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Chapter 19 - Reinforced Concrete Box Culverts and Similar Structures
CHAPTER 19 REINFORCED CONCRETE BOX CULVERTS AND SIMILAR STRUCTURES (SPANS UP TO 12.2 m)
19.2 SELECTION CRITERIA
19.3 FOUNDATIONS
19.3.1 Rock
19.3.2 Earth or Granular Soil
19.4 DESIGN GUIDELINES FOR BOX CULVERTS
19.4.1 .............................................
19.4.2 Analysis Method
19.4.3 ..............................................
19.4.4 Dead Load and Earth Pressure
19.4.5 .................................................
19.4.6 Live Load Impact Factor
19.4.7 Wall Thickness Requirements
19.4.8 Concrete Strength
19.4.9 Reinforcement Requirements
19.4.10 Skewed Precast Sections
19.4.11 Detailing Requirements
19.5 COMPUTER DESIGN AND ANALYSIS PROGRAM
19.6 DESIGN AND DETAILS OF CONCRETE CULVERTS
19.6.1 ............................................
19.6.2 Design Procedure
19.6.3 ............................................
19.6.4 Headwalls/Edge Beams
19.7 PRECAST CONCRETE CULVERTS (SPECIAL DETAILS AND REQUIREMENTS)
19.7.1 Contract Plans
19.7.2 Reinforcement ............................................
19.7.3 Design and Fabrication
19.7.4 Precast Arches and Frames
19.7.5 General Notes for Precast Culverts
19.8 GUIDE RAILING
19.9 CUT-OFF WALL
19.10 LOW FLOW DISH
19.11 APRONS
19.12 SUBBASE DRAINAGE
Typical Cross Sections, Cast-In-Place
Wingwalls Plan and Elevation, Precast or Cast-In-Place
Contraction & Construction Joint, Cast-In-Place
Longitudinal Section, Cast-In-Place
Wingwall Plan, Culvert on Skew, Precast or Cast-In-Place
Wingwall Aprons, Cast-In-Place
Typical Cross Sections - Precast
Typical Cross Section - Precast
19-10 Headwall Details - Precast 19-11 Headwall and Wingwall Connection - Precast 19-12 Precast Arch and Frame 19-13 Approach Railing Details, Transition to Box Beam Guide Railing 19-14 Approach Railing Details 19-15 Culvert Railing Details, Cast-In-Place Connection
The purpose of this Chapter is to provide a good working knowledge of the requirements for designing and preparing contract plans for reinforced concrete culverts.
This Chapter is intended to provide guidelines, minimum recommendations and the available references needed to complete a design. As with any structural engineering design, alternate design methods are available. The designer has the ultimate responsibility to provide an efficient, safe design. It is not possible to provide guidance in this chapter for all conditions. Guidance is provided for the typical design.
Engineering judgment will need to be used for most projects. The history of the project site must always be evaluated and must be factored into the design when appropriate. Each location will usually have some unique character (floods, scour, surroundings etc.). Also, each Region may have their own preferences. For example, some Regions only specify heavy stone filling in stream beds and on the side slopes. Other Regions prefer stone gutters instead of sod gutters. In Regions 10 and 11, some designs need to consider the effects of salt water. Another potential problem can be polluted water. Unique environments need to be thoroughly evaluated and all environmental requirements satisfied.
The reinforcement and section properties charts in the previous edition of this Chapter have been eliminated in favor of a Reinforced Concrete Box Culvert Design and Analysis Computer Program (see Section 19.5).
Wingwall alignment is highly dependent on site conditions and should be evaluated on a case by case basis. Wingwall design is not included in this Chapter. Information on retaining wall design can be found in Chapter 20. Computer programs available for wall design are BRADD2 and WALLRUN. If assistance is required, contact the Regional Structures Engineer or the Structures Design and Construction Division.
In the previous edition of this Chapter, there were references to different designs or details when the skew exceeded 20°. These references have been removed. All culverts, regardless of skew, shall be individually designed to determine the required reinforcement and segment sizes.
This chapter is entirely in metric units, except the references to bar reinforcement sizes. At this time, the bar reinforcement manufacturers are not producing bar reinforcement in hard metric sizes. All references to reinforcing bar size in this Chapter are in customary english units. Bar spacings are in metric units. Should the situation change in the future, the designer should be aware that an area of reinforcement per spacing length may need to be provided.
Definitions (explanations of the terminology used in this chapter) follow:
A culvert is defined in the Standard Specifications as any structure, whether of single or multiple span construction, with an interior width of 6.1 m or less when the measurement is made horizontally along the center line of the roadway from face to face of abutments or sidewalls. Structures spanning more than 6.1 m along the center line of the roadway are considered bridges. Various precast structures spanning up to 12.2 m are covered in this Chapter.
While it is recognized that any structure with a span over 6.1 m is technically not a culvert, for simplicity, all structures in this Chapter will be referred to as culverts. However, the procedure for the hydraulic analysis differs based on the span length. For guidance on hydraulic design for structures with spans of 6.1 m or less, see Chapter 8. Any structure with a span greater than 6.1 m will require a more detailed analysis. The procedures for structures with a span greater than 6.1 m is outlined in the AASHTO Model Drainage Manual with NYSDOT modifications. The NYSDOT modifications for the AASHTO Model Drainage Manual and assistance in hydraulic design procedures for structures greater than 6.1 m may be obtained from the Structures Design and Construction Division or the Regional Hydraulics Engineer.
