12.3-i

Table of Contents

Chapter 12 ..................................................................................................................................................

Special Bridges 

 12.3 Concrete Pipe Culverts......................................................................... 12.3.1 

 

 12.3.1 Introduction.......................................................................... 12.3.1 

 

 12.3.2 Design Characteristics ......................................................... 12.3.2 

 Structural Behavior ....................................................... 12.3.2 

 12.3.3 Types and Shapes of Concrete Pipe Culverts ...................... 12.3.4 

 

 12.3.4 Hazards of Culvert Inspection ............................................. 12.3.4 

 12.3.5 Overview of Common Defects ............................................ 12.3.4 

 

 12.4.6 Inspection Procedures and Locations .................................. 12.3.5 

 Procedures..................................................................... 12.3.6 

 Visual...................................................................... 12.3.6 

 Physical................................................................... 12.3.6 

 Advanced Techniques............................................. 12.3.6 

 Locations....................................................................... 12.3.7 

 Misalignment .......................................................... 12.3.7 

 Joint Defects ........................................................... 12.3.8 

 Cracks ................................................................... 12.3.11 

 Spalls .................................................................... 12.3.14 

 Slabbing................................................................ 12.3.15 

 Durability.............................................................. 12.3.15 

 End Section Drop-off............................................ 12.3.15 

 Wingwalls and Headwalls........................................... 12.3.17 

 

 12.3.7 Evaluation.......................................................................... 12.3.17 

 NBI Rating Guidelines................................................ 12.3.17 

 Element Level Condition State Assessment................ 12.3.17 

 
SECTION 12: Special Bridges 

TOPIC 12.3: Concrete Pipe Culverts 

12.3-ii

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12.3.1 

Topic 12.3 Concrete Pipe Culverts 

12.3.1 

Introduction A common type of culvert used today is the concrete pipe culvert (see Figure 

12.3.1). The concrete pipe culvert is typically circular or elliptical in shape. In 

situations where the required size of the opening is very large, two or more 

concrete pipe culverts may be used (see Figure 12.3.2). 

Figure 12.3.1 Concrete Pipe Culvert 

Culverts are somewhat protected by the soil backfill from rapid fluctuations in 

surface temperature and direct application chloride (salts) used for deicing. As a 

result they are generally more resistant to surface deterioration than concrete bridge 

elements. 

See Topic P.3, Culverts, for a detailed presentation of various culvert types. 
SECTION 12: Special Bridges 

TOPIC 12.3: Concrete Pipe Culverts 

12.3.2 

Figure 12.3.2 Twin Concrete Pipe Culvert 

12.3.2 

Design 

Characteristics 

Structural Behavior Concrete culverts are classified as rigid structures because they do not bend or 

deflect appreciably. 

The load carrying capability of rigid culverts is essentially provided by the 

structural strength of the pipe itself and little benefit from the surrounding soil is 

required. When vertical loads are applied to rigid culverts, tension and 

compression zones are created (see Figure 12.3.3). Reinforcing steel is added to 

the tension zones to increase the tensile strength of the pipe. Shear stress in the 

haunch or “bell” area where the pipe sections are joined, can be critical for heavily 

loaded rigid pipe on hard foundations, especially if the pipe bed preparation is 

inadequate. Because rigid pipe is stiffer than the surrounding soil, it carries a 

substantial portion of the load. 
SECTION 12: Special Bridges 

TOPIC 12.3: Concrete Pipe Culverts 

12.3.3 

DL & LL

Tension and Compression 

Zones in Rigid Pipes

Compression

Tension

Figure 12.3.3 Rigid Culvert Stresses 

The weight of earth that must be carried varies with soil characteristics and 

installation conditions. The installation conditions can have a significant influence 

on the loads that must be carried by a rigid culvert. There are two major classes of 

installation conditions: 1) trench, where culverts are placed in natural ground or 

compacted fill with a controlled trench width and 2) embankment, where culverts 

are placed in or covered by an embankment. 

In narrow trench installations, the pipe is placed in a relatively narrow trench and 

covered with backfill material. The backfill tends to settle more than the 

undisturbed soil beside the trench. Friction between the backfill material and the 

sides of the trench tends to help support the backfill material reducing the load on 

the pipe. In effect the width of the soil column over the pipe is decreased. 

As the trench width increases, the effect of the friction at the sides of the trench is 

reduced and dead load on the pipe is increased. The amount that the loading is 

increased depends on trench width and the amount of backfill settlement, which is 

related to compaction. Poorly compacted soil will settle more than well compacted 

soil. In a trench that is too wide, poor compaction can result in an increase in the 

dead load on the pipe. Pipes placed in a shallow bedding on top of the original 

ground surface and then covered by the embankment material will have loads 

similar to the very wide trench. Pipes placed in trenches in the original ground 

prior to being covered by embankment have reduced earth loads similar to those 

described for the narrow trench installations. 
SECTION 12: Special Bridges 

TOPIC 12.3: Concrete Pipe Culverts 

12.3.4 

12.3.3 

Types and Shapes 

of Concrete Pipe 

Culverts 

The size of the opening of the pipe is determined by the peak flow of the channel. 

The circular shape is the most common shape manufactured for pipe culverts. It is 

hydraulically and structurally efficient under most conditions. Possible hydraulic 

drawbacks are that circular pipe generally causes some reduction in stream width 

during low flows. It may also be more prone to clogging than some other shapes. 

Elliptical shapes are used in situations where horizontal or vertical clearance is 

limited. The oblong shape allows the pipe to fit where a circular pipe may not, but 

still allows for the necessary size opening. Elliptical shaped pipe culverts may 

also be used when a wider section is desirable for low flow levels. As with 

circular shaped pipe culverts, these shapes also are prone to clogging as the depth 

of flow increases. 

Concrete culvert pipe is manufactured in up to five standard strength 

classifications. Higher classification numbers indicate higher strength. All of 

these standard shapes are manufactured in a wide range of sizes. Circular and 

elliptical pipes are available with standard sizes as large as 3.7 meters (12 feet) in 

diameter, with larger sizes available for special designs. Several factors such as 

span length, vertical and horizontal clearance, peak stream flow and terrain 

determine which shape of pipe culvert is used. 

Precast concrete pipe culverts are manufactured in three standard shapes: 

3⁄4 Circular 

3⁄4 Horizontal elliptical 

3⁄4 Vertical elliptical 

See pages 12.3.18 – 12.3.20 for the different standard sizes for concrete pipe 

culvert shapes. 

12.3.4 

Hazards of Culvert 

Inspection 

The bridge inspector should be alerted to the following hazards when inspecting a 

culvert. 

3⁄4 Inadequate ventilation 

3⁄4 Drowning 

3⁄4 Toxic chemicals 

3⁄4 Animals 

3⁄4 Quick conditions at the outlet

Refer to Topic 3.2.5 for a detailed discussion of each hazard. 

12.3.5 

Overview of 

Common Defects 

Common defects that occur on concrete pipe culverts include: 

3⁄4 Cracking (flexure, shear, temperature, shrinkage, mass concrete) 
SECTION 12: Special Bridges 

TOPIC 12.3: Concrete Pipe Culverts 

12.3.5 

3⁄4 Scaling 

3⁄4 Delamination 

3⁄4 Spalling 

3⁄4 Chloride contamination 

3⁄4 Efflorescence 

3⁄4 Ettringite formation 

3⁄4 Honeycombs 

3⁄4 Pop-outs 

3⁄4 Wear 

3⁄4 Collision damage 

3⁄4 Abrasion 

3⁄4 Overload damage 

3⁄4 Reinforcing steel corrosion 

3⁄4 Embankment erosion at culvert entrance and exit 

3⁄4 Roadway settlement 

3⁄4 Foundation Failure 

3⁄4 Scour / Undermining 

3⁄4 Misalignment 

3⁄4 Settlement of pipe sections 

Refer to Topic 2.2 for a detailed explanation of the properties of concrete, types 

and causes of concrete deterioration, and the examination of concrete. 

12.3.6 

Inspection 

Procedures and 

Locations 

Safety is the most important reason that culverts should be inspected. For a more 

detailed discussion on reasons for inspecting culverts, see Topic P.3.1. 

Previous inspection reports and as-built plans, when available, should be reviewed 

prior to, and possibly during, the field inspection. A review of previous reports 

will familiarize the inspector with the structure and make detection of changed 

conditions easier. A review will also indicate critical areas that need special 

attention and the possible need for special equipment. 

A logical sequence for inspecting culverts helps ensure that a thorough and 

complete inspection will be conducted. In addition to the culvert components, the 

inspector should also look for highwater marks, changes in the drainage area, 

settlement of the roadway, and other indications of potential problems. In this 

regard, the inspection of culverts is similar to the inspection of bridges. 

For typical installations, it is usually convenient to begin the field inspection with 

general observations of the overall condition of the structure and inspection of the 

approach roadway. The inspector should select one end of the culvert and inspect 

the embankment, waterway, headwalls, wingwalls, and culvert barrel. The 

inspector should then move to the other end of the culvert. The following 

sequence is applicable to all culvert inspections: 

3⁄4 Review available information 

3⁄4 Observe overall condition 
SECTION 12: Special Bridges 

TOPIC 12.3: Concrete Pipe Culverts 

12.3.6 

3⁄4 Inspect approach roadway and embankment 

3⁄4 Inspect waterway (see Topic 11.2) 

3⁄4 Inspect end treatments 

3⁄4 Inspect culvert barrel 

Procedures Visual 

The inspection of concrete pipe culverts for cracks, spalls, and other defects is 

primarily a visual activity. 

Physical 

Sounding by a hammer can be used to detect delaminated areas. A delaminated 

area will have a distinctive hollow “clacking” sound when tapped with a hammer. 

A hammer hitting sound concrete will result in a solid “pinging” type sound. 

Advanced Inspection Techniques 

Several advanced techniques are available for concrete inspection. Nondestructive 

methods, described in Topic 13.2.2, include: 

3⁄4 Acoustic wave sonic/ultrasonic velocity measurements 

3⁄4 Delamination detection machinery 

3⁄4 Electrical methods 

3⁄4 Electromagnetic methods 

3⁄4 Pulse velocity 

3⁄4 Flat jack testing 

3⁄4 Ground-penetrating radar 

3⁄4 Impact-echo testing 

3⁄4 Infrared thermography 

3⁄4 Laser ultrasonic testing 

3⁄4 Magnetic field disturbance 

3⁄4 Neutron probe for detection of chlorides 

3⁄4 Nuclear methods 

3⁄4 Pachometer 

3⁄4 Rebound and penetration methods 

3⁄4 Ultrasonic testing 

3⁄4 Radiography 

Other methods, described in Topic 13.2.3, include: 

3⁄4 Core sampling 

3⁄4 Carbonation 

3⁄4 Concrete permeability 

3⁄4 Concrete strength 

3⁄4 Endoscopes and videoscopes 

3⁄4 Moisture content 

3⁄4 Petrographic examination 
SECTION 12: Special Bridges 

TOPIC 12.3: Concrete Pipe Culverts 

12.3.7 

3⁄4 Reinforcing steel strength 

3⁄4 Chloride test 

3⁄4 Matrix analysis 

3⁄4 ASR evaluation 

Locations Rigid culverts such as precast concrete pipe do not deflect appreciably before 

cracking or fracturing. As a result, shape inspections, while very important in 

flexible structures, are of little value in inspecting concrete culverts. 

Although the need for soil stability and side support is obviously important with 

flexible pipe, it is less important with rigid pipe. However, adequate stability of the 

surrounding soil is necessary to prevent settlement around the culvert and to 

achieve load carrying capability. The inspector should therefore look for any 

indications of a lack of soil stability such as settlement or misalignment as well as 

signs of structural distress such as cracking. Descriptions of the types of distress to 

look for during inspection are provided in the following paragraphs. Guidelines for 

condition ratings of concrete pipe are included at the end of this Topic. 

The following is a list of areas that should be inspected in concrete pipe culverts. 

3⁄4 Misalignment 

3⁄4 Joint Defects 

3⁄4 Cracks 

3⁄4 Spalls 

3⁄4 Slabbing 

3⁄4 Durability 

3⁄4 End Section Drop-off 

The following excerpts are from a reproduction of the out-of-print Culvert 

Inspection Manual Report No.-IP-86-2 – Chapter 5, Section 8.

Misalignment 

Misalignment may indicate the presence of serious problems in the supporting soil. 

The vertical and horizontal alignment of the culvert barrel should be checked by 

sighting along the crown and sides of the culvert and by checking for differential 

movement or settlement at joints between pipe sections. Vertical alignment should 

be checked for sags, faulting, and heaving. The inspector should be aware that 

pipes are occasionally laid with a camber or a grade change (broken back grade) to 

allow for fill settlement. 

