Chapter 7
Substructure Design
To design a substructure properly, a designer must know or assume the loads acting on the substructure. Assumed loads must be verified before completing the final design. Since many loads can act on the substructure, not all loads will act with full intensity at the same time. To apply the appropriate loads to the substructure design, the designer must determine the critical combination of loads for given conditions.
ABUTMENT DESIGN PROCEDURE
71. There are separate design procedures depending on the type of load acting on the abutment. Determine the vertical or horizontal load as discussed below.
VERTICAL LOADS
72. For abutments constructed of piles or posts and footings, determine the total loads acting on the entire abutment. For continuous abutments (such as mass or reinforced concrete) (Figure 71), determine the load per foot of abutment length. The dead load acts vertically through the centerline of the bearing plate (Figure 72).
Figure 71. Typical Concrete Abutment
Figure 72. Dead and LiveLoad Action on an Abutment
Superstructure Dead Load
73. Compute the superstructure dead load as follows:
Live Load
74. The live load acts vertically through the centerline of the bearing plate (Figure 72). Impact loads are not included. Compute the live load as follows:
Abutment Weight
75. The weight of timber or steel abutments is negligible since it is small in comparison to other vertical abutment loads. However, the weight of concrete abutments should be included in the design loads. Divide the crosssectional shape of an abutment into sections of known size, shape, and crosssectional area (Figure 73). The weight of any section acts vertically through the centroid of that section. Compute the weight of any section per foot of abutment length as follows:
Figure 73. Abutment Sectioning, Load Centers, and Soil Forces
Soil Forces
76. Use equation 75 and compute the weight of soil acting on the rear face and heel of the abutment (Appendix I). To account for vehicular traffic approaching the abutment, assume placing a hypothetical layer of soil (surcharge) over the backfill at the abutment. Figure 73 shows vertical soil forces. Compute the height of the surcharge as follows:
Hydrostatic Uplift
77. Properly locate the abutment, and provide drainage to avoid hydrostatic uplift forces on the abutment. Eliminate hydrostatic uplift forces by locating the abutment well above the flood stage, by providing granular material behind and under the abutment, and by installing weep holes in the abutment. Methods for draining water from behind and under abutments are discussed in paragraphs 731 and 744.
HORIZONTAL LOADS
78. Horizontal loads acting on abutments are created by soil, water, temperature, and vehicles. These loads are computed per foot of abutment length.
Soil Forces
79. Use the Rankine equations from Introductory Soil Mechanics and Foundations: Geotechnic Engineering to compute the horizontal surcharge and backfill forces as shown below. Figure 73 shows the actions of these forces. In most cases, passive pressure is so negligible that it is not considered, resulting in a somewhat conservative design.
 Coefficient of active earth pressure.
 Coefficient of passive earth pressure.
 Soil forces and point of application.
Water Forces
710. Water that collects behind an abutment will create a horizontal force that can be eliminated by proper drainage. See paragraph 745 for methods of draining water from behind and under abutments.
Temperature Forces
711. Temperature forces are negligible in simply supported stringer bridges designed for military purposes. Bearing plates support each end of every steel stringer. One end of the stringer is rigidly attached so that there is no movement between the stringer and the support. The other end is attached so that movement is allowed along the length of the stringer. As the stringer expands or contracts due to temperature changes, temperature forces are dissipated at the free end. The fixed bearing plate is normally placed on the abutment end of the stringer.
Vehicular Longitudinal Forces
712. As vehicles accelerate or brake on a bridge span, a longitudinal force is transmitted to the substructure through the fixed bearing plates. Compute the vehicular force as follows:
713. The number of vehicles allowed per lane is based on the span length and the vehicle spacing (at least 100 feet). Since vehicles traveling in opposite directions have canceling effects on loads, the worst loading conditions occur when vehicles move in only one direction. Therefore, the number of vehicles on a span is for one lane only. At the expansion bearing plate, the vehicle force is zero. The vehicle force acts horizontally on the fixed bearing plate (Figure 74).
Figure 74. Vehicular Longitudinal Force
LATERAL LOADS
714. Lateral loads acting on abutments are negligible.
ABUTMENT SELECTION
715. Abutments may be timber, steel, concrete, or a combination of these materials. The abutments may rest directly on the soil (as in the case of concrete), or they may rest on footings or piles. Figure 75 and Figures 76 through 711 show typical abutments. Table 71 gives a general guide to the types of abutments used under various conditions.
Figure 75. TimberSill Abutment
Figure 76.TimberBent Abutment
Figure 77. TimberPile Abutment
Figure 78. SteelPile Abutment
Figure 79. MassConcrete Abutment
Figure 710. Concrete Abutment on Piles
Figure 711. Reinforced Concrete Abutment
716. The general principles of abutment design are applicable to any type of abutment resting on the ground or on footings, with only a few modifications for abutments resting on piles. These principles also apply directly to the design of retaining walls. Abutments must be designed to avoid the following types of failure:
 Overturning.
 Sliding.