The largest culverts are typically not boxes, rather they are frames or arches and are discussed in Section 19.7.4.
Concrete box culverts have prismatic members, i.e., the wall and slab thickness dimensions are uniform. They have top slabs, sidewalls and usually bottom slabs and cutoff walls if there is a bottom slab. Frames and arches typically have varying wall and/or slab thicknesses.
There are two types of concrete culverts: open and closed.
Open concrete culverts do not have bottom slabs and are typically used at locations where the footings can be founded on rock at or near the stream bed. Use of open concrete culverts where rock is not at stream bed would require piles under the footings or some other form of scour protection. Open concrete culverts on spread footings may be used for cattle passes, bicycle/pedestrian paths and other uses that do not convey water, i.e., they do not have scour vulnerability.
Closed concrete culverts have bottom slabs (invert slabs) of either cast-in-place or precast concrete. Closed concrete culverts are typically used where the stream bed is earth or granular soil and rock is not close enough to the stream bed.
The clear span is the perpendicular distance between the inside of the sidewalls. The maximum clear span recommended for a concrete box culvert is 7.3 m.
The design span is the perpendicular distance between the center of the sidewalls. A design span for a concrete box culvert in excess of 7.5 m may prove to be uneconomical. For culverts with skewed ends, the distance controlling design will be between the center of the sidewalls parallel to the highway. If an extreme skew (e.g., >30E) is necessary, squaring the ends should be considered if site conditions will allow. Squaring the ends will reduce the required top slab
thickness, the reinforcement and edge beam requirements. Under certain conditions (e.g., stage construction, R.O.W. problems, utility conflicts, detour conflicts, etc.) the sections may need to be skewed.
The most appropriate type of short span structure must be determined. The choices are a corrugated metal structure, concrete box culvert, concrete frame or arch and a short span bridge. While the site conditions should be the primary deciding factor for structure selection, economics are also very important.
Precast and cast-in-place concrete culverts are usually more expensive in first cost than a corrugated metal structure. However, concrete culverts should not be rejected as an alternative without making an engineering analysis that includes suitability to the site and life cycle cost estimates. The advantages of concrete culverts are superior durability for most environmental conditions, greater resistance to corrosion and damage due to debris, greater hydraulic efficiency and typically longer life spans (i.e. potentially lower life cycle costs).
At sites with limited headroom, concrete culverts may be the least expensive option. Corrugated metal structures may not fit the site conditions without appreciable changes to the roadway profile. Corrugated metal structures typically require a minimum height of cover of 600 mm or more. Concrete culverts can have asphalt pavement placed directly on the top slab. Corrugated metal structures will also typically require taller structures to provide adequate waterway area below design high water than concrete culverts. If a corrugated metal structure is a viable option, an engineering and cost analysis should be done. Hydraulics may dictate the need for a concrete culvert because of excessive back water, excessive water velocity or ice and debris.
When a concrete culvert is selected, a precast option should be considered whenever possible. Speed of erection, maintenance of traffic, stream diversion problems and site constraints can be minimized when this option is chosen.
Before a final determination is made to use a large concrete culvert, the use of a short span bridge with laid back slopes and integral abutments should be investigated. Possible advantages of a bridge may be: minimized work in the stream, speed of erection, no interference with existing structure foundation and easier construction when there is staged construction.
Information on corrugated metal structures (steel and aluminum) is available in Chapter 8.
At this time, all structures discussed in this chapter, regardless of span and height of cover, are considered buried structures in regard to foundation design, thus there is no requirement for seismic analysis. This may change in the future as more research is completed. For culverts with spans
greater than 6.1 m, questions should be directed to the Structures Design and Construction Division.
When sound rock is at or near the surface of a stream bed, no invert slab will be required. Concrete footings are either keyed or doweled into rock based on a consultation with an Engineering Geologist from the Geotechnical Engineering Bureau.
If the elevation of the rock surface varies by 600 mm or less, the wall height should be constant and the footing height varied. If the variation in rock surface elevation exceeds 600 mm, the height of the culvert wall may be varied at a construction joint or at a precast segment joint. In some cases, it may be required to use walls of unequal heights in the same segment, but this should be avoided if possible.
When a concrete culvert cannot be founded on rock, an invert slab is typically required. However, in areas of compact soil and low stream velocities, open concrete culverts may be used if they have scour protection (piles, sheeting, stone lined invert or deeper footings).
In areas with a significant potential for cobbles and boulders to move with the bed load, an open culvert with a strip footing on piles should be investigated. The movement of cobbles and boulders may damage a concrete invert.
To avoid differential settlement, closed concrete culverts should never be founded partially on rock and partially on earth. If rock is encountered in a limited area, it should be removed to a minimum depth of 300 mm below the bottom of the bottom slab and backfilled with either select granular material or crushed stone. Concrete culverts are rigid frames and do not perform well when subjected to differential settlement. If differential settlement cannot be avoided, a concrete culvert should not be used.