Sags which trap water may aggravate settlement problems by saturating the 

supporting soil. Horizontal alignment should be checked for straightness or smooth 

curvature for those culverts constructed with a curved alignment. Alignment 

problems may be caused by improper installation, undermining, or uneven 

settlement of fill. The inspector should attempt to determine which of those 

problems is causing the misalignment. If undermining is determined to be the 

probable cause, maintenance forces should be notified since damage will continue 

until the problem is corrected. The inspector should also try to determine whether 

the undermining is due to piping, water exfiltration, or infiltration of backfill 
SECTION 12: Special Bridges 

TOPIC 12.3: Concrete Pipe Culverts 

12.3.8 

material. When the misalignment is due to improper installation or uneven 

settlement, repeat inspections may be needed to determine if the settlement is still 

progressing or has stabilized. 

Joint defects 

Joint defects are fairly common and can range from minor problems to problems 

that are serious in nature. Typical joint defects include leakage (exfiltration and 

infiltration), cracks, and joint separation. Past and current criteria should be 

reviewed as some agencies design culverts with open joints to perform as 

subdrains. 

 (1) Exfiltration - Exfiltration occurs when leaking joints allow 

water flowing through the pipe to leak into the supporting 

material. Many culverts are built with joints that are not 

watertight or with mortar joints that crack with minor deflection, 

movement, or settlement of the pipe sections. Minor leakage 

may not always be a significant problem unless soils are quite 

erosive. However, if leaking joints contribute to or cause piping, 

then serious misalignment of the culvert or even failure may 

result. Leaking joints may be detected during low flows by 

visual observation of the joints and by checking around the ends 

of the culvert for evidence of piping. 

 (2) Infiltration – Infiltration is the opposite of exfiltration. Many 

culverts are essentially empty except during peak flows. When 

the water table is higher than the culvert invert, water may seep 

into the culvert between storms. This infiltration of water can 

cause settlement and misalignment problems if it carries fine 

grained soil particles from the surrounding backfill. Infiltration 

may be difficult to detect visually in its early stages although it 

may be indicated by open joints, staining at the joints on the 

sides and top of the culvert, deposits of soil in the invert, or by 

depressions over the culvert, as shown in Figure 12.3.4 (Exhibit 

105). 
SECTION 12: Special Bridges 

TOPIC 12.3: Concrete Pipe Culverts 

12.3.9 

 (3) Cracks – Cracks in the joint area may be caused by improper 

handling during installation, improper gasket placement, and 

movement or settlement of the pipe sections. Cracked joints are 

more than likely not watertight even if gaskets were used. 

However, if no other problems are evident, such as differential 

movement between pipe sections, and the cracks are not open or 

spalling, they may be considered a minor problem to only be 

noted in the inspection report. Severe joint cracks are similar in 

significance to separated joints.

Figure 12.3.4 (Exhibit 105) Surface Indications of Infiltration 

 (4) Separated Joints - Joint separations may be caused by the same 

forces described under misalignment (settlement, undermining, 

or improper installation). Joint separations are significant 

because they accelerate damage caused by exfiltration and 

infiltration resulting in the erosion of the backfill material. 

Examples of severe infiltration through separate joints are 

shown in Exhibits 106 and 107. Separated joints are often found 

when severe misalignment is found. In fact either problem may 

cause or aggravate the other. Movement of the soil in the 

general direction of the culvert's centerline may cause sections 

to gradually pull apart. Embankment slippage may also cause 

separations to occur. 
SECTION 12: Special Bridges 

TOPIC 12.3: Concrete Pipe Culverts 

12.3.10 

Figure 12.3.5 (Exhibit 106) Example of Severe Infiltration of Backfill 

Material through Separated Joints 

Figure12.3.6 (Exhibit 107) Severe Infiltration of Ground Water Through

Separated Joint 
SECTION 12: Special Bridges 

TOPIC 12.3: Concrete Pipe Culverts 

12.3.11 

 Cracks 

Longitudinal Cracks – Concrete is strong in compression but weak in tension. 

Reinforcing steel is provided to handle the tensile stresses. Hairline longitudinal 

cracks in the crown or invert indicate that the steel has accepted part of the load. 

Cracks less than 0.01 inches in width are minor and only need to be noted in the 

inspection report. Cracks greater than hairline cracks, or those more than 0.01 inch 

in width but less than 0.1 inches, should be described in the inspection report and 

noted as possible candidates for maintenance. Longitudinal cracking in excess of 

0.1 inch in width may indicate overloading or poor bedding. If the pipe is placed 

on hard material and backfill is not adequately compacted around the pipe or under 

the haunches of the pipe, loads will be concentrated along the bottom of the pipe 

and may result in flexure or shear cracking, as illustrated in Exhibit 108. 

Figure 12.3.7 (Exhibit 108) Results of Poor and Good Side Support, Rigid 

Pipe 

Other signs of distress such as differential movement, efflorescence, spalling, or 

rust stains should also be noted. Examples of longitudinal cracking are shown in

Exhibits 109 and 110. When cracks are wider than 0.1 inch measurements should 

be taken of fill height and the diameter of the pipe both horizontally and vertically 

to permit analysis of the original design. Crack measurements and photographs 

may be useful for monitoring conditions during subsequent inspections. 
SECTION 12: Special Bridges 

TOPIC 12.3: Concrete Pipe Culverts 

12.3.12 

Figure 12.3.8 (Exhibit 109) Minor Longitudinal Crack with Efflorescence 

Figure 12.3.9 (Exhibit 110) Severe Longitudinal Cracks with Differential 

Movement and Spalling 
SECTION 12: Special Bridges 

TOPIC 12.3: Concrete Pipe Culverts 

12.3.13 

Transverse Cracks – Transverse or circumferential cracks may also be caused by 

poor bedding. Cracks can occur across the bottom of the pipe (broken bell) when 

the pipe is only supported at the ends of each section. This is generally the result of 

poor installation practices such as not providing indentions (bell holes) in hard 

foundation material for the ends of bell and spigot-type pipe or not providing a 

sufficient depth of suitable bedding material. Cracks may occur across the top of 

pipe (broken back) when settlement occurs and rocks or other areas of hard 

foundation material near the midpoint of a pipe section are not adequately covered 

with suitable bedding material. Transverse cracking is illustrated in Exhibit 111. 

Figure 12.3.10 (Exhibit 111) Transverse or Circumferential Cracks 
SECTION 12: Special Bridges 

TOPIC 12.3: Concrete Pipe Culverts 

12.3.14 

 Spalls 

Spalling is a fracture of the concrete parallel or inclined to the surface of the 

concrete. In precast concrete pipe, spalls often occur along the edges of either 

longitudinal or transverse cracks when the crack is due to overloading or poor 

support rather than simple tension cracking. Spalling may also be caused by the 

corrosion of the steel reinforcing when water is able to reach the steel through 

cracks or shallow cover. As the steel corrodes, the oxidized steel expands, causing 

the concrete covering the steel to spall. Spalling may be detected by visual 

examination of the concrete along the edges of cracks. Tapping with a hammer 

should be performed along cracks to check for areas that have fractured but are not 

visibly separated. Such areas will produce a hollow sound when tapped. These 

areas may be referred to as delaminations or incipient spalls. Exhibit 112 shows 

spalling with reinforcing steel exposed. 

Figure 12.3.11 (Exhibit 112) Spalling Exposing Reinforcing Steel 
SECTION 12: Special Bridges 

TOPIC 12.3: Concrete Pipe Culverts 

12.3.15 

 Slabbing 

The terms slabbing, shear slabbing, or slab shear refer to a radial failure of the 

concrete which occurs from straightening of the reinforcement cage due to 

excessive deflection. It is characterized by large slabs of concrete "peeling" away 

from the sides of the pipe and a straightening of the reinforcing steel as shown in 

Exhibit 113. Slabbing is a serious problem that may occur under high fills. 

Figure 12.3.12 (Exhibit 113) Shear Slabbing 

 Durability 

Durability is a measure of a culvert's ability to withstand chemical attack and 

abrasion. Concrete pipes are subject to chemical attack in strongly acidic 

environments such as drainage from mines and may also be damaged by abrasion. 

Abrasion damage is a wearing away of the concrete surface by sediment and debris 

being transported by the stream. Mild deterioration or abrasion less than 1/4 inch 

deep should be noted in the report. More severe surface deterioration should be 

reported as a potential candidate for maintenance. In severe cases where the invert 

is completely deteriorated, maintenance forces should be given immediate 

notification. When linings are used to protect against chemical attack or abrasion 

the condition of the lining should be noted in the report. 

End Section Drop-off 

This type of distress is usually due to outlet erosion as discussed earlier in the 

sections on end treatments and waterways. It is caused by the erosion of the 

material supporting the pipe sections on the outlet end of the culvert barrel. 
SECTION 12: Special Bridges 

TOPIC 12.3: Concrete Pipe Culverts 

12.3.16 

Figure 12.3.13 (Exhibit 114) Condition Rating Guidelines 
SECTION 12: Special Bridges 

TOPIC 12.3: Concrete Pipe Culverts 

12.3.17 

Wingwalls and Headwalls Wingwalls are provided to support the embankment around the openings of the 

culvert. Wingwalls should be inspected to ensure they are in proper vertical 

alignment. Wingwalls may be tilted due to settlement, slides or scour. See Topic 

10.1 for a detailed description of defects and inspection procedures of wingwalls. 

12.3.7 

Evaluation State and federal rating guideline systems have been developed to aid in the 

inspection of concrete pipe culverts. The two major rating guideline systems 

currently in use are the FHWA's Recording and Coding Guide for the Structural 

Inventory and Appraisal of the Nation's Bridges used for the National Bridge 

Inventory (NBI) component rating method and the AASHTO element level 

condition state assessment method. 

NBI Rating Guidelines Using NBI rating guidelines, a 1-digit code on the Federal Structure Inventory and 

Appraisal (SI&A) sheet indicates the condition of the culvert (Item 62). This item 

evaluates the alignment, settlement, joints, structural condition, scour, and other 

items associated with culverts. Rating codes range from 9 to 0 where 9 is the best 

rating possible. See Topic 4.2 (Item 62) for additional details about NBI Rating 

Guidelines. The rating code is intended to be an overall evaluation of the culvert. 

Integral wingwalls to the first construction or expansion joint shall be included in 

the evaluation. It is also important to note that Items 58-Deck, 59-Superstructure, 

and 60-Substructure shall be coded “N” for all culverts. 

The previous inspection data should be considered along with current inspection 

findings to determine the correct rating. 

Element Level Condition 

State Assessment 

In an element level condition state assessment of a concrete pipe culvert, the 

AASHTO CoRe element is: 

Element No. Description

241 Reinforced Concrete Culvert 

The unit quantity for culverts is meters or feet of culvert length along the barrel. 

The total quantity equals the culvert length times the number of barrels. The 

inspector must visually evaluate each 1 m (1 ft) slice of the culvert barrel(s) and 

assign the appropriate condition state description. The total length must be 

distributed among the four available condition states depending on the extent and 

severity of deterioration. The sum of the individual condition state quantities must 

equal the total element quantity. Condition state 1 is the best possible rating. See 

the AASHTO Guide for Commonly Recognized (CoRe) Structural Elements for 

condition state descriptions. 

A Smart Flag is used when a specific condition exists, which is not described in 

the CoRe element condition state. The severity of the damage is captured by 

coding the appropriate Smart Flag condition state. The Smart Flag quantities are 

measured as each, with only one each of any given Smart Flag per bridge. 

