail two bar lengths if 2 feet ormore of bar steel can be saved.

When two bar lengths are detailed in multi-cell culverts, run every other positive bar acrossthe entire width of box. If this requires a length longer than 40 feet, lap them over an interiorwall.

The cutoff points of alternate bars are determined from the maximum positive momentenvelope for the top slab. These same points are used in the bottom slab.

36.5.3 Negative Moment Steel over Interior Walls

Figure 36.5-3Layout of Negative Moment Steel

The area of steel is determined from the moment at the C/L of the interior wall and theeffective depth of slab at the face of the interior wall. If the slab is haunched the steel arearequired at the end of the haunch must also be checked. Top and bottom steel may be ofdifferent size and spacing. If a savings in bar length of more than 2 feet can be realized, twobar lengths are detailed.

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36.6.2 Type B, C, D

Type C & D are of equal efficiency but Type C is used most frequently. Type D is used forinlets when the water is entering the culvert at a very abrupt angle. Type B is used foroutlets.

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Figure 36.6-2Wing Type B, C, D (Angles vs. Skew)

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36.9 Box Culvert Headers

For skews of 20 degrees and less the main reinforcing steel is parallel to the end of thebarrel. A header is not required for structural purposes but is used to prevent the fill materialfrom spilling into the apron. A 12 inch wide by 6 inch high (above the top of top slab) header

with nominal steel is therefore used for skews of 20 degrees and less on the top slab. Noheader is used on the bottom slab.

For skews over 20 degrees the main reinforcing is not parallel to the end of the barrel. Thepositive reinforcing steel terminates in the header and thus the header must support, inaddition to its own dead load, an additional load from the dead load of the slab and fill aboveit. A portion of the live load may also have to be supported by the header.

The calculation of the actual load that a header must support becomes a highlyindeterminate problem. For this reason a rational approach is used to determine the amountof reinforcement required in the headers. The design moment strength of the header must beequal to or greater than 1.3 times the header dead load moment (based on simple span) plus

1.3 times a live load moment from a 16 kip load assuming 0.5 fixity at ends.

To prevent a traffic hazard, culvert headers are designed not to protrude above the groundline. For this reason the height of the header above the top of the top slab is allowed to beonly 6 inches. The width of the header is standardized at 18 inches.

The header in the following figure gives the design moment strengths listed using d = 8.5inches.

Figure 36.9-1Header Details (Skews > 20)

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The following size bars are recommended for the listed header lengths where "HeaderLength" equals the distance between C/L of walls in one cell measured along the skew.

Header Length Bar Size1

To 10 #7

Over 10 to 13 #8

Over 13 to 16 #9

Over 16 to 20 #10

Table 36.9-1Header Reinforcement

1 Use the bar size listed in each header and place 2 bars on the top and 2 bars on thebottom. Use a header on both the top and bottom slab. See the Standard Box Culvert

Details in Chapter 36 for details.

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36.10 Construction Issues

36.10.1 Weepholes

Investigate the need for weepholes for culverts in cohesive soils. These holes are to relieve

the hydrostatic pressure on the sides of the culverts. Where used, place the weepholes 1foot above normal water elevation but a minimum of 1 foot above the lower sidewallconstruction joint. Do not use weepholes if they are closer than 1 foot to the top slab.

36.10.2 Cutoff Walls

Where dewatering the cutoff wall in sandy terrain is a problem, the concrete may be pouredin the water and this can be noted on the plans.

36.10.3 Nameplate

Designate a location on the wingwall for placement of the nameplate. Locate nameplate on

the first right wing traveling in the Cardinal direction (North/East).

36.10.4 Plans Policy

Box Culvert plans will be detailed for only cast-in-place reinforced concrete where sitegeometry, hydraulic, and soils conditions are acceptable.

Precast concrete box culverts have not been cost effective in comparison to cast-in-placeconcrete and in general will not be detailed on plans. If an exception occurs warranting theuse of precast box sections; contact the Bureau of Structures for further guidance.

36.10.5 Rubberized Membrane Waterproofing

When the Standard requires membrane waterproofing, place the bid item "RubberizedMembrane Waterproofing" on the final plans along with the bid quantity in square yards.

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