All closed precast concrete box culverts should have a designed undercut and backfill. The Regional Geotechnical Engineer or the Geotechnical Engineering Bureau should be consulted to determine the depth of the undercut and type of backfill material required.
A closed concrete culvert can be considered if settlement is expected and the foundation material is fairly uniform. However, the culvert should be designed to accommodate additional wearing surface(s) which may be needed to accommodate the settlement of the box.
If the foundation material is extremely poor and it is desirable to limit settlement, the problem should be referred to the Regional Geotechnical Engineer or the Geotechnical Engineering Bureau to determine the best course of action. A typical remedy might be removal of unsuitable or unstable material and replacement with suitable material.
Reinforced concrete box culverts (precast or cast-in-place) subjected to either earth fill and/or highway vehicle loading shall be designed in accordance with the guidelines in Sections 19.4 -
The following Sections and Articles of the AASHTO Standard Specifications for Highway Bridges - Fifteenth Edition have been used to develop these guidelines:
Reinforced Concrete Box, Cast-In-Place
Reinforced Concrete Box, Precast
19.4.1 Design Method
The design method shall be either the Service Load Design Method (Allowable Stress Design) or the Strength Design Method (Load Factor Design) as described in Articles 8.15 and 8.16 of the AASHTO Standard Specifications for Highway Bridges - Fifteenth Edition (AASHTO) with NYSDOT modifications described in Sections 19.4.2 - 19.4.11.
The analysis of reinforced concrete box culverts shall be in accordance with AASHTO Article 8.8.2 and modified as follows:
In analysis, distance to the geometric centers of members shall be used in the determination of moments. Moments at faces of support, however, may be used for member design. Face of support shall be defined as the inside face of an exterior or interior wall. When a fillet is built monolithic with the member and support, no portion of the fillet shall be considered as adding to the shear and moment capacity of the member.
19.4.3 Load Factors
The product of the load factors [gamma (() x beta ($)] for the Strength Design Method (Load Factor Design) shall be equal to 1.3 for Dead and Earth Loads and 2.17 for Live Loads in accordance with AASHTO Article 3.22 and Table 3.22.1A Group X.
The dead load on the top slab shall consist of the mass densities of the pavement, soil and the concrete slab. For simplicity, the computer program assumes the pavement as soil.
The following mass densities, as given in AASHTO Articles 3.3.6 and 6.2, shall be used in determining dead load and earth pressures for design:
1925 kg/m 3
Concrete mass density
960 kg/m 3 max., 480 kg/m 3 min.
Maximum and minimum values of lateral earth pressure shall be investigated in accordance with AASHTO Articles 3.20.2 and 6.2.1.B.
Soil mass density shall be modified when using the Strength Design Method (Load Factor Design) in accordance with AASHTO Articles 17.6.4.2 and 17.7.4.2 Modification of Earth Loads for Soil Structure Interaction (Embankment Installations).
19.4.5 Live Load
Reinforced concrete box culverts shall be designed for MS-23 vehicle live load.
When the depth of fill is less than 600 mm, wheel loads shall be distributed in accordance with AASHTO Article 3.24.3.2, Case B and modified as follows:
Wheel loads shall be distributed over a distribution slab width, E (measured in meters), equal to 1.22 + 0.06S, where S is the perpendicular distance in meters between wall centerlines.
When the culvert is skewed relative to the over roadway, the distribution width, E, shall be reduced by multiplying E by the cosine of the skew angle. In no instance shall the distribution width exceed 2.13 m nor the section length of precast units.
A live load surcharge pressure equivalent to 600 mm of earth fill shall be added to the lateral earth pressure.
When the depth of fill is 600 mm or more, wheel loads shall be distributed in accordance with AASHTO Articles 6.4.1 and 6.4.2:
Wheel loads shall be considered as uniformly distributed over a square with sides equal to 1.75 times the depth of fill. When such areas from several concentrations overlap, the total load shall be uniformly distributed over the area defined by the outside limits of the individual areas, but the total width of distribution shall not exceed the total width of the supporting slab.
The Live Load Impact Factor, I, described in AASHTO Article 3.8.2 shall be modified as follows:
For fill heights < 900 mm, the Impact Factor, I, shall be equal to 1.3 for S #
12.2 m,
where S is the perpendicular distance in meters between wall
For fill heights $ 900 mm, the Impact Factor shall be equal to 1.0.
Exterior wall thickness requirements for reinforced concrete box culverts shall be controlled by design, except that minimum exterior wall thickness requirements have been established to allow for a better distribution of negative moment corner reinforcement as follows:
CLEAR SPAN <2.45 m $2.45 m & <4.25 m $4.25 m & <6.10 m $6.10 m
MINIMUM WALL THICKNESS 150 mm 200 mm 250 mm 300 mm
Interior wall thickness, in multi-cell applications, shall be controlled by design but shall not be less than 150 mm in any instance.
Reinforced concrete box culverts shall be designed for the following concrete strengths:
f ' c = 35 MPa
f ' c = 25 MPa
Reinforcement shall be either bar reinforcement, welded wire fabric (plain), or welded wire fabric (deformed). Designs shall use a yield strength of 415 MPa for bar reinforcement and 450 MPa for welded wire fabric in accordance with AASHTO Articles 17.6.2.2 and 17.7.2.2.