For settlement of the culvert, the “Settlement” Smart Flag, Element No. 360, can 

be used and one of three condition states assigned. For channel scour at the 

culvert ends, the “Scour” Smart Flag, Element No. 361, can be used and one of 

three condition states assigned. 
SECTION 12: Special Bridges 

TOPIC 12.3: Concrete Pipe Culverts 

12.3.18 

Dimensions and Approximate Weights of Concrete Pipe 

*ASTM C 76 – Reinforced concrete Culvert, Storm Drain and Sewer Pipe, 

Tongue and Groove Joints 

WALL A WALL B WALL C 

Internal 

Diameter 

inches 

Minimum 

Wall 

Thickness, 

inches 

Approximate 

Weight, 

pounds per 

foot 

Minimum 

Wall 

Thickness, 

inches 

Approximate 

Weight, 

pounds per 

foot 

Minimum 

Wall 

Thickness, 

inches 

Approximate 

Weight, 

pounds per 

foot 

96 8 2710 9 3090 9 3⁄4 3355 

102 8 1⁄2 3078 9 1⁄2 3480 10 1⁄4 3760 

108 9 3446 10 3865 10 3⁄4 4160 

Large Sizes of Pipe Tongue and Groove Joint 

Internal 

Diameter 

Inches 

Internal 

Diameter 

Feet 

Wall 

Thickness 

Inches 

Approximate 

Weight, pounds 

per foot 

114 9 1⁄2 9 1⁄2 3840 

120 10 10 4263 

126 10 1⁄2 10 1⁄2 4690 

132 11 11 5148 

138 11 1⁄2 11 1⁄2 5627 

144 12 12 6126 

150 12 1⁄2 12 1⁄2 6647 

156 13 13 7190 

162 13 1⁄2 13 1⁄2 7754 

168 14 14 8339 

174 14 1⁄2 14 1⁄2 8942 

180 15 15 9572 

* For description of ASTM C 76 see page 12.3.20

Figure 12.3.14 Standard Sized for Concrete Pipe (Source: American Concrete Pipe Association)
SECTION 12: Special Bridges 

TOPIC 12.3: Concrete Pipe Culverts 

12.3.19 

Typical Cross Section of Arch Pipe 

Horizontal 

and 

Vertical 

Ellipse 

Pipe 

Dimensions and Approximate Weights of Elliptical Concrete Pipe 

*ASTM C 507 – Reinforced Concrete Elliptical Culvert, Storm Drain and Sewer Pipe 

Equivalent 

Round Size, 

inches 

Minor Axis, 

inches 

Major Axis, 

inches 

Minimum 

Wall 

Thickness, 

 inches 

Water-Way 

Area, 

square feet 

Approximate 

Weight, 

pounds per foot 

96 77 121 9 1⁄2 52.4 3420 

102 82 128 9 3⁄4 59.2 3725 

108 87 136 10 66.4 4050 

114 92 143 10 1⁄2 74.0 4470 

120 97 151 11 82.0 4930 

132 106 166 12 99.2 5900 

144 116 180 13 118.6 7000 

* For description of ASTM C 507 see page 12.3.20

Figure 12.3.14 Standard Sized for Concrete Pipe (Source: American Concrete Pipe Association), continued
SECTION 12: Special Bridges 

TOPIC 12.3: Concrete Pipe Culverts 

12.3.20 

Typical Cross Section of Arch Pipe 

Dimensions and Approximate Weights of Concrete Arch Pipe 

*ASTM C 506 – Reinforced Concrete Arch Culvert, Storm Drain and Sewer Pipe 

Equivalent 

Round Size, 

inches 

Minimum Rise, 

 inches 

Minimum 

Span, 

inches 

Minimum 

Wall 

Thickness, 

 inches 

Water-Way 

Area, 

square feet 

Approximate 

Weight, 

pounds per foot 

96 77 1/4 122 9 51.7 3110 

108 87 1/8 138 10 66.0 3850 

120 96 7/8 154 11 81.8 5040 

132 106 1/2 168 3/4 10 99.1 5220 

* For description of ASTM C 506 see page 12.3.20

Figure 12.3.14 Standard Sized for Concrete Pipe (Source: American Concrete Pipe Association), continued
SECTION 12: Special Bridges 

TOPIC 12.3: Concrete Pipe Culverts 

12.3.21 

American Society for Testing and Materials (ASTM) descriptions for select rigid pipe culverts 

ASTM C 76 Reinforced concrete Culvert, Storm Drain, and Sewer Pipe: Covers reinforced concrete pipe 

intended to be used for the conveyance of sewage, industrial wastes, and storm waters, and 

for the construction of culverts. Class I – 60 inches through 144 inches in diameter; Class II, 

III, IV and V – 12 inches through 144 inches in diameter. Larger sizes and higher classes are 

available as special designs. 

ASTM C 506 Reinforced Concrete Arch Culvert, Storm Drain, and Sewer Pipe: Covers pipe to be used for 

the conveyance of sewage, industrial waste, and storm water and for the construction of 

culverts in sizes from 15 inch through 132 inch equivalent circular diameter. Larger sizes are 

available as special designs. 

ASTM C 507 Reinforced Concrete Elliptical Culvert, Storm Drain, and Sewer Pipe: Covers reinforced 

elliptically shaped concrete pipe to be used for the conveyance of sewage, industrial waste 

and storm water, and for the construction of culverts. Five standard classes of horizontal 

elliptical, 18 inches through 144 inches in equivalent circular diameter and five standard 

classes of vertical elliptical, 36 inches through 144 inches in equivalent circular diameter are 

included. Larger sizes are available as special designs. 

 
SECTION 12: Special Bridges 

TOPIC 12.3: Concrete Pipe Culverts 

12.3.22 

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12.4-i

Table of Contents

Chapter 12 ..................................................................................................................................................

Special Bridges 

 12.4 Flexible Culverts ................................................................................ 12.4.1 

 

 12.4.1 Introduction................................................................... 12.4.1 

 

 12.4.2 Design Characteristics................................................... 12.4.2 

 Structural Behavior................................................. 12.4.2 

 

 12.4.3 Types and Shapes of Flexible Culverts......................... 12.4.3 

 Corrugated Pipe ...................................................... 12.4.5 

 Structural Plate ....................................................... 12.4.5 

 Box Culverts........................................................... 12.4.5 

 Long Span Culverts ................................................ 12.4.6 

 

 12.4.4 Hazards of Culvert Inspection....................................... 12.4.6 

 12.4.5 Overview of Common Defects...................................... 12.4.6 

 

 12.4.6 Inspection Procedures and Locations............................ 12.4.7 

 Procedures .............................................................. 12.4.7 

 Visual ............................................................... 12.4.7 

 Physical ............................................................ 12.4.7 

 Advanced Techniques ...................................... 12.4.8 

 Locations ................................................................ 12.4.8 

 Inspect End Treatments.................................... 12.4.8 

 Corrugated Metal Culverts............................... 12.4.9 

Shape Inspection of Corrugated Metal 

Barrels ............................................................ 12.4.24 

 Corrugated Metal Long Span Culverts........... 12.4.41 

 

 12.4.7 Evaluation ................................................................... 12.4.55 

 NBI Rating Guidelines ......................................... 12.4.55 

 Element Level Condition State Assessment ......... 12.4.55 

 

 
SECTION 12: Special Bridges 

TOPIC 12.4: Flexible Culverts 

12.4-ii

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12.4.1 

Topic 12.4 Flexible Culverts 

12.4.1 

Introduction Like all culverts, flexible culverts are designed for full flow. Unlike bridges, 

culverts have no distinction between substructure and superstructure and because 

earth backfill separates the culvert structure from the riding surface, culverts have 

no "deck." Most flexible culverts have a circular or elliptical configuration (see 

Figure 12.4.1). There are some flexible box and arch culverts in use today. From 

their design nature, flexible culverts have little structural bending strength without 

proper backfill. The material from which they are made, such as corrugated steel 

or aluminum can be flexed or bent and can be distorted significantly without 

cracking. Consequently, flexible culverts depend on the backfill support to resist 

bending. In flexible culvert designs, proper interaction between the soil and 

structure is critical. 

Figure 12.4.1 Pipe Arch Flexible Culvert 
SECTION 12: Special Bridges 

TOPIC 12.4: Flexible Culverts 

12.4.2 

12.4.2 

Design 

Characteristics 

Structural Behavior A flexible culvert is a composite structure made up of the culvert barrel and the 

surrounding soil. The barrel and the soil are both vital elements to the structural 

performance of the culvert. 

Flexible pipe has relatively little bending stiffness or bending strength on its own. 

Flexible culvert materials include steel, aluminum, and plastic. As loads are 

applied to the culvert, it attempts to deflect. In the case of a round pipe, the vertical 

diameter decreases and the horizontal diameter increases (see Figure 12.4.2). 

When good embankment material is well compacted around the culvert, the 

increase in horizontal diameter of the culvert is resisted by the lateral soil pressure. 

With round pipe the result is a relatively uniform radial pressure around the pipe 

which creates a compressive thrust in the pipe walls. As illustrated in Figure 

12.4.3, the compressive thrust is approximately equal to vertical pressure times 

one-half the span length. 

DL & LL

Soil 

Pressure

Deflection of 

Flexible Culverts

Figure 12.4.2 Flexible Culvert Deflection 
SECTION 12: Special Bridges 

TOPIC 12.4: Flexible Culverts 

12.4.3 

Figure 12.4.3 Formula for Ring Compression 

An arc of a flexible round pipe, or other shape will be stable as long as adequate 

soil pressures are achieved, and as long as the soil pressure is resisted by the 

compressive force C on each end of the arc. Good quality backfill material and 

proper installation are critical in obtaining a stable soil envelope around a flexible 

culvert. 

In long span culverts the radius (R) is usually large. To prevent excessive 

deflection due to dead and/or live loads, longitudinal or circumferential stiffeners 

are sometimes added. The circumferential stiffeners are usually metal ribs bolted 

to the outside of the culvert. Longitudinal stiffeners may be metal or reinforced 

concrete. Concrete thrust beams provide some circumferential stiffening as well 

as longitudinal stiffening. The thrust beams are added to the structure prior to 

backfill. They also provide a solid vertical surface for soil pressures to act on and a 

surface which is easier to backfill against. The use of concrete stress relieving 

slabs is another method used to achieve longer spans or reduce minimum cover. A 

stress-relieving slab is cast over the top of the backfill above the structure to 

distribute live loads to the adjacent soil. 

12.4.3 

Types and Shapes 

of Flexible Culverts 

Flexible culverts are constructed from corrugated steel or aluminum pipe or field 

assembled structural plate products. Structural plate steel products are available as 

structural plate pipes, box culverts, or long span structures. See Figure 12.4.4 for 

standard shapes for corrugated flexible culverts. 
SECTION 12: Special Bridges 

TOPIC 12.4: Flexible Culverts 

12.4.4 

Figure 12.4.4 (Exhibit 11 Culvert Inspection Manual Report No. FHWA-IP86-2) Standard Corrugated Steel Culvert Shapes (Source: Handbook of Steel 

Drainage and Highway Construction Products, American Iron and Steel Institute) 
SECTION 12: Special Bridges 

TOPIC 12.4: Flexible Culverts 

12.4.5 

Corrugated Pipe Factory-made pipe is produced in two basic shapes: round and pipe arch. Both 

shapes are produced in several wall thicknesses, several corrugation sizes, and 

with annular (circumferential) or helical (spiral) corrugations. Pipes with helical 

corrugations have continuously welded seams or lock seams. Both round and arch 

steel pipe shapes are available in a wide range of standard sizes. Round pipe is 

available in standard sizes up to 3.7 m (12 feet) in diameter. Standard sizes for 

pipe arch are available in sizes up to the equivalent of a 3 m (10 feet) diameter 

round pipe. Round aluminum pipe is available in standard sizes up to3 m (10 feet) 

in nominal diameter. Aluminum pipe arch is available in sizes up to the equivalent 

of an 2.4 m (8 feet) diameter round pipe. 

Flexible aluminum culverts are constructed from factory assembled corrugated 

aluminum pipe or field assembled from structural plates. Structural plate 

aluminum culverts are available as conventional structural plate structures, box 

culverts, or long span structures. 

Structural Plate Structural plate steel pipes are field assembled from standard corrugated 

galvanized steel plates. Standard plates have corrugations with a 150 mm (6-inch) 

pitch and a depth of 50 mm (2 inches). Plates are manufactured in a variety of 

thicknesses and are pre-curved for the size and shape of structure to be erected. 

Structural steel plate pipes are available in four basic shapes: 

3⁄4 Round 

3⁄4 Pipe arch 

3⁄4 Arch 

3⁄4 Underpass 

The standard sizes available range in span from 1.5 to 7.9 m (5 feet to 26 feet). 

Structural plate aluminum pipes are field assembled with a 230 mm (9-inch) pitch 

and a depth of 65 mm (2.5 inches). 

Structural plate aluminum pipes are produced in five basic shapes: 

3⁄4 Round 

3⁄4 Pipe arch 

3⁄4 Arch 

3⁄4 Pedestrian/animal underpass 

3⁄4 Vehicle underpass 

A wide range of standard sizes is available for each shape. Spans as large as 9.1 m 

(30 feet) can be obtained for the arch shape. 

Box Culverts Corrugated steel box sections use standard corrugated galvanized steel plates with 

special reinforcing elements applied to the areas of maximum moments. Steel box 

culverts are available with spans that range from to 2.9 to 6.3 m (9 feet 8 inches to 

20 feet 9 inches). 