 Soil bearing.
 Soil shear.
 Material.
717. Since abutments are subjected to various types of loadings during construction and use, the load cases must be considered (Table 72). Figure 712 shows a sample chart of abutment loads and moments.
Figure 712. Sample Chart of Abutment Loads and Moments
ABUTMENT DESIGNS
TIMBERFOOTING AND TIMBERBENT ABUTMENTS
718. Determine the loads using the procedures in paragraphs 72 through 74. Consider only live and dead loads. Compute the total load for the abutment as follows:
Abutments
719. Choose the abutment length so that the following requirement is met:
Posts
720. Compute the allowable bearing capacity per post and the required number of posts as follows:
 Allowable bearing capacity per post.
 Required number of posts. The number of posts must be greater than or equal to the number of stringers.
Caps and Sills
721. The following equations apply to both caps and sills. The absolute minimum size for a cap or sill is 6 x 8 inches. If a sill is used, it should be the same size as the cap.
 Width. After determining the cap or sill width for stringers, substitute the post size for the stringer size in the following equation and use the larger of the two results as the cap or sill width:
 Depth. Compute the depth as follows:
Footings
722. Each post or stringer (whichever is used) must rest on a footing. Compute the allowable length, the area, the capacity, and the number of footings as follows:
 Allowable length.
 Area.
 Capacity.
 Number of footings. The required number of footings will be greater than or equal to the number of posts or stringers.
End Dam and Deadmen
723. Material used in an end dam should be at least 3 inches thick to prevent failure from earth pressure. Install deadmen if using a timber bent (see paragraph 758 for a deadman design).
TIMBER AND STEELPILE ABUTMENT DESIGNS
724. Follow the procedures described for the design of pile foundations in paragraph 7100.
MASSCONCRETE ABUTMENT DESIGN
725. Massconcrete abutments are used when the abutment will or is expected to be in contact with the stream or if the required height is greater than ten feet. Design massconcrete abutments by trial and error.
Dimensions
726. Base the estimate of the overall height of an abutment on local site conditions. An abutment should extend far enough below the ground's surface so that it rests on firm soil. If the overall height exceeds 20 feet, a massconcrete structure becomes uneconomical and a pile foundation should be considered. Once the abutment height is determined, use the following guidelines to estimate other dimensions (Figure 713). Always check the safety of any abutment designed by these guidelines. Assume the following dimensions and loads:
 The abutment width is 40 to 60 percent of the abutment height, in feet.
 The footing height is 1 to 2 feet.
 The toe and heel width are to 1 feet and are less than or equal to 75 percent of the footing height.
 The seat width is greater than or equal to the cap or sill width, and the cap or sill width is greater than or equal to foot.
 The superstructure height is measured from the bottom of the bearing plate to the top of the deck, in feet.
Figure 713. MassConcreteAbutment Design Dimensions
727. If any of the safety criteria fail, choose new dimensions. A heel and toe are not mandatory for massconcrete abutments, but they are recommended because they provide additional stability and aid in drainage. Once the preliminary dimensions are selected, determine all the loads that act on the abutment, using the methods described in paragraphs 72 and 78.
Loading Case Values
728. For easy computations, set the values for the loads, lever arms, and moments as shown in Figure 712. The moment arm is the perpendicular distance from the abutment toe to the line of action of the given load. Determine the moments about the toe, since this is the critical point for overturning. Use the tabulated values to check the safety of the abutment against sliding, overturning, and soilbearing failure.
Sliding Check
729. Compute the safety factor of sliding for loading Cases I and III as follows:
Overturning Check
730. Compute the safety factor of overturning for loading Cases I and III as follows:
Sliding and Overturning Prevention
731. Do one or more of the actions discussed below to prevent sliding and overturning.
732. Modify the Abutment or Heel. Increase the size of the abutment or the length of the heel so that more weight acts vertically.
733. Modify the Superstructure Placement. If sliding or overturning is critical for Case I (unloaded) only, place the superstructure onto the abutment before backfilling the space behind the abutment.
734. Modify the Friction Coefficient. Increase the friction coefficient by adding a layer of gravel under the abutment. Use a filter layer to prevent fine material from washing out through the gravel.
735. Install a Deadmen. Install a deadmen behind the abutment. Paragraph 758 describes a deadman design.
736. Construct a Key. Construct a key as shown in Figure 714. This key will develop passive earth forces that will help resist sliding. Use half the value of the horizontal force in safety computations, since the full value will probably never be developed.
Figure 714. Abutment Key
 Compute the dimensions. If necessary, choose and modify the preliminary key dimensions as follows:
 Reinforce the key to prevent failure due to concrete stresses. Use No. 4 bars with 1foot, centertocenter spacing. Extend the bars 1 foot into the body of the abutment. Use a concrete cover (minimum of 3 inches) over the reinforcing steel.
 Modify the design. If placing the abutment on piles, design the piles to carry the full load (including the abutment weight). Paragraph 7100 describes pile foundations.