When the fill height over the box culvert is less than 600 mm, all reinforcing steel in the top mat of the top slab shall be epoxy coated.
The maximum service load stress in the design reinforcing steel for crack control shall be in accordance with AASHTO Articles 17.6.4.7 and 17.7.4.7.
Design reinforcement stresses at service loads shall be limited to satisfy the requirements for fatigue in accordance with AASHTO Article 8.16.8.3.
Minimum reinforcement shall be provided in accordance with AASHTO Article 8.17.1 at all cross sections subject to flexural tension, including the inside face of walls.
Distribution reinforcement as described in AASHTO Article 3.24.10 shall be modified as follows:
To provide for the lateral distribution of loads, steel reinforcement shall be placed transverse to the main design steel in both the top and bottom slabs of reinforced concrete box culverts for fill heights less than 600 mm.
The amount of distribution reinforcement required shall be in accordance with AASHTO Article 3.24.10.2 Equation (3-21).
All faces of reinforced concrete box culverts not requiring design or distribution steel shall be reinforced with the equivalent of #4 bar reinforcement at 300 mm centers in each direction. Under no circumstances shall any reinforcement be spaced greater than 300 mm.
Shear reinforcement shall not be used in reinforced concrete box culverts. Slab and wall thickness shall be designed to have adequate shear capacity in accordance with AASHTO Articles 8.15.5 or 8.16.6.
Skewed precast culvert sections should be avoided if practical. Precast concrete culverts should have square ends whenever possible. Skewed sections are sometimes required to satisfy right-of-way constraints and/or stage construction requirements for skewed alignments. In the event they are necessary, skewed precast culvert sections shall be designed for the skewed end clear span. Large skews may lead to sections that require additional reinforcement and/or greater wall and slab thickness than typical square sections with the same clear opening. Fabricators should be contacted for information on maximum skews available.
The minimum reinforcing bar cover requirements for precast box culverts shall be as follows:
Exterior concrete cover top slab
Exterior concrete cover bottom slab Exterior concrete cover walls Interior concrete cover
50 mm (Fill < 610 mm) 25 mm (Fill $ 610 mm) 25 mm 25 mm 25 mm
The minimum reinforcing bar cover requirements for cast-in-place box culverts shall be as follows:
Exterior concrete cover bottom slab
Exterior concrete cover walls
Interior concrete cover
The minimum bending radius of negative moment reinforcing steel (outside corners top and bottom slabs) shall be in accordance with AASHTO Article 8.23.2 Minimum Bend Diameters.
Top and bottom slab outside face transverse steel shall be full length bars unless spliced to top and bottom slab corner reinforcing steel.
A Reinforced Concrete Box Culvert Design and Analysis Program is used by the Structures Design and Construction Division. It has been distributed to NYSDOT Regional Structures personnel and the Precast Concrete Association of New York (PCANY). Questions regarding the use of this program or how to obtain a copy and/or a Users Manual should be addressed to the Structures Design and Construction Division.
This program will design and/or analyze a one, two, three or four cell reinforced concrete box culvert with prismatic members (precast or cast-in-place) with or without bottom slab in accordance with the design criteria in Section 19.4. All cells are assumed to be the same size for any one culvert and the clear opening dimensions remain constant. By knowing the span, rise, and fill height, the program will design the box culvert by either Service Load Design or Load Factor Design. The program will design wall and slab thickness and required reinforcement. The bar schedule will be displayed for the entire length of a cast-in-place box culvert or one unit of a precast box culvert.
19-10 CONCRETE CULVERTS
Standard details for concrete culverts are shown in Figures 19-1 through 19-11.
When a cast-in-place concrete culvert is proposed for a site, the designer is required to provide a complete design for the contract plans. If a precast concrete culvert is proposed, the contractor/fabricator will be required to submit the design and fabrication details to the State for approval. This shall typically be done within 45 days of the contract award date.
If alternate designs (i.e., cast-in-place vs. precast) are proposed for a site, the bar list table for the cast-in-place culvert unit may be omitted from the contract plans. Once the contract has been awarded and an alternate is chosen, the designer must provide a complete design if the cast-in-place alternate is selected. This shall typically be done within 45 days of the contract award date.
19.6.1 Contract Plans
The contract plans shall include these minimum design details:
1. Live Loading Requirements: MS-23 unless another loading is required.
2. A Plan View showing the alignment, skew angle of the culvert relative to a perpendicular to the centerline of roadway, stationing along the culvert centerline, the equality stations for the intersection with the highway centerline, and wingwall orientation.
3. An Elevation View indicating the culvert clear span and rise, slope protection, general foundation treatment and any clearance requirements.
4. For culverts that are categorized by definition as bridges, a table of hydraulic data and the minimum hydraulic area perpendicular to flow below Design High Water .
5. A Longitudinal Section showing the slope of the culvert, a typical highway section, culvert end treatments, foundation treatment (footing on rock or piles, 4-sided box with cut-off walls), type of apron, and any utilities attached, in the embankment or sidewalk.
6. Construction Staging Information (determines lengths of segments and potential need for skewed segments).
7. Earth Cover: measured from the top of the top slab to the top of pavement.
8. Headwall, Cut-off Wall, Wingwall, Apron Slab and Nosing Information: Provide geometry, reinforcement, location on culvert and connection details.