The aluminum box culvert utilizes standard aluminum structural plates with 
SECTION 12: Special Bridges 

TOPIC 12.4: Flexible Culverts 

12.4.6 

aluminum rib reinforcing added in the areas of maximum bending stresses. Ribs 

are bolted to the exterior of the aluminum shell during installation. Aluminum box 

culverts are suitable for shallow depths of fill and are available with spans ranging 

from to 2.7 to 7.7 m (8 feet 9 inches to 25 feet 5 inches). 

Long Span Culverts Long span steel culverts are assembled using conventional 150 by 50 mm (6 by 2 

inch) corrugated galvanized steel plates and longitudinal and circumferential 

stiffening members. There are five standard shapes for long span steel structures: 

3⁄4 Horizontal elliptical 

3⁄4 Pipe arch 

3⁄4 Low profile arch 

3⁄4 High profile arch 

3⁄4 Pear shape 

The span lengths of typical sections range from 5.9 to 12.2 m (19 feet 4 inches to 

40 feet). Longer spans are available for some shapes as special designs. It should 

be noted that each long span installation represents, to a certain extent, a custom 

design. The inspector should therefore use design or as-built plans when checking 

dimensions of existing long span structures. 

Long span aluminum structures are assembled using conventional 230 by 65 mm 

(9 by 2 1/2 inch) corrugated aluminum plates and aluminum rib stiffeners. Long 

span aluminum structures are essentially the same size and available in the same 

five basic shapes as steel long spans. 

See the end of this Topic for the different standard sizes for each flexible culvert 

shape (pg 164-193 Culvert Inspection Manual Report No. FHWA-IP-86-2) 

12.4.4 

Hazards of Culvert 

Inspection 

The bridge inspector should be alert to the following hazards when inspecting a 

culvert. 

3⁄4 Inadequate ventilation 

3⁄4 Drowning 

3⁄4 Toxic chemicals 

3⁄4 Animals 

3⁄4 Quick conditions at the outlet 

Refer to Topic 3.2.5 for a detailed discussion of each hazard. 

12.4.5 

Overview of 

Common Defects 

Common defects that can occur to flexible culvert materials include the following: 

3⁄4 Pitting 

3⁄4 Surface Rust 
SECTION 12: Special Bridges 

TOPIC 12.4: Flexible Culverts 

12.4.7 

3⁄4 Section Loss 

3⁄4 Overload Damage 

3⁄4 Heat Damage 

3⁄4 Buckling 

3⁄4 Embankment erosion at culvert entrance and exit 

3⁄4 Roadway settlement 

Refer to Topic 2.3 for a more detailed presentation of the properties of steel, types 

and causes of steel deterioration, and the examination of steel. 

12.4.6 

Inspection 

Procedures and 

Locations 

Safety is an important reason that culverts should be inspected. For a more 

detailed discussion on reasons for inspecting culverts, see Topic P.3.1. 

A logical sequence for inspecting culverts helps ensure that a thorough and 

complete inspection will be conducted. In addition to the culvert components, the 

inspector should also look for highwater marks, changes in the drainage area, 

settlement of the roadway, and other indications of potential problems. In this 

regard, the inspection of culverts is similar to the inspection of bridges. 

For typical installations, it is usually convenient to begin the field inspection with 

general observations of the overall condition of the structure and inspection of the 

approach roadway. The inspector should select one end of the culvert and inspect 

the embankment, waterway, headwalls, wingwalls, and culvert barrel. The 

inspector should then move to the other end of the culvert. The following 

sequence is applicable to all culvert inspections: 

3⁄4 Review available information 

3⁄4 Observe overall condition 

3⁄4 Inspect approach roadway and embankment 

3⁄4 Inspect waterway (see in Topic 11.2) 

3⁄4 Inspect end treatments 

3⁄4 Inspect culvert barrel 

Procedures Visual 

Most defects in flexible culverts are first detected by visual inspection. In order 

for this to occur, a hands-on inspection, or inspection where the inspector is close 

enough to touch the area being inspected, is required. The types of defects to look 

for when inspecting the culvert barrel will depend upon the type of culvert being 

inspected. In general, corrugated metal culvert barrels should be inspected for 

cross-sectional shape and barrel defects such as joint defects, seam defects, plate 

buckling, lateral shifting, missing or loose bolts, corrosion, excessive abrasion, 

material defects, and localized construction damage. A critical area for the 

inspection of long span metal culverts is at the 2 o’clock and 10 o’clock locations. 

An inward bulge at these locations may indicate potential failure of the structure. 

Physical 

In a steel culvert, the bolts on the longitudinal seams should be checked by tapping 
SECTION 12: Special Bridges 

TOPIC 12.4: Flexible Culverts 

12.4.8 

the nuts with a hammer. For aluminum structural plate, the bolts should be 

checked with a torque wrench. 

A geologist’s pick hammer can be used to scrape off heavy deposits of rust and 

scale. The hammer can then be used to locate areas of corrosion by striking the 

culvert walls. The walls will deform or the hammer will break through the culvert 

wall if severe corrosion exists. 

Sometimes surveying the culvert is necessary to determine if there is any shape 

distortion, and if there is distortion how much exists. 

Advanced Inspection Techniques 

In metal culverts, visual inspections can only point out surface defects. Therefore, 

advanced inspection techniques may be used to achieve a more rigorous and 

thorough inspection of the flexible culvert, including: 

Several advanced techniques are available for steel inspection. Nondestructive 

methods, described in Topic 13.3.2, include: 

3⁄4 Acoustic emissions testing 

3⁄4 Computer programs 

3⁄4 Computer tomography 

3⁄4 Corrosion sensors 

3⁄4 Smart paint 1 

3⁄4 Smart paint 2 

3⁄4 Dye penetrant 

3⁄4 Magnetic particle 

3⁄4 Radiographic testing 

3⁄4 Robotic inspection 

3⁄4 Ultrasonic testing 

3⁄4 Eddy current 

Other methods, described in Topic 13.3.3, include: 

3⁄4 Brinell hardness test 

3⁄4 Charpy impact test 

3⁄4 Chemical analysis 

3⁄4 Tensile strength test 

Locations Inspect End Treatments 

End treatments should be inspected like any other structural component. Their 

effectiveness can directly affect the performance of the culvert. 

The most common types of end treatments for flexible culverts are: 
SECTION 12: Special Bridges 

TOPIC 12.4: Flexible Culverts 

12.4.9 

3⁄4 Projections 

3⁄4 Mitered 

3⁄4 Pipe end section 

Projections - The inspector should indicate the location and extent of any scour or 

undermining around the culvert ends. The depth of any scouring should be 

measured with a probing rod. In low flow conditions scour holes have a tendency 

to fill up with debris or sediment. If no probing rod is used an inspector could 

mistakenly report less scour than has taken place. 

Water flowing along the outside of a culvert can remove supporting material. This 

is referred to as piping and it can lead to the culvert end being unsupported. If not 

repaired in time, piping can cause cantilevered end portions of the culvert to bend 

down and restrict the stream flow. 

Mitered Ends - Inspection items for mitered ends are the same as for projecting 

ends. Additional care should be taken to measure any deformation of the end. 

Mitering the end of corrugated pipe culvert reduces its structural capacity. 

Pipe End Sections - These are typically used on relatively smaller culverts. For 

inspection purposes, treat the pipe end section as you would a projected end. 

The following excerpts are from a reproduction of the out-of-print Culvert 

Inspection Manual Report No.-IP-86-2 – Chapter 5, Section 4. 

Section 4 - CORRUGATED METAL CULVERTS 

5-4.0 General 

Corrugated aluminum and corrugated steel culverts are classified as flexible 

structures because they respond to and depend upon the soil backfill to provide 

structural stability and support to the culvert. The flexible corrugated metal acts 

essentially as a liner. The liner acts mainly in compression and can carry large ring 

compression thrust, but very little bending or moment force. (Rib reinforced box 

culverts are exceptions.) Inspection of the culvert determines whether the soil 

envelope provides adequate structural stability for the culvert and verifies that the 

“liner” is capable of carrying the compressive forces and protecting the soil 

backfill from water flowing through the culvert. Verification of the stability of the 

soil envelope is accomplished by checking culvert shape. Verification of the 

integrity of the “liner” is accomplished by checking for pipe and plate culvert 

barrel defects. 

This section contains discussions on inspecting corrugated metal structures for 

shape and barrel defects. Because shape inspection requirements do vary 

somewhat for different shapes, separate sections with detailed guidelines are 

provided for corrugated metal pipe culvert shapes and long-span culvert shapes. 

Section 5 of this chapter addresses corrugated metal pipe culverts, and section 6 

covers long-span corrugated metal culverts. 
SECTION 12: Special Bridges 

TOPIC 12.4: Flexible Culverts 

12.4.10 

5-4.1 Shape Inspections 

The single most important feature to observe and measure when inspecting 

corrugated metal culverts is the cross-sectional shape of the culvert barrel. The 

corrugated metal culvert barrel depends on the backfill or embankment to 

maintain its proper shape and stability. When the backfill does not provide the 

required support, the culvert will deflect, settle, or distort. Shape changes in the 

culvert therefore provide a direct indication of the adequacy and stability of the 

supporting soil envelope. By periodic observation and measurement of the 

culvert's shape, it is possible to verify the adequacy of the backfill. The design or 

theoretical cross-section of the culvert should be the standard against which field 

measurements and visual observations are compared. If the design cross section is 

unknown, a comparison can be made between the unloaded culvert ends and the 

loaded sections beneath the roadway or deep fills. This can often provide an 

indication of structure deflection or settlement. Symmetrical shape and uniform 

curvature around the perimeter are generally the critical factors. If the curvature 

around the structure becomes too flat, and/or the soil continues to yield under 

load, the culvert wall may not be able to carry the ring thrust without either 

buckling inward or deflecting excessively to the point of reverse curvature. Either 

of these events leads to partial or total failure. 

As explained in earlier in this Topic, an arc of a circular pipe or other shape 

structure will be stable and perform as long as the soil pressure on the outside of 

the pipe is resisted by the compression force in the pipe at each end of the arc. 

Corrugated metal pipes can change shape safely within reasonable limits as long 

as there is adequate exterior soil pressure to balance the ring compression. 

Therefore, size and shape measurements taken at any one time do not provide 

conclusive data on backfill instability even when there is significant deviation 

from the design shape. Current backfill stability cannot be reliably determined 

unless changes in shape are measured over time. It is therefore necessary to 

identify current or recent shape changes to reliably check backfill stability. If there 

is instability of the backfill, the pipe will continue to change shape. 

In general, the inspection process for checking shape will include visual 

observations for symmetrical shape and uniform curvature as well as 

measurements of important dimensions. The specific measurements to be obtained 

depend upon factors such as the size, shape, and condition of the structure. If 

shape changes are observed, more measurements may be necessary. For small 

structures in good condition, one or two simple measurements may be sufficient, 

for example, measuring the horizontal diameter on round pipe. For larger 

structures such as long span culverts, key measurements may be difficult to 

obtain. Horizontal diameters may be both high and large. The inspection process 

for long span culverts generally requires that elevations be established for key 

points on the structure. Although some direct measurements may also be required 

for long-span structures, elevations are needed to check for settlement and for 

calculating vertical distances such as the middle ordinate of the top arc. For 

structures with shallow cover, observations of the culvert with a few live loads 

passing over are recommended. Discernible movement in the structure may 

indicate possible instability and a need for more in-depth investigation. 

The number of measurement locations depends upon the size and condition of the 
SECTION 12: Special Bridges 

TOPIC 12.4: Flexible Culverts 

12.4.11 

structure. Long-span culverts should normally be measured at the end and at 7.6 m 

(25 foot) intervals. Measurements may be required at more frequent intervals if 

significant shape changes are observed. The smaller pipe culverts can usually be 

measured at longer intervals than long-span culverts. 

Locations in sectional pipe can be referenced by using pipe joints as stations to 

establish the stationing of specific cross-sections. Stations should start with 

number 1 at the outlet and increase going upstream to the inlet. The location of 

points on a circular cross section can be referenced like hours on a clock. The 

clock should be oriented looking upstream. On structural plate corrugated metal 

culverts, points can be referenced to bolted circumferential and longitudinal 

seams. 

It is extremely important to tie down exact locations of measurement points. 

Unless the same point is checked on each inspection, changes cannot be accurately 

monitored. The inspection report must, therefore, include precise descriptions of 

reference point locations. It is safest to use the joints, seams, and plates as the

reference grid for measurement points. Exact point locations can then be easily 

described in the report as well as physically marked on the structures. This guards 

against loss of paint or scribe marks and makes points easy to find or reestablish. 

All dimensions in structures should be measured to the inside crest of corrugation. 

When possible, measurement points on structural plate should be located at the 

center of a longitudinal seam. However, some measurement points are not on a 

seam. 

When distortion or curve flattening is apparent, the extent of the flattened area, in 

terms of arc length, length of culvert affected, and the location of the flattened 

area should be described in the inspection report. The length of the chord across 

the flattened area and the middle ordinate of the chord should be measured and 

recorded. The chord and middle ordinate measurements can be used to calculate 

the curvature of the flattened area using the formula shown in Exhibit 66. 
SECTION 12: Special Bridges 

TOPIC 12.4: Flexible Culverts 

12.4.12 

Figure 12.4.5 (Exhibit 66) Checking Curvature by Curve and Middle Ordinate 

5-4.2 Inspecting Barrel Defects 

The structural integrity of corrugated metal culverts and long-span structures is 

dependent upon their ability to perform in ring compression and their interaction 

with the surrounding soil envelope. Defects in the culvert barrel itself, which can 

influence the culvert's structural and hydraulic performance, are discussed in the 

following paragraphs. Rating guidelines are provided in the sections dealing with 

specific shapes. 

a. Misalignment - The inspector should check the vertical and horizontal 

alignment of the culvert. The vertical alignment should be checked visually for 

sags and deflection at joints. Poor vertical alignment may indicate problems 

with the subgrade beneath the pipe bedding. Sags trap debris and sediment and 

may impede flow. Since most highway culverts do not have watertight joints, 

sags which pocket water could saturate the soil beneath and around the culvert, 

reducing the soil's stability. The horizontal alignment should be checked by 

sighting along the sides for straightness. Vertical alignment can be checked by 

sighting along bolt lines. Minor horizontal and vertical misalignment is 

generally not a significant problem in corrugated metal structures unless it 

causes shape or joint problems. Occasionally culverts are intentionally 
SECTION 12: Special Bridges 

TOPIC 12.4: Flexible Culverts 

12.4.13 

installed with a change in gradient. 

b. Joint Defects - Field joints are generally only found with factory manufactured 

pipe. There are ordinarily no joints in structural plate culverts, only seams. (In 

a few cases, preassembled lengths of structural plate pipe have been coupled 

or banded together like factory pipe.) 

Field joints in factory pipe serve to maintain the water conveyance of the 

culvert from section to section, to keep the pipe sections in alignment, keep the 

backfill soil from infiltrating, and to help prevent sections from pulling apart. 

Joint separation may indicate a lack of slope stability as described in section 5-

4.2 e., circumferential seams. Key factors to look for in the inspection of joints 

are indications of backfill infiltration and water exfiltration. Excessive seepage 

through an open joint can cause soil infiltration or erosion of the surrounding 

backfill material reducing lateral support. Open joints may be probed with a 

small rod or flat rule to check for voids. Indications of joint defects include 

open joints, deflection, seepage at the joints, and surface sinkholes over the 

culvert as illustrated in Exhibits 67 and 68. Any evidence of joint defects 

should be recorded. Culverts in good condition should have no open joints, 

those in fair condition may have a few open joints but no evidence of soil 

infiltration, and those in marginal to poor condition will show evidence of soil 

infiltration. 

Figure 12.4.6 (Exhibit 67) Surface Indications of Infiltration 

Effect on Unpaved 

Areas

Effect on 

Pavement

Permeable 

Soil
SECTION 12: Special Bridges 

TOPIC 12.4: Flexible Culverts 

12.4.14 

Figure 12.4.7 (Exhibit 68) Surface Hole Above Open Joint 

c. Seam Defects in Fabricated Pipe - Pipe seams in helical pipe do not carry a 

significant amount of the ring compression thrust in the pipe. That is the 

reason that a lock seam is an acceptable seam. Helical seams should be 

inspected for cracking and separation. An open seam could result in a loss of 

backfill into the pipe, or exfiltration of water. Either condition could reduce 

the stability of the surrounding soil. 

In riveted or spot welded pipes, the seams are longitudinal and carry the full 

ring compression in the pipe. These seams, then, must be sound and capable of 

handling high compression forces. They should be inspected for the same 

types of defects as those described in the text for structural plate culverts, 

Section 12.4.3, Structural Pipe. When inspecting the longitudinal seams of 

bituminous-coated corrugated metal culverts, cracking in the bituminous 

coating may indicate seam separation. 

d. Longitudinal Seam Defects in Structural Plate Culverts - Longitudinal seams 

should be visually inspected for open seams, cracking at bolt holes, plate 

distortion around the bolts, bolt tipping, cocked seams, cusped seams, and for 

significant metal loss in the fasteners due to corrosion. 

Culverts in good condition should have only minor joint defects. Those in fair 

condition may have minor cracking at a few bolt holes or minor opening at 

seams that could lead to infiltration or exfiltration. Marginal to poor culvert 

barrel conditions are indicated by significant cracking at bolt holes, or 

deflection of the structure due to infiltration of backfill through an open seam. 

Cracks 3 inches (76 mm) long on each side of the bolts indicate very poor to 

critical conditions. 

 (1) Loose Fasteners - Seams should be checked for loose or missing 

fasteners as shown in Exhibit 69. For steel structures the 
SECTION 12: Special Bridges 

TOPIC 12.4: Flexible Culverts 

12.4.15 

longitudinal seams are bolted together with high-strength bolts in 

two rows; one row in the crests and one row in the valleys of the 

corrugations. These are bearing type connections and are not 

dependent on a minimum clamping force of bolt tension to 

develop interface friction between the plates. Fasteners in steel 

structural plate may be checked for tightness by tapping lightly 

with a hammer and checking for movement. 

Figure 12.4.8 (Exhibit 69) Close-Up of Loose and Missing Bolts at a Cusped 

Seam; Loose Fasteners are Usually Detected by Tapping the Nuts with a Hammer 

For aluminum structural plate, the longitudinal seams are bolted together with 

normal strength bolts in two rows with bolts in the crests and valleys of both 

rows. These seams function as bearing connections, utilizing bearing of the 

bolts on the edges of holes and friction between the plates. The seams in 

aluminum structural plate should be checked with a torque wrench (125 ft-lbs 

(169 Joules) minimum to 150 ft-lbs (203 Joules) maximum). If a torque 

wrench is not available fasteners can be checked for tightness with a hammer 

as described for steel plates. 

 (2) Cocked and Cusped Seams - The longitudinal seams of structural 

plate are the principal difference from factory pipe. The shape and 

curvature of the structure is affected by the lapped, bolted 

longitudinal seam. Improper erection or fabrication can result in 

cocked seams or cusped effects in the structure at the seam, as 

illustrated in Exhibit 70. Slight cases of these conditions are fairly 

common and frequently not significant. However, severe cases 

can result in failure of the seam or structure. When a cusped seam 

is significant the structure's shape appearance and key dimensions 

will differ significantly from the design shape and dimensions. 
SECTION 12: Special Bridges 

TOPIC 12.4: Flexible Culverts 

12.4.16 

The cusp effect should cause the structure to receive very low 

ratings on the shape inspection if it is a serious problem. A cocked 

seam can result in loss of backfill and may reduce the ultimate 

ring compression strength of the seam. 

Figure 12.4.9 (Exhibit 70) Cocked Seam with Cusp Effect 

(3) Seam Cracking - Cracking along the bolt holes of longitudinal 

seams can be serious if allowed to progress. As cracking 

progresses, the plate may be completely severed and the ring 

compression capability of the seam lost. This could result in 

deformation or possible failure of the structure. Longitudinal

cracks are most serious when accompanied by significant 

deflection, distortion, and other conditions indicative of backfill or 

soil problems. Longitudinal cracks are caused by excessive 

bending strain, usually the result of deflection, Exhibit 71. 

Cracking may occasionally be caused by improper erection 

practices such as using bolting force to "lay down" a badly cocked 

seam. 
SECTION 12: Special Bridges 

TOPIC 12.4: Flexible Culverts 

12.4.17 

Figure 12.4.10 (Exhibit 71) Cracking Due to Deflection 

(4) Bolt Tipping - The bolted seams in structural plate culverts only 

develop their ultimate strength under compression. Bolt tipping 

occurs when the plates slip. As the plates begin to slip, the bolts 

tip, and the bolt holes are plastically elongated by the bolt shank. 

High compressive stress is required to cause bolt tipping. 

Structures have rarely been designed with loads high enough to 

produce a ring compression that will cause bolt tip. However, 

seams should be examined for bolt tip particularly in structures 

under higher fills. Excessive compression on a seam could result 

in plate deformations around the tipped bolts and failure is 

reached when the bolts are eventually pulled through the plates. 

e. Circumferential Seams - The circumferential seams, like joints in factory 

pipe, do not carry ring compression. They do make the conduit one 

continuous structure. Distress in these seams is rare and will ordinarily be a 

result of a severe differential deflection or distortion problem or some other 

manifestation of soil failure. For example, a steep sloping structure through

an embankment may be pulled apart longitudinally if the embankment moves 

down as shown in Exhibit 72. Plates should be installed with the upstream 

plate overlapping the downstream plate to provide a "shingle" effect in the 
SECTION 12: Special Bridges 

TOPIC 12.4: Flexible Culverts 

12.4.18 

direction of flow. 

Figure 12.4.11 (Exhibit 72) Circumferential Seam Failure Due to Embankment 

Slippage 

The circumferential seam at one or more locations would be distressed by the 

movement of the fill. Such distress is important to note during inspections 

since it would indicate a basic problem of stability in the fill. Circumferential 

seam distress can also be a result of foundation failure, but in such cases 

should be clearly evident by the vertical alignment. 

f. Dents and Localized Damage - All corrugated metal culverts should be 

inspected for localized damage. Pipe wall damage such as dents, bulges, 

creases, cracks, and tears can be serious if the defects are extensive and can 

impair either the integrity of the barrel in ring compression or permit 

infiltration of backfill. Small, localized examples are not ordinarily critical. 

When the deformation type damages are critical, they will usually result in a 

poorly shaped cross section. The inspector should document the type, extent, 

and location of all significant wall damage defects. When examining dents in 

corrugated steel culverts, the opposite side of the plate should be checked, if 

possible, for cracking or disbonding of the protective coating. 

g. Durability (Wall Deterioration) - Durability refers to the ability of a material to 

resist corrosion and abrasion. Corrosion is the deterioration of metal due to 

electrochemical or chemical reactions. Abrasion is the wearing away of culvert 

materials by the erosive action of bedload carried in the stream. 

Abrasion is generally most serious in steep or mountainous areas where high flow 

rates carry sand and rocks that wear away the culvert invert. Abrasion can also 

accelerate corrosion by wearing away protective coatings. 

Metal culverts are subject to corrosion in certain aggressive environments. For 

example, steel rapidly corrodes in salt water and in environments with highly 

acidic (low pH) conditions in the soil and water. Aluminum is fairly resistant to 

salt water but will corrode rapidly in highly alkaline (high pH) environments,

particularly if metals such as iron or copper and their salts are present. The 
SECTION 12: Special Bridges 

TOPIC 12.4: Flexible Culverts 

12.4.19 

electrical resistivity of soil and water also provide an indication of the likelihood 

of corrosion. Many agencies have established guidelines in terms of pH and 

resistivity that are based on local performance. The FHWA has also published 

guidelines for aluminum and steel culverts including various protective coatings. 

Corrosion and abrasion of corrugated metal culverts can be a serious problem with 

adverse effects on structural performance. Damage due to corrosion and abrasion 

is the most common cause for culvert replacement. The inspection should include 

visual observations of metal corrosion and abrasion. As steel corrodes it expands 

considerably. Relatively shallow corrosion can produce thick deposits of scale. A 

geologist's pick-hammer can be used to scrape off heavy deposits of rust and scale 

permitting better observation of the metal. A hammer can also be used to locate 

unsound areas of exterior corrosion by striking the culvert wall with the pick end 

of the hammer. When severe corrosion is present, the pick will deform the wall or 

break through it. Protective coatings should be examined for abrasion damage, 

tearing, cracking, and removal. The inspector should document the extent and 

location of surface deterioration problems. 

When heavy corrosion is found by observation or sounding, special inspection 

methods such as pH testing, electrical resistivity measurement, and obtaining cores 

from the pipe wall are recommended. A routine program for testing pH and 

electrical resistivity should be considered since it is relatively easy to perform and 

provides valuable information. 

Durability problems are the most common cause for the replacement of pipe 

culverts. The condition of the metal in corrugated metal culverts and any coatings, 

if used, should be considered when assigning a rating to the culvert barrel. 