9. End Section Treatment: Provide details for square, skewed, beveled or open end sections, if applicable.
10. Railing Details: Locate the railing on the culvert and indicate how it is to be attached. See Section 19.8 and Figures 19-13, 19-14 and 19-15.
11. Chamfers: Indicate chamfer size and locations.
12. Foundation: All pertinent foundation details.
13. Installation of Draw Connectors: If Draw Connectors are to be left in place they should be galvanized in accordance with §719-01 of the Standard Specifications and a note must state they are being left in place. (Precast only)
Determine the clear span and rise of the culvert using proper design procedures for the feature crossed. See Chapter 8 for information on designing the opening of structures with spans of
6.1 m or less crossing water features.
Determine the height of the fill, which is the distance from the top of the roadway to the top of the top slab of the culvert.
For a cast-in-place culvert, use the Reinforced Concrete Box Culvert Design and Analysis Program or manual calculations to determine the wall and slab thickness and required reinforcement.
For precast design requirements, see Section 19.7.
Standard details for cast-in-place culverts are shown in Figures 19-1 to 19-6. Standard details for precast culverts are shown in Figures 19-2, 19-5 and 19-8 to 19-11.
Figure 19-1 Typical Cross Sections, Cast-In-Place.
Figure 19-2 Wingwalls Plan and Elevation, Precast or Cast-In-Place
Figure 19-3 Contraction and Construction Joint, Cast-In-Place
Figure 19-4 Longitudinal Section, Cast-In-Place
Figure 19-5 Wingwall Plan, Culvert on Skew, Precast or Cast-In-Place
Figure 19-6 Wingwall Aprons, Cast-In-Place
19.6.3 Reinforcement
The main reinforcement in the top and bottom slabs shall be perpendicular to the sidewalls in cast-in-place culverts and non skewed sections of precast culverts. In a cast-in-place concrete culvert with a skewed end section, the top and bottom slab reinforcement will be "cut" to length to fit the skewed ends. The "cut" transverse bars have the support of only one culvert sidewall and must be supported at the other end by the edge beam or cut-off wall. See Figure 19-7. For reinforcement requirements of skewed precast culverts, see Section 19.7.2.
When the fill height over the culvert is less than 600 mm, all reinforcing steel in the top mat of the top slab shall be epoxy coated.
All reinforcement contained within the headwall or edge beam including the reinforcement that extends into the top slab of the culvert shall be epoxy coated and shall meet the requirements of § 709-04 of the Standard Specifications. If the headwall or edge beam is a significant distance from the highway where it is not in danger from chlorides, the epoxy coating can be eliminated.
The following bar label criteria shall be used:
Bar Identification Schedule (see Figures 19-1 and 19-8)
Top Corner Bars (design steel)
Bottom Corner Bars (design steel)
Top Slab, inside face transverse bars (design steel)
Bottom Slab, inside face transverse bars (design steel)
Top Slab, outside face transverse bars (design steel for multiple cells)
Bottom Slab, outside face transverse bars (design steel for multiple cells)
Exterior wall, inside face vertical bars (design steel)
Exterior wall, outside face vertical bars (design steel)
Interior wall, vertical bars both faces (design steel)
Top Slab, bottom slab and wall longitudinal bars (temperature reinforcement)
Top Slab, inside face longitudinal bars (design distribution steel)
Bottom Slab, inside face longitudinal bars (design distribution steel)
See the Computer Program Users Manual for bars used less frequently. This bar schedule is also valid for precast culverts.
Figure 19-7 Reinforcement Diagram
Headwalls are normally used on all culverts. A headwall helps retain the embankment in deep fills. In shallow fills, the headwall may retain the subbase and/or highway pavement and provide the anchorage area for the railing system.
Headwalls that are over 300 mm in height or have a railing attachment should be cast-in-place. Headwalls 300 mm or less in height with no railing attachment may be either precast or cast-in-place.
If possible the maximum height of the headwall should not exceed 900 mm. Greater heights are attainable but should only be used in special cases.
Cast-in-Place culverts with skewed ends may require additional stiffening of the top and bottom slabs by what is most commonly called an "edge beam" in the top slab and a "cut-off wall" in the bottom slab. An edge beam is very similar to a headwall in that it may be used to anchor guide railing posts or retain earth fill. Its main purpose, however, is to stiffen the top slab of cast-in-place culverts that lose their rigid frame action as a result of having a skewed end. A cut-off wall will stiffen the bottom slab as well as prevent water from undermining the culvert (see Section 19.9).
When additional strength is required in the concrete edge beam, the following criteria shall be used:
If there is a 1 on 2 slope to the edge beam, it will be more economical to increase the depth of the edge beam in order to meet the required design.
When the edge beam is at shoulder elevation, the edge beam height should be maintained and the width of the edge beam should be increased.
Assistance in edge beam and cut-off wall design may be obtained from the Structures Design and Construction Division.
Precast concrete culverts are fabricated in a plant where the ability to control placement and curing conditions typically results in higher strength and more durable concrete. Precasting permits efficient mass production of concrete units. The advantages usually more than offset the cost of handling and transporting the units to the site. The majority of concrete culverts installed are precast.