Suggested rating guidelines for metal culverts with metallic coatings are shown in 

Exhibit 73. Modification of these guidelines may be required when inspecting 

culverts with non-metallic coatings. Aluminum culvert barrels may be rated as 

being in good condition if there is superficial corrosion. Steel culverts rated as in 

good condition may have superficial rust with no pitting. Perforation of the invert 

as shown in Exhibit 74 would indicate poor condition. Complete deterioration of 

the invert in all or part of the culvert barrel would indicate a critical condition as 

shown in Exhibit 75. Culverts with deteriorated inverts may function as an arch 

structurally, but are highly susceptible to failure due to erosion of the bedding. 
SECTION 12: Special Bridges 

TOPIC 12.4: Flexible Culverts 

12.4.20 

Figure 12.4.12 (Exhibit 73) Suggested Rating Criteria for Condition of 

Corrugated Metal 

Figure 12.4.13 (Exhibit 74) Perforation of the Invert Due to Corrosion 
SECTION 12: Special Bridges 

TOPIC 12.4: Flexible Culverts 

12.4.21 

Figure 12.4.14 (Exhibit 75) Invert Deterioration 

h. Concrete Footing Defects - Structural plate arches, long-span arches, and box 

culverts use concrete footings. Metal footings are occasionally used for the 

arch and box culvert shapes. The metal "superstructure" is dependent upon the 

footing to transmit the vertical load into the foundation. The structural plate 

arch is usually bolted in a base channel which is secured in the footing. 

The most probable structural defect in the footing is differential settlement. 

One section of a footing settling more than the rest of the footing can cause 

wrinkling or other distortion in the arch. Flexible corrugated metal culverts can 

tolerate some differential settlement but will be damaged by excessive 

differential settlement. Uniform settlement will not ordinarily affect a metal 

arch but can affect the clearances in a grade separation structure if the footings 

settle and the road does not. The significance of differential footing settlement 

increases as the amount of the difference in settlement increases, the length it 

is spread over decreases, and the height of the arch decreases. This concept is 

illustrated in Exhibit 76. 
SECTION 12: Special Bridges 

TOPIC 12.4: Flexible Culverts 

12.4.22 

Figure 12.4.15 (Exhibit 76) Differential Footing Settlement 

The inspection of footings in structural plate and long-span arches should 

include a check for differential settlement along the length of a footing. This 

might show up in severe cracking, spalling, or crushing across the footing at 

the critical spot. If severe enough, it might be evidenced by compression or 

stretching of the corrugations in the culvert barrel. Deterioration may occur in 

concrete and masonry footings which is not related to settlement but is caused 

by the concrete or mortar. In arches with no invert slab, the inspector should 

check for erosion and undermining of the footings and look for any indication 

of rotation of the footing as illustrated in Exhibits 77 and 78. 

Figure 12.4.16 (Exhibit 77) Footing Rotation due to Undermining 
SECTION 12: Special Bridges 

TOPIC 12.4: Flexible Culverts 

12.4.23 

Figure 12.4.17 (Exhibit 78) Erosion of Invert Undermining footing of Arch 

Culverts rated in good condition may have minor footing damage. Poor to critical 

condition would be indicated by severe footing undermining, damage, or rotation, 

or by differential settlement causing distortion and circumferential kinking in the 

corrugated metal as shown in Exhibit 79. 
SECTION 12: Special Bridges 

TOPIC 12.4: Flexible Culverts 

12.4.24 

Figure 12.4.18 (Exhibit 79) Erosion Damage to Concrete Invert 

i. Defects in Concrete Inverts - Concrete inverts in arches are usually 

floating slabs used to carry water or traffic. Invert slabs provide 

protection against erosion and undercutting, and are also used to improve 

hydraulic efficiency. Concrete inverts are sometimes used in circular, as 

well as other culvert shapes, to protect the metal from severe abrasive or 

severe corrosive action. Concrete invert slabs in arches should be 

checked for undermining and damage such as spalls, open cracks, and 

missing portions. The significance of damage will depend upon its effect 

on the footings and corrugated metal. 

The following excerpts are from a reproduction of the out-of-print Culvert 

Inspection Manual (Supplement to Manual 70), July 1986 – Chapter 5, Section 5.

Section 5 - SHAPE INSPECTION OF CORRUGATED METAL CULVERT 

BARRELS 

5-5.0 General 

This section deals with shape inspections of common culvert shapes including 

round and vertical elongated, pipe arches, arches, and box culvert shapes. Specific 

guidelines for recommended measurements to be taken for each location are 

provided for each typical culvert shape. Additional measurements are also 

recommended when field measurements differ from the design dimensions or 

when significant shape changes are observed. Rating guidelines are also provided 

for each shape. The guidelines include condition descriptions with shape and 

barrel defects defined for each rating. 
SECTION 12: Special Bridges 

TOPIC 12.4: Flexible Culverts 

12.4.25 

5-5.1 Using the Rating Guidelines

When using the rating guidelines, the inspector should keep the following factors 

in mind: 

 a. The inspector should select the lowest rating which best describes 

either the shape condition or the barrel condition. Structure shape is 

the most critical factor in flexible culverts, and this should be kept in 

mind when selecting the rating. 

 b. The shape criteria described for each numerical rating should be 

considered as a group rather than as separate criteria for each 

measurement check listed. Good curvature and the rate of change are 

critical. Significant changes in shape since the last inspection should 

be carefully evaluated even if the structure is still in fairly good 

condition. 

 c. The guidelines merely offer a starting point for the inspector. The 

inspector must still use judgment in assigning the appropriate 

numerical rating. The numerical rating should be related to the actions 

required. The inspector may wish to refer to Section 4.2 of this 

manual. 

5-5.2 Round and Vertical Elongated Pipe 

Round and vertically elongated pipes are expected to deflect vertically during 

construction resulting in a slightly increased horizontal span. Round pipes are 

sometimes vertically elongated five percent to compensate for settlement during 

construction. It is frequently difficult to determine in the field if a pipe was round 

or elongated when installed. Large round pipes may appear to be elongated if they 

were subjected to minor flattening of the sides during backfill. 

Vehicular underpasses sometimes use 10 percent vertically elongated very large 

pipe which is susceptible to side flattening during installation. In shallow cover 

situations, adequate curvature in the sides is the important factor. The soil 

pressures on the sides may be greater than the weight of the shallow fill over the 

pipe. The result is a tendency to push the sides inward rather than outward as in 

deeper buried or round pipes. Side flattening, such as that shown in Exhibit 80, can 

be caused by unstable backfill. A deteriorated invert may have contributed to the 

problem by reducing the pipe's ability to transmit compressive forces. 
SECTION 12: Special Bridges 

TOPIC 12.4: Flexible Culverts 

12.4.26 

Figure 12.4.19 (Exhibit 80) Excessive Side Deflection 

Flattening of the top arc is an indication of possible distress. Flattening of the 

invert is not as serious. Pipes not installed on shaped bedding will often exhibit 

minor flattening of the invert arc. However, severe flattening of the bottom arc 

would indicate possible distress. 

The inspector should note the visual appearance of the culvert's shape and measure 

the horizontal span as shown in Exhibit 81. Almost all round or vertical elongated 

pipe can be directly measured and will not require elevations. Exceptions are large 

vertical elongated grade separation structures. On such structures, elevations 

should be obtained similar to those recommended for the long-span pear shape. 
SECTION 12: Special Bridges 

TOPIC 12.4: Flexible Culverts 

12.4.27 

Figure 12.4.20 (Exhibit 81) Shape Inspection Circular and Vertical Elongated 

Pipe 

If the visual appearance or measured horizontal diameter differ significantly from 

the design specifications, additional measurement, such as vertical diameter, 

should be taken. Flattened areas should be checked by measuring a chord and the 

mid ordinate of the chord. The chord length and ordinate measurement should be 

noted in the report with a description of the location and extent of the flattened 

area. 

Round and vertically elongated pipe with good to fair shape will have a generally

good shape appearance. Good shape appearance means that the culvert's shape 

appears to match the design shape, with smooth, symmetrical curvature and no 

visible deformations. The horizontal span should be within 10 percent of the 

design span. Pipe with marginal shape will be indicated by characteristics such as 

a fair or marginal general shape appearance, distortion in the upper half of the 

pipe, severe flattening in the lower half of the pipe, or horizontal spans 10 to 15 

percent greater than design. 

Pipe with poor to critical shape will have a poor shape appearance that does not 

match the design shape, does not have smooth or symmetrical curvature, and may 

have obvious deformations. Severe distortion in the upper half of the pipe, a 
SECTION 12: Special Bridges 

TOPIC 12.4: Flexible Culverts 

12.4.28 

horizontal diameter more than 15 percent to 20 percent greater than the design 

diameter, or flattening of the crown to an arc with a radius of 20 to 30 feet or more 

would indicate poor to critical condition. It should be noted that pipes with 

deflection of less than 15 to 20 percent may be rated as critical based on poor 

shape appearance. Guidelines for rating round corrugated metal culvert are 

presented in Exhibit 82. 
SECTION 12: Special Bridges 

TOPIC 12.4: Flexible Culverts 

12.4.29 

Figure 12.4.21 (Exhibit 82) Condition Rating Guidelines 
SECTION 12: Special Bridges 

TOPIC 12.4: Flexible Culverts 

12.4.30 

5-5.3 Pipe Arch 

The pipe arch is a completely closed structure but is essentially an arch. The load 

is transmitted to the foundation principally at the corners. The corners are much 

like footings of an arch. There is relatively little force or pressure on the large 

radius bottom plate. The principal type of distress in a pipe arch is a result of 

inadequate soil support at the corners where the pressure is relatively high. The 

corner may push down or out into the sail while the bottom stays in place. The 

effect will appear as if the bottom pushed up. This problem is illustrated in 

Exhibits 83 and 84. 

Figure 12.4.22 (Exhibit 83) Bottom Distortion in Pipe Arches 
SECTION 12: Special Bridges 

TOPIC 12.4: Flexible Culverts 

12.4.31 

Figure 12.4.23 (Exhibit 84) Bottom and Corners of this Pipe Arch have Settled 

The bottom arc should be inspected for signs of flattening and the bottom corners 

for signs of spreading. The extent and location of bottom flattening and corner 

spreading should be noted in the inspection report. 

Complete reversal of the bottom arc can occur without failure if corner movement 

into the foundation has stabilized. The top arc of the structure is supporting the 

load above and its curvature is an important factor. However, if the “footing” 

corner should fail, the top arc would also fail. The spreading of the corners is 

therefore very important as it affects the curvature of the top arc. 

The inspector should record the visual appearance of the shape and measure both 

the span and the rise. If the span exceeds the design span by more than 3 percent, 

the span of the top arc, the mid ordinate of the top arc, and the mid ordinate of the 

bottom arc should also be measured. Recommended measurements are shown in 

Exhibit 85. 
SECTION 12: Special Bridges 

TOPIC 12.4: Flexible Culverts 

12.4.32 

Figure 12.4.24 (Exhibit 85) Shape Inspection Structural Plate Pipe Arch 

Pipe arches in fair to good condition will have a symmetrical appearance, smooth 

curvature in the top of the pipe, and a span less than five percent greater than 

theoretical. The bottom may be flattened but should still have curvature. Pipe 

arches in marginal condition will have fair to marginal shape appearance, with 

distortion in the top half of the pipe, slight reverse curvature in the bottom of the 

pipe, and a horizontal span five to seven percent greater than theoretical. Pipe in 

poor to critical condition will have characteristics such as a poor shape appearance, 

severe deflection or distortion in the top half of the pipe, severe reverse curvature 

in the bottom of the pipe, flattening of one side, flattening of the crown to an arc 

with a radius of 6.1 to 9.1 m (20 to 30 feet), or a horizontal span more than seven 

percent greater than theoretical. Guidelines for rating pipe arches are shown in 

Exhibit 86. 
SECTION 12: Special Bridges 

TOPIC 12.4: Flexible Culverts 

12.4.33 

Figure 12.4.25 (Exhibit 86) Condition Rating Guidelines 
SECTION 12: Special Bridges 

TOPIC 12.4: Flexible Culverts 

12.4.34 

5-5.4 Arches. 

Arches are fixed on concrete footings, usually below or at the springline. The 

springline is a line connecting the outermost points on the sides of a culvert. This 

difference between pipes and arches means that an arch tends to deflect differently 

during backfill. Backfill forces tend to flatten the arch sides and peak its top 

because the springline cannot move inward like the wall of a round pipe as shown 

in Exhibit 87. As a result, important shape factors to look for in an arch are 

flattened sides, peaked crown, and flattened top arc. 

Figure 12.4.26 (Exhibit 87) Arch Deflection During Installation 

Another important shape factor in arches is symmetrical shape. If the arch was 

erected with the base channels not square to the centerline, it causes a racking of 

the cross section. A racked cross-section is one that is not symmetrical about the 

centerline of the culvert. One side tends to flatten, the other side tends to curve 

more while the crown moves laterally and possibly upward. If these distortions are 

not corrected before backfilling the arch, they usually get worse during backfill. 