CONCRETE CULVERTS 19-21
Precast units are limited to certain sizes and skews. Transportation and handling limits the size of units. Skewed sections may need more reinforcement and larger slab and/or sidewall widths. The use of skewed sections will increase the cost of the culvert due to increased fabrication costs.
Figures 19-8 and 19-9 show some of the typical section details for precast concrete culverts. Headwall and wingwall connection details are shown in Figures 19-10 and 19-11.
In culverts with end sections squared off, each unit will routinely be square unless the designer has included special requirements in the Contract documents. Stage construction is one example of a special requirement which may require skewed interior sections. The units that meet at the division of the stages may need to be skewed to provide adequate width for travel lane(s).
In culverts with skewed end sections, the interior sections will routinely be square and the end sections skewed at the outside end. However, this will usually be determined by the manufacturer of the precast units unless the designer has included special requirements in the Contract documents.
Concern has been raised about the use of precast culverts on steep grades. No maximum slope is recommended for closed box culverts because of the need to match the slope of the stream. However, larger open box culverts and the frames and arches discussed in Section 19.7.4 should be limited to approximately 2%. Precast fabricators should be contacted for the maximum grade that can be fabricated. If matching a steep stream bed slope is necessary for an open culvert, the footings can be stepped and the length of the sidewall varied.
When two or more single cell precast concrete culverts are placed side by side, it is usually not possible to place the walls of adjacent cells tightly together. It is reasonable to detail a 50 mm to 100 mm gap between the walls of adjacent cells. This gap should be filled with any concrete item in the project or Class D concrete if no other concrete item is available.
Dimensions of the sidewalls and top slab and reinforcement size and spacing should not be shown on the plans, unless necessary. If sidewall or top slab dimensions are dictated by site conditions, show only affected dimensions and indicate if they are minimums, maximums or specifically required dimensions.
A note in the Contract plans shall require the manufacturer, through the contractor, to provide all design details not included in the contract plans for checking by the State. This method should result in the most economical culvert design.
Figure 19-8 Typical Cross Sections - Precast
Figure 19-9 Typical Cross Sections - Precast
Figure 19-10 Headwall Details - Precast
Figure 19-11 Headwall and Wingwall Connection - Precast
19.7.2 Reinforcement
The main reinforcement in the top and bottom slabs shall be perpendicular to the sidewalls except in skewed sections. Precast concrete culverts with skewed ends are unable to use edge beams as stiffening members because of forming restrictions. For this reason, transverse reinforcement is unable to be "cut" (as in cast-in-place culverts) to fit the skewed end section.
When a precast end section is skewed, the transverse reinforcement must be splayed to fit the geometry of the skew, (see Figure 19-7). This splaying of the reinforcement will increase the length of the transverse bars and, more importantly, the clear span of the end section. For small skews, the splayed reinforcement is usually more than adequate. However, large skews may require more reinforcement and can increase the clear span to the point where increased slab thickness may be necessary.
See Section 19.6.3 for bar identification schedule.
When contract plans do not contain complete design details for the precast concrete culvert the contractor shall be responsible for providing them. All design submissions from the contractor shall include a complete set of working drawings and a complete set of design calculations. The drawings and the design calculations shall be stamped by a Professional Engineer licensed to practice in New York State. If the Reinforced Concrete Box Culvert Design and Analysis Program is used to design the culvert, the output shall be substituted for the design calculations.
Fabrication requirements of precast concrete box culverts are contained in Section 706-17 of the Standard Specifications.
In addition to box sections, there are various types of proprietary precast concrete arches and frames available. These sections are typically used when larger culverts ($6 m ±) are required and are usually founded on footings on rock. They can be considered when hydraulics can be accommodated and/or aesthetics are a consideration. Where appropriate, they may be placed on a combined invert footing/slab or a pile supported footing.
The advantages of the precast concrete arches and frames are the same as for the precast concrete box culverts, except that longer spans (up to 12.2 m) are possible. See Figure 19-12.
Figure 19-12 Precast Arch and Frame
Fabrication requirements for precast concrete arches and frames can be found in Special Specifications available from the Structures Design and Construction Division. The contractor shall be responsible for providing all design computations and details for these units. The drawings and the design calculations shall be stamped by a Professional Engineer licensed to practice in New York State.
1. For headwalls where H > 1.8 m, specific design information must be provided in the contract documents. H is the height of the headwall above the top of the top slab. (See Figure 19-10.)
2. Reinforcement for headwalls without guide rail shall be the same as shown for headwalls with guide rail. See Figure 19-10, section A-A.
3. Headwalls where 0.3 m # H # 1.8 m are to be attached to the box culvert by use of mechanical connectors for reinforcing bar splices meeting the requirements of § 709-10 (epoxy coated) of the Standard Specifications. The female threaded portion of the connector is cast into the box culvert.
4. Threaded inserts, where detailed, shall be designed for use with #5 and #6 reinforcing steel. Inserts shall be non-corrosive and, when used in 35 MPa concrete, able to resist minimum pull out loads of 49 kN for #5 reinforcing and 71 kN for #6 reinforcing.
5. Dowel bars used to attach the cut-off wall to the box culvert shall be drilled and grouted per the requirements of Standard Specification § 586. The keyway shall be grouted with the same material as the dowels.