Exhibit 88 illustrates racked or peaked arches. 
SECTION 12: Special Bridges 

TOPIC 12.4: Flexible Culverts 

12.4.35 

Figure 12.4.27 (Exhibit 88) Racked and Peaked Arch 

Visual observation of the shape should involve looking for flattening of the sides, 

peaking or flattening of the crown, or racking to one side. The measurements to be 

recorded are illustrated in Exhibit 89. Minimum measurements include the vertical 

distance from the crown to the bottom of the base channels and the horizontal 

distances from each of the base channels to a vertical line from the highest point 

on the crown. These horizontal distances should be equal. When they differ by 

more than 10 inches or 5 percent of the span, whichever is less, racking has 

occurred and the curvature on the flatter side of the arch should be checked by 

recording chord and midordinate measurements. Racking can occur when the rise 

checks with the design rise. When the rise is more than 5 percent less than the 

design rise, the curvature of the top arc should be checked. 
SECTION 12: Special Bridges 

TOPIC 12.4: Flexible Culverts 

12.4.36 

Figure 12.4.28 (Exhibit 89) Shape Inspection Structural Plate Arch 

Arches in fair to good condition will have the following characteristics: a good 

shape appearance with smooth and symmetrical curvature, and a rise within three 

to four percent of theoretical. Marginal condition would be indicated when the 

arch is significantly non-symmetrical, when arch height is five to seven percent 

less or greater than theoretical, or when side or top plate flattening has occurred 

such that the plate radius is 50 to 100 percent greater than theoretical. Arches in 

poor to critical condition will have a poor shape appearance including significant 

distortion and deflection, extremely non-symmetrical shape, severe flattening 

(radius more than 100 percent greater than theoretical) of sides or top plates, or a 

rise more than eight percent greater or less than the theoretical rise. Guidelines for 

rating structural plate arches are shown in Exhibit 90. 
SECTION 12: Special Bridges 

TOPIC 12.4: Flexible Culverts 

12.4.37 

Figure 12.4.29 (Exhibit 90) Condition Rating Guidelines 
SECTION 12: Special Bridges 

TOPIC 12.4: Flexible Culverts 

12.4.38 

5-5.5 Corrugated Metal Box Culverts. 

The box culvert is not like the other flexible buried metal structures. It behaves as 

a combination of ring compression action and conventional structure action. The 

sides are straight, not curved and the plates are heavily reinforced and have 

moment or bending strength that is quite significant in relation to the loads carried.

The key shape factor in a box culvert is the top arc. The design geometry is clearly 

very “flat” to begin with and therefore cannot be allowed to deflect much. The 

span at the top is also important and cannot be allowed to increase much. 

The side plates often deflect slightly inward or outward. Generally an inward 

deflection would be the more critical as an outward movement would be restrained 

by soil. 

Shape factors to be checked visually include flattening of top arc, outward 

movement of sides, or inward deflection of the sides. The inspector should note the 

visual appearance of the shape and should measure and record the rise and the 

horizontal span at the top of the straight legs as shown in Exhibit 91. If the rise is 

more or less than 1 1⁄2 percent of the design rise, the curvature of the large top 

radius should be checked. 

Figure 12.4.30 (Exhibit 91) Shape Inspection Structural Plate Box Culverts 

The radius points are not necessarily located at the longitudinal seams. Many box 

culverts use double radius plates and the points where the radius changes must be 

estimated by the inspector or can be determined from the manufacturer’s literature. 

These points can still be referenced to the bolt pattern to describe exactly where 

they are. Since these are all low structures, the spots should also be marked and 
SECTION 12: Special Bridges 

TOPIC 12.4: Flexible Culverts 

12.4.39 

painted for convenient repeat inspection. 

Box culverts in fair to good condition will appear to be symmetrical with smooth 

curves, slight or no deflection of the straight legs, a horizontal span length within 

five percent of the design span and the middle ordinate of the tops are within ten 

percent of the design. Culverts in marginal condition may appear to be nonsymmetrical, have noticeable deflection in the straight legs, have spans that differ 

from design by five percent, or have a middle ordinate of the top arc that differ 

from design by 20 to 30 percent. Poor to critical conditions exist when the culvert 

shape appears poor, the culvert has severe deflections of the straight legs, a 

horizontal span that differs from design by more than five percent, or a middle 

ordinate of the top arc that differs from the theoretical by more than 40 to 50 

percent. Guidelines for rating structural plate box culverts are shown in Exhibit 92.
SECTION 12: Special Bridges 

TOPIC 12.4: Flexible Culverts 

12.4.40 

Figure 12.4.31 (Exhibit 92) Condition Rating Guidelines 
SECTION 12: Special Bridges 

TOPIC 12.4: Flexible Culverts 

12.4.41 

The following excerpts are from a reproduction of the out-of-print Culvert 

Inspection Manual (Supplement to Manual 70), July 1986 – Chapter 5, Section 6.

Section 6. CORRUGATED METAL LONG-SPAN CULVERTS 

5-6.0 General. 

This section describes procedures for conducting shape inspections of long-span 

structures. The long-span structures addressed include four typical shapes: low 

profile arch, horizontal ellipse, high profile arch, and pear. These shapes are 

illustrated in Exhibit 93. The evaluation of shape characteristics of long-spans will 

vary somewhat depending upon the typical shape being inspected. However, the 

top or crown sections of all long-span structures have very similar geometry. The 

crown sections on all long-span structures can be inspected using the same criteria. 

This section therefore includes separate discussions on the crown section and on 

each of the typical long-span shapes. Guidelines are also provided for rating the 

condition of each shape in terms of shape characteristics and barrel defects. The 

procedures for using the rating guidelines are the same as those described in 

section 5-5.1. 

Figure 12.4.32 (Exhibit 93) Typical Long-Span Shapes
SECTION 12: Special Bridges 

TOPIC 12.4: Flexible Culverts 

12.4.42 

Shape inspections of long-span structures will generally consist of 1) visual 

observations of shape characteristics such as smooth or distorted curvature and 

symmetrical or non-symmetrical shape, 2) measurements of key dimensions, and 

3) elevations of key points. Additional measurements may be necessary if 

measurements or observed shape differ significantly from design. 

The visual observations are extremely important to evaluate the shape of the total 

cross section. Simple measurements such as rise and span do not describe 

curvature, yet adequate curvature is essential, as shown in Exhibit 94. However, 

measurements and elevations are also needed to document the current shape so 

that the rate change, if any, can be monitored. 

Figure 12.4.33 (Exhibit 94) Erosion Damage to Concrete Invert 

Many long-spans will be too large to allow simple direct measuring. Vertical 

heights may be as large as 6.1 to 9.1 m (20 to 30 feet) and horizontal spans may be 

large and as high as 3.7 to 4.6 m (12 to 15 feet) above inverts. Culverts may have 

flowing water obscuring the invert and any reference points there. It is, therefore, 

in general desirable to have instrument survey points, which can be quickly 

checked for elevation. When direct measuring is practical a 7.6 m (25 foot) 

telescoping extension rod can be used for measuring. Such rods can also serve as 

level rods for taking elevations. 
SECTION 12: Special Bridges 

TOPIC 12.4: Flexible Culverts 

12.4.43 

5-6.1 Long-Span Crown Section - Shape Inspection. 

As previously mentioned, the section above the springline is essentially the same 

for most long-span shapes. With the exception of pear shapes, the standard top 

geometry uses a large radius top arc of approximately 80 degrees with a radius of 

4.6 to 7.6 m (15 to 25 feet). The adjacent corner or side plates are from one-half to 

one-fifth the top arc radius. The most important part of a long-span shape is the 

standard top arch geometry. Adequate curvature of the large radius top arc is 

critical. Inspection of the crown section should consist of a visual inspection of the 

general shape for smooth curvature (no distortion, flattening, peaks, or cusps) and 

symmetrical shape (no racking). 

An inspection should also include key measurements such as the middle ordinate 

of the top arc. Recommended measurements and elevations are shown in exhibit 

95. 

Figure 12.4.34 (Exhibit 95) Shape Inspection Crown Section of Long Span 

Structures 
SECTION 12: Special Bridges 

TOPIC 12.4: Flexible Culverts 

12.4.44 

The initial inspection should establish elevations for the radius points and the top 

of the crown. From these elevations the middle ordinate for the top arc can be 

calculated. If the actual middle ordinate is 10 percent more or less than the 

theoretical design mid-ordinate the horizontal span for the top arc should also be 

measured. For standard 80 degree arcs the theoretical middle ordinate is equal to 

0.234 times the theoretical radius of the top arc. This span is not easy to measure 

on many long-span structures and need not be measured if the top arc mid-ordinate 

is within 10 percent of theoretical. Even if it is convenient and practical to direct 

measure the vertical heights of the points on the top arc from the bottom of the 

structure, it is wise to also establish their elevations from a reliable benchmark. 

Bottom reference points can be wiped out by erosion, covered with debris, or 

covered by water. When direct vertical measuring is practical, the shape may be 

checked on subsequent inspections with direct measurement. However, it is still 

important to establish elevations in case bottom reference points are lost or 

inaccessible. 

Crown sections in good condition will have a shape appearance that is good, with 

smooth and symmetrical curvature. The actual middle ordinate should be within 10 

percent of the theoretical, and the horizontal span (if measured) should be within 

five percent of theoretical. Crown sections in fair condition will have a fair to good 

shape appearance, smooth curvature but possibly slightly non-symmetrical. Middle 

ordinates of the top arc may be within 11 to 15 percent of theoretical and the 

horizontal span may differ by more than 5 percent of theoretical. 

Crown sections in marginal condition will have measurements similar to those 

described for fair shape. However, the shape appearance will be only fair to 

marginal with noticeable distortion, deflection, or non-symmetrical curvature. 

When the curvature is noticeably distorted or non-symmetrical, the sides should be 

checked for flattening by measuring the middle ordinates of the halves of the top 

arc. Crown sections with marginal shape may have middle ordinates for top half 

arcs that are 30 to 50 percent less than theoretical. 

Crown sections in poor to critical condition will have a poor to critical shape 

appearance with severe distortion or deflection. The middle ordinate of the top arc 

may be as much as 20 percent less than theoretical, while middle ordinates of the 

top arc halves may be 50 to 70 percent less than theoretical. 

5-6.2 Low Profile Long-Span Arch - Shape Inspection. 

The low profile arch is essentially the same as the crown section except that the 

sides are carried about 10 degrees below the springline to the footing. These 

structures are low and can be measured more easily than other long-span shapes. 

Recommended measurements and elevations are shown in exhibit 96. Rating 

guidelines are listed in exhibit 97. 
SECTION 12: Special Bridges 

TOPIC 12.4: Flexible Culverts 

12.4.45 

Figure 12.4.35 (Exhibit 96) Shape Inspection Low Profile Long Span Arch 
SECTION 12: Special Bridges 

TOPIC 12.4: Flexible Culverts 

12.4.46 

Figure 12.4.36 (Exhibit 97) Condition Rating Guidelines 
SECTION 12: Special Bridges 

TOPIC 12.4: Flexible Culverts 

12.4.47 

Because arches are fixed on concrete footings, backfill pressures will try to flatten 

the sides and peak the top. Another important shape factor is symmetry. If the base 

channels are not square to the centerline of the structure racking may occur during 

erection. In racked structures, the crown moves laterally and the curvature in one 

side becomes flatter while the curvature in the other side increases. Backfill 

pressures may cause this condition to worsen. 

5-6.3 High Profile Long-Span Arch – Shape Inspection. 

High profile arches have a standard crown section geometry but have high large 

radius side walls below the springline. Curvature in these side plates is important. 

In shallow fills or minimum covers, the lateral soil pressures may approach or 

exceed the loads over the culvert. Excessive lateral forces could cause the sidewall

to flatten or buckle inward. 

Inspectors should visually inspect high profile arches for flattening of the side 

plates. Additionally, high profile arches have the same tendencies as regular arches 

for peaking and racking, so inspectors must also look for peaked top arcs and nonsymmetrical or racked arches. 

Recommended measurements and elevations are shown in Exhibit 98. The shape 

of the crown section is the most important shape factor. It can be measured and 

evaluated using the same criteria as that described for the standard crown section. 

If flattening is observed in the high sidewall the curvature of the sides should be 

checked by measuring the middle ordinate of the side walls. If the sidewall middle 

ordinate is no more than 50 to 70 percent less than the theoretical middle ordinate 

and no other shape problems are found the arch’s shape may be considered fair. 