The anchorage of the guide railing is determined by the amount of fill over the top of the unit. If there is less than 900 mm of fill, guide railing shall be anchored into the headwall, edge beam or individual concrete pedestals. If there is more than 900 mm of fill (i.e., enough for standard length post embedment) regular highway guide rail can be used. The offset from the end of culvert to the back of the guide rail should be considered when choosing the type of highway guide rail, i.e., the guide rail deflection characteristics should be reviewed.
When the recommended offsets from the back of the posts to the shoulder break can not be achieved or the embankment slopes away from the normal shoulder break steeper than a 1:2 slope, extra long posts are required. In these situations, the 900 mm criteria is no longer valid. See Chapter 10 for guidance on the required length of posts.
When the guide rail is anchored to the headwall, edge beam or pedestal, either culvert rail, bridge rail, or equivalent can be used. Bridge rail is recommended for culvert spans over 6 m. The choice may be made on the basis of types of railings being used in the project or Regional preference. For example, Region 10 prefers using heavy post blocked-out corrugated beam guide railing.
The approach railing details in Figure 19-13 have been modified to match the current BDD sheet for Steel Bridge Rail-Two Rail. For culvert rail details, see Figures 19-13, 19-14 and 19-15.
The following construction notes, as appropriate, shall be placed on the plans:
Standard Culvert Railing Construction Notes:
After the nuts on the anchor bolts have been tightened to the satisfaction of the Engineer, the anchor bolts shall be flame cut 25 mm above the nut and the threads above the nut shall be damaged as directed by the Engineer to prevent removal. All other nuts shall be either tack welded in place or have lock washers, as determined by the Engineer. Galvanizing damaged by flame cutting and/or tack welding shall be repaired according to §719-01 of the Standard Specifications.
Rail terminus details shall be in accordance with the latest approved details. (For information see Figure 19-13 and 19-14 or the latest Bridge Design Data Sheets.)
All railing is to be fabricated and erected so that the rails are parallel to the roadway and the posts are truly vertical.
The box beam rail elements shall be long enough to span the entire culvert or be a minimum of 6 m in length.
For material requirements and construction details, see §568 of the Standard Specifications.
Anchor rods shall be cast into the concrete or grouted into 40 mm N through holes made with a core drill. The grout used shall meet the requirements of the Standard Specifications § 701-05, Concrete Grouting Materials, and appear on the Department's Approved List.
Figure 19-13 Approach Railing Details, Transition to Box Beam Guide Railing
Figure 19-14 Approach Railing Details
Figure 19-15 Culvert Railing Details, Cast-In-Place Connection
A minimum 450 mm wide cut-off wall is required in all culverts with invert slabs to prevent undermining. The cutoff wall should be 1.2 m deep or to the top of sound rock if the rock is closer. For culverts with skewed ends, the cut-off wall also provides stiffening to the bottom slab. The 1.2 m is measured from the lowest elevation of the top surface of the invert slab. See Figures 19-2, 19-4 and 19-6.
When cut-off walls are required, they shall always be specified at each end of the barrel. When a concrete apron (Section 19.11) is specified, an additional cut-off wall shall also be specified at the end of the apron. The bottom of all wingwall footings should be at or below the bottom of the cut-off wall to prevent scour around the edges of the cut-off wall.
When a precast culvert is specified, the cut-off wall may be precast or cast-in-place. When precast concrete cut-off walls are specified, their cost should be included in the cost of the culvert barrel. No separate item is required. When a cast-in-place culvert is specified, the cut-off wall should be cast-in-place.
Closed box culverts shall have a low flow dish whenever the stream is classified as a fishing stream by the Department of Environmental Conservation and a low flow dish is requested. The Regional Hydraulics Engineer can assist in communications with the Department of Environmental Conservation. The depth of the dish may be as small as 150 mm or as large as 300 mm. The depth of the dish depends on the quantity of flow. Lower flows may require a deeper dish to provide adequate depth of flow. A typical dish is shown in Detail E of Figures 19-1 and 19-8.
The dish is usually in the center of the invert slab, but there may be times when the dish will be at or near the sidewall. This may happen when the stream is on a curved alignment and the low flow of the stream is on the outside of the curve.
There are times when the Department of Environmental Conservation may require native stream bed material over the top of the bottom slab. The typical requirement is for a 300 mm depth. One problem that may develop is the movement of the native material during high flows. Typically, however, material washed out during the high flow will be replaced with new material as the water recedes. This periodic movement of material may abrade the surface of the concrete culvert. The depth of the concrete covering the reinforcing bar should be increased if this situation is anticipated. Note that covering the bottom slab with native material will make it very difficult, if not impossible, to inspect the bottom slab.
The Reinforced Concrete Box Culvert Design and Analysis Program does not design the bottom slab of a culvert with a low flow dish since it is not a prismatic member. The additional triangle of concrete should be considered as not contributing to the strength of the section.
19-34 CONCRETE CULVERTS
Box culverts can significantly increase the stream flow velocity because the concrete has a roughness coefficient significantly lower (i.e., smoother) than the stream bed and banks. The longer the culvert and the steeper the slope, the more the velocity will be increased. To dissipate this increase in energy and to prevent scour, a stone apron shall be placed at the outlet of all culverts. In addition, a stone apron should be specified at the inlet of all culverts to prevent scour caused by the constriction of flow.