When the middle ordinate approaches 75 to 80 percent less than theoretical, the 

shape should be considered marginal. If the middle ordinate is more than 80 to 90 

percent less than theoretical the shape should be considered poor to critical. Rating 

guidelines are provided in Exhibit 99. 
SECTION 12: Special Bridges 

TOPIC 12.4: Flexible Culverts 

12.4.48 

Figure 12.4.37 (Exhibit 98) Shape Inspection High Profile Long-Span Arch 
SECTION 12: Special Bridges 

TOPIC 12.4: Flexible Culverts 

12.4.49 

Figure 12.4.38 (Exhibit 99) Condition Rating Guidelines 
SECTION 12: Special Bridges 

TOPIC 12.4: Flexible Culverts 

12.4.50 

5-6.4 Pear Shape Long-Span – Shape Inspection. 

The crown section of the pear shape differs from the standard top arch in that 

smaller radius corner arcs stop short of the horizontal springline. The large radius 

sides extend above the plane of the horizontal span. In checking curvature of the 

sides, the entire arc should be checked. Side flattening, particularly in shallow fills, 

is the most critical shape factor. 

The pear shape behaves similarly to the high profile arch. It is essentially a high 

profile with a metal bottom instead of concrete footings. Pears may be inspected 

using the criteria for a high profile arch. The recommended measurements and 

elevations are shown in Exhibit 100. Rating guidelines are provided in Exhibit 

101. 

Figure 12.4.39 (Exhibit 100) Shape Inspection Long Span Pear-Shape 
SECTION 12: Special Bridges 

TOPIC 12.4: Flexible Culverts 

12.4.51 

Figure 12.4.40 (Exhibit 101) Condition Rating Guidelines 
SECTION 12: Special Bridges 

TOPIC 12.4: Flexible Culverts 

12.4.52 

5-6.5 Horizontal Ellipse – Shape Inspections. 

For horizontal ellipses the most important shape factor is adequate curvature in the 

crown section. The crown section uses the standard long-span crown geometry. 

The sides and bottom behave similar to the corners and bottom of pipe arches. The 

invert has relatively minor pressure when compared with the sides, which may 

have several times the bearing pressure of the invert. As a result the corners and 

sides have the tendency to push down into the soil while the bottom does not 

move. The effect is as if the bottom pushed up. Inspectors should look for 

indications of bottom flattening and differential settlement between the side and 

bottom sections, as illustrated in Exhibit 102. 

Figure 12.4.41 (Exhibit 102) Potential for Differential Settlement in Horizontal 

Ellipse 

The recommended measurements and evaluations for a shape inspection of 

horizontal ellipse are shown in Exhibit 103. The measurements are essentially the 

same as those recommended for a standard crown section. Shape evaluation of an 

ellipse is also essentially the same as the evaluation of a standard crown section 

except that the curvature of the bottom should also be evaluated. Marginal shape 

would be indicated when the bottom is flat in the center and corners are beginning 

to deflect downward or outward. Critical shape conditions would be indicated by 

reverse curvature in the bottom arc. Guidelines for rating horizontal ellipse shape 

culverts are provided in Exhibit 104. 
SECTION 12: Special Bridges 

TOPIC 12.4: Flexible Culverts 

12.4.53 

Figure 12.4.42 (Exhibit 103) Shape Inspection Long-Span Horizontal Ellipse 
SECTION 12: Special Bridges 

TOPIC 12.4: Flexible Culverts 

12.4.54 

Figure 12.4.43 (Exhibit 104) Condition Rating Guidelines 
SECTION 12: Special Bridges 

TOPIC 12.4: Flexible Culverts 

12.4.55 

12.4.7 

Evaluation State and federal rating guideline systems have been developed to aid in the 

inspection of flexible culverts. The two major rating guideline systems currently 

in use are the FHWA's Recording and Coding Guide for the Structural Inventory 

and Appraisal of the Nation's Bridges used for the National Bridge Inventory 

(NBI) component rating method and the AASHTO element level condition state 

assessment method. 

NBI Rating Guidelines Using NBI rating guidelines, a 1-digit code on the Federal Structure Inventory and 

Appraisal (SI&A) sheet indicates the condition of the culvert (Item 62). This item 

evaluates the alignment, settlement, joints, structural condition, scour, and other 

items associated with culverts. Rating codes range from 9 to 0 where 9 is the best 

rating possible. See Topic 4.2 (Item 62) for general descriptive codes and Topics 

12.4.6 through 12.4.8 for specific codes for the various flexible culverts. The 

rating code is intended to be an overall evaluation of the culvert. Integral 

wingwalls to the first construction or expansion joint shall be included in the 

evaluation. It is also important to note that Items 58-Deck, 59-Superstructure, and 

60-Substructure shall be coded “N” for all culverts. 

The previous inspection data should be considered along with current inspection 

findings to determine the correct rating. 

Element Level Condition 

State Assessment 

In an element level condition state assessment of a flexible culvert, the AASHTO 

CoRe element is: 

Element No. Description

240 

243 

Unpainted Steel Culvert 

Culvert: Other 

The unit quantity for culverts is meters or feet of culvert length along the barrel. 

The total quantity equals the culvert length times the number of barrels. The 

inspector must visually evaluate each 1 m (1 ft) slice of the culvert barrel(s) and 

assign the appropriate condition state description. The total length must be 

distributed among the four available condition states depending on the extent and 

severity of deterioration. The sum of the individual condition state quantities must 

equal the total element quantity. Condition state 1 is the best possible rating. See 

the AASHTO Guide for Commonly Recognized (CoRe) Structural Elements for 

condition state descriptions. 

A Smart Flag is used when a specific condition exists, which is not described in 

the CoRe element condition state. The severity of the damage is captured by 

coding the appropriate Smart Flag condition state. The Smart Flag quantities are 

measured as each, with only one each of any given Smart Flag per bridge. 

For settlement of the culvert, the “Settlement” Smart Flag, Element No. 360, can 

be used and one of three condition states assigned. For channel scour at the 

culvert ends, the “Scour” Smart Flag, Element No. 361, can be used and one of 

three condition states assigned. 
SECTION 12: Special Bridges 

TOPIC 12.4: Flexible Culverts 

12.4.56 

Figure 12.4.44 Standard Sizes for Corrugated Steel Culverts (Source: American 

Iron and Steel Institute) 
SECTION 12: Special Bridges 

TOPIC 12.4: Flexible Culverts 

12.4.57 

Figure 12.4.44 Standard Sizes for Corrugated Steel Culverts (Source: American 

Iron and Steel Institute), continued 
SECTION 12: Special Bridges 

TOPIC 12.4: Flexible Culverts 

12.4.58 

Figure 12.4.44 Standard Sizes for Corrugated Steel (Source: American Iron and 

Steel Institute), continued 
SECTION 12: Special Bridges 

TOPIC 12.4: Flexible Culverts 

12.4.59 

Figure 12.4.44 Standard Sizes for Corrugated Steel Culverts (Source: American 

Iron and Steel Institute), continued 
SECTION 12: Special Bridges 

TOPIC 12.4: Flexible Culverts 

12.4.60 

Figure 12.4.44 Standard Sizes for Corrugated Steel Culverts (Source: American 

Iron and Steel Institute), continued 
SECTION 12: Special Bridges 

TOPIC 12.4: Flexible Culverts 

12.4.61 

Figure 12.4.44 Standard Sizes for Corrugated Steel Culverts (Source: American 

Iron and Steel Institute), continued 
SECTION 12: Special Bridges 

TOPIC 12.4: Flexible Culverts 

12.4.62 

Figure 12.4.44 Standard Sizes for Corrugated Steel Culverts (Source: American 

Iron and Steel Institute), continued 
SECTION 12: Special Bridges 

TOPIC 12.4: Flexible Culverts 

12.4.63 

Figure 12.4.44 Standard Sizes for Corrugated Steel Culverts (Source: American 

Iron and Steel Institute), continued 
SECTION 12: Special Bridges 

TOPIC 12.4: Flexible Culverts 

12.4.64 

Figure 12.4.44 Standard Sizes for Corrugated Steel Culverts (Source: American 

Iron and Steel Institute), continued 
SECTION 12: Special Bridges 

TOPIC 12.4: Flexible Culverts 

12.4.65 

Figure 12.4.44 Standard Sizes for Corrugated Steel Culverts (Source: American 

Iron and Steel Institute), continued 

 
SECTION 12: Special Bridges 

TOPIC 12.4: Flexible Culverts 

12.4.66 

Figure 12.4.44 Standard Sizes for Corrugated Steel Culverts (Source: American 

Iron and Steel Institute), continued 
SECTION 12: Special Bridges 

TOPIC 12.4: Flexible Culverts 

12.4.67 

Figure 12.4.44 Standard Sizes for Corrugated Steel Culverts (Source: American 

Iron and Steel Institute), continued 
SECTION 12: Special Bridges 

TOPIC 12.4: Flexible Culverts 

12.4.68 

Figure 12.4.44 Standard Sizes for Corrugated Steel Culverts (Source: American 

Iron and Steel Institute), continued 
SECTION 12: Special Bridges 

TOPIC 12.4: Flexible Culverts 

12.4.69 

Figure 12.4.44 Standard Sizes for Corrugated Steel Culverts (Source: American 

Iron and Steel Institute), continued 
SECTION 12: Special Bridges 

TOPIC 12.4: Flexible Culverts 

12.4.70 

Figure 12.4.44 Standard Sizes for Corrugated Steel Culverts (Source: American 

Iron and Steel Institute), continued 
SECTION 12: Special Bridges 

TOPIC 12.4: Flexible Culverts 

12.4.71 

Figure 12.4.45 Standard Sizes for Aluminum Culvert (Source: Aluminum 

Association) 
SECTION 12: Special Bridges 

TOPIC 12.4: Flexible Culverts 

12.4.72 

Figure 12.4.45 Standard Sizes for Aluminum Culvert (Source: Aluminum 

Association), continued 
SECTION 12: Special Bridges 

TOPIC 12.4: Flexible Culverts 

12.4.73 

Figure 12.4.45 Standard Sizes for Aluminum Culvert (Source: Aluminum 

Association), continued 
SECTION 12: Special Bridges 

TOPIC 12.4: Flexible Culverts 

12.4.74 

Figure 12.4.45 Standard Sizes for Aluminum Culvert (Source: Aluminum 

Association), continued 
SECTION 12: Special Bridges 

TOPIC 12.4: Flexible Culverts 

12.4.75 

Figure 12.4.45 Standard Sizes for Aluminum Culvert (Source: Aluminum 

Association), continued 
SECTION 12: Special Bridges 

TOPIC 12.4: Flexible Culverts 

12.4.76 

Figure 12.4.45 Standard Sizes for Aluminum Culvert (Source: Aluminum 

Association), continued 
SECTION 12: Special Bridges 

TOPIC 12.4: Flexible Culverts 

12.4.77 

Figure 12.4.45 Standard Sizes for Aluminum Culvert (Source: Aluminum 

Association), continued 
SECTION 12: Special Bridges 

TOPIC 12.4: Flexible Culverts 

12.4.78 

Figure 12.4.45 Standard Sizes for Aluminum Culvert (Source: Aluminum 

Association), continued 
SECTION 12: Special Bridges 

TOPIC 12.4: Flexible Culverts 

12.4.79 

Figure 12.4.45 Standard Sizes for Aluminum Culvert (Source: Aluminum 

Association), continued 
SECTION 12: Special Bridges 

TOPIC 12.4: Flexible Culverts 

12.4.80 

Figure 12.4.45 Standard Sizes for Aluminum Culvert (Source: Aluminum 

Association), continued 
SECTION 12: Special Bridges 

TOPIC 12.4: Flexible Culverts 

12.4.81 

Figure 12.4.45 Standard Sizes for Aluminum Culvert (Source: Aluminum 

Association), continued 
SECTION 12: Special Bridges 

TOPIC 12.4: Flexible Culverts 

12.4.82 

Figure 12.4.45 Standard Sizes for Aluminum Culvert (Source: Aluminum 

Association), continued 
SECTION 12: Special Bridges 

TOPIC 12.4: Flexible Culverts 

12.4.83 

Figure 12.4.45 Standard Sizes for Aluminum Culvert (Source: Aluminum 

Association), continued 
SECTION 12: Special Bridges 

TOPIC 12.4: Flexible Culverts 

12.4.84 

Figure 12.4.45 Standard Sizes for Aluminum Culvert (Source: Aluminum 

Association), continued 
SECTION 12: Special Bridges 

TOPIC 12.4: Flexible Culverts 

12.4.85 

Figure 12.4.45 Standard Sizes for Aluminum Culvert (Source: Aluminum 

Association), continued 
SECTION 12: Special Bridges 

TOPIC 12.4: Flexible Culverts 

12.4.86 

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