The recommended minimum length of the stone apron is 7.5 m. The stone apron should cover the full width of the stream bed. In addition, stone filling should be placed on the side slopes to an elevation 300 mm± above Design High Water. A 1.5 m wide by 1.2 m deep key of stone filling should be placed in the stream bed at the end of the apron away from the culvert. Stone filling should also be required to stabilize all disturbed slopes to an elevation 300 mm± above Design High Water.
The Regional Hydraulics Engineer or the Structures Design and Construction Division Hydraulics Unit should be consulted to determine an appropriate length apron for special situations such as locations where excessive scour has occurred.
The stone apron will begin at the end of the barrel if no concrete apron is specified. If a concrete apron is specified, the stone apron will begin at the end of the concrete apron. The size of the stone filling shall be determined by the design velocity or Regional preference, whichever is larger. For design velocities of 3 m/s or higher, heavy stone filling should be specified. For design velocities less than 3 m/s, medium stone filling may be used.
When stream velocities are very high (>5 m/s), where special site conditions exist or where the existing soils are very poor, a concrete apron may be used. It should extend to the end of the wingwalls. See Figure 19-6. A cut-off wall (section 19.9) is required at the end of the concrete apron and at the end of the culvert barrel. The cut-off wall at the end of the culvert barrel is added protection if the apron fails or separates from the barrel. Concrete aprons may fail from frost heaves, bed load material abrading the apron or other unique situations.
Where there is significant movement of cobbles and boulders in the bed load, an open culvert on a strip footing on piles should be investigated. The movement of cobbles and boulders may damage a concrete invert.
For some unique situations with very high velocities, steep profiles, or special circumstances, energy dissipators (baffles) may be required to reduce the velocity. Baffles are concrete sections that extend 300 mm to 750 mm above the bottom slab. The Regional Hydraulics Engineer or the Structures Design and Construction Division Hydraulics Unit should be consulted to determine the need for and the design of energy dissipators.
When a precast culvert is specified, the concrete apron and wingwalls may be precast or cast-in-place. When precast concrete aprons and/or wingwalls are specified, their cost should
be included in the cost of the culvert barrel. No separate item is required. When a cast-in-place culvert is specified, the concrete apron and wingwalls should be cast-in-place.
Draining surface and ground water away from the culvert through the subbase is just as important as the conveyance of water through the culvert. All culverts should have a minimum longitudinal slope of approximately 1%, if possible, to drain the water that permeates through the pavement and subbase away from the top of the culvert. The slope is very important, in situations where there is low fill (0.3 m±) or asphalt directly on the culvert, to limit the likelihood that water will not pond, freeze and cause potholes or other problems.
If a longitudinal slope is not possible, a 1% slope (wash), perpendicular to the centerline of the culvert, can be used. The wash can be from the centerline to each side or all in one direction. The wash can be formed into a cast-in-place culvert but is difficult to form on precast culverts. On precast culverts, the wash can be added after the culvert is in place by placing a shim course of asphalt or concrete.
In some instances, culverts will be used to allow the passage of things other than water such as pedestrians, bicycles, trains, golf carts or farm animals. In cases where it is desirable to have a dry environment, a waterproof membrane should be used to cover the joints between precast culvert sections or to cover the construction joints in cast-in-place culverts. Even though a joint sealer is always placed between individual precast concrete culvert sections and the sections are pulled tightly together, water may seep through the joint. The minimum requirement for waterproofing these joints is to provide a membrane strip, having a minimum width of 600 mm, centered on the joints, covering the top slab, and then extending down the sidewalls to the footing. In this application, the membrane is not intended to protect the concrete.
A waterproof membrane may be used to cover the joints of precast concrete culverts that convey water through the culvert. The Designer must understand that the purpose of the waterproofing membrane is to restrict seepage of water or migration of backfill material through the joints in the culverts and it is not intended to protect the concrete.
When watertight joints are required, contact the Materials Bureau for the proper waterproof membrane item to be included in the contract. This item contains all the material and construction details necessary for the application of the system.
1. Standard Specifications for Highway Bridges 15th Edition (1992) as amended by the Interim Specifications - Bridges - 1993, American Association of State Highway and Transportation Officials, 444 North Capital Street, N. W. Suite 249, Washington, D. C.
2. Reinforced Concrete Box Culvert Design and Analysis Program Users Manual, January 1995, Structures Design and Construction Division, New York State Department of Transportation, State Campus, Albany, NY 12232.
3. Standard Specifications of Construction and Materials, January 2, 1995, Design Quality Assurance Bureau, New York State Department of Transportation, State Campus, Albany, NY 12232.
4. Model Drainage Manual - 1991, American Association of State Highway and Transportation Officials, 444 North Capital Street, N. W. Suite 225, Washington, D. C.
19.9, 19.11
Box Culvert Computer Design Program
Cast-In-Place Culverts
19.2, 19.6
19.6, 19.6.1, 19.7.1
Culvert Railing
19.6.3, 19.9, 19.11
Low Flow Dish
19.2, 19.6, 19.7
Skewed Culverts
19.4.10, 19.6.3, 19.6.4, 19.7, 19.7.2
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