Soil Bearing Capacity
737. Determine the allowable soil bearing capacity for the specific site, if possible. Otherwise, use Appendix H to estimate the soil bearing capacity. The criteria for proper soil bearing capacity are the—
 Eccentric distance is less than or equal to onesixth the abutment width for all three cases.
 Minimum pressure of eccentricity is less than or equal to the soil bearing capacity for Case III.
738. Eccentric Distance. Since horizontal and vertical forces act on an abutment, the pressure it exerts on the soil varies. The resultant force due to soil pressure acts at an eccentric distance from the geometric center of the abutment.
739. Compute the eccentric distance as follows:
740. If the sign of the eccentric distance is positive, the resultant force is to the left of the centerline and maximum pressure occurs at the toe (Figure 715A). If the sign of the eccentric distance is negative, the resultant force is to the right of the centerline and maximum pressure occurs at the heel (Figure 715B) .
Figure 715. Eccentric Distance
741. Load Eccentricity. The resultant force must be within the middle third of the abutment. If the eccentric distance is less than onesixth of the abutment width, the resultant force is within the middle third of the abutment. If the eccentric distance is greater than onesixth of the abutment width, the resultant force is outside the middle third, indicating that the soil is in tension (Figure 716). However, since soil cannot take tension, the resultant load is spread over a smaller area, which increases the maximum pressure and possibly leads to failure. Determine the eccentricity of load for each loading case to ensure that the eccentric distance is less than or equal to onesixth of the abutment width. To correct excessive load eccentricity, increase the toe or heel length so that the eccentric distance is within the middle third of the abutment.
Figure 716. Eccentricity Outside the Middle Third of the Abutment
742. Maximum Soil Pressure. The maximum soil pressure must not exceed the allowable soil bearing capacity. Determine the maximum pressure for Case III only, since the worst loading condition occurs for this case. Compute the maximum pressure for eccentricity within the middle third of the abutment as follows:
743. Minimum Soil Pressure. Compute the minimum soil pressure for eccentricity within the middle third of the abutment as follows:
Soil Shear Failure
744. Soil shear failure is a landslidetype failure involving the abutment and the mass of earth under and behind the abutment. Figure 717 shows typical soil shear failure. The primary causes of soil shear failure are—
 The slope in front of the abutment is too steep.
 Scour occurs under the abutment.
 The backfill is saturated with water or eroded from the sides of the abutment.
Figure 717. Soil Shear Failure
745. To prevent soil shear failure, ensure that the slope in front of the abutment is no more than 45 degrees and preferably less than 35 degrees (Figure 718A). Place riprap in front of the abutment to prevent scour. If riprap is not available or the slope cannot be made less than 45 degrees, drive sheet piles in front of the abutment. The sheet piles must extend below the bottom of the gap and be anchored near the top with a deadman (Figure 718B). The abutment can be placed on friction or bearing piles. Backfill the abutment with sand and gravel and install weep holes for drainage. Protect weep holes with a filter. Also, install granular material under the abutment, especially if the soil contains clay. Install wing walls to prevent erosion of the earth from the sides of the abutment (Figure 718C)
Figure 718. Preventing Soil Shear Failure
Concrete Failure
746. Figure 719 shows the critical sections in a massconcrete abutment. Section AA is especially critical because Point C is subject to vehicle impact loads if the backfill settles. To prevent failure, reinforce Section AA with steel bars (Figure 720). See Figure 721 for concretestress checkpoints.
Figure 719. Critical Sections in a MassConcrete Abutment
Figure 720. Reinforcement of Section AA
Figure 721. ConcreteStress Checkpoints
REINFORCED CONCRETE ABUTMENT
747. Use reinforced concrete abutments if economy of materials is desired. Concrete abutments are subject to the same types of failure as other abutments. Preventing failure (except for concrete failure) requires the same procedures as for massconcrete abutments. Figure 722 shows a typical reinforced concrete abutment. Check Sections AA, BB, DD, and EE for potential failure. Follow the procedures discussed below when designing reinforced concrete abutments.
Figure 722. Reinforced Concrete Abutment With Critical Cross Sections
Dimensions
748. Local soil conditions govern the abutment height. Choose a height that allows the footing to rest on firm soil. Choose the other dimensions using the following limits (check all critical sections for safety after choosing the dimensions):
 The curbtocurb roadway width is less than the abutment length, and the abutment length is less than the roadway width plus 4 feet.
 The abutment width is 50 to 70 percent of the abutment height.
 The stem width is 15 percent of the abutment height.
 The toe and heel heights are 10 percent of the abutment height.
 The seat width is greater than or equal to the cap or sill width, and the cap or sill width is greater than or equal to foot.
 The superstructure depth is measured from the bottom of the bearing plate to the top of the deck.
 The toe width is 10 percent of the abutment height.
Loading Case Values
749. Using the preliminary dimensions outlined above, determine the vertical and horizontal abutment loads using the methods described in paragraphs 72 and 78. Determine the soil pressure at Points A and D (Figure 723) using the methods in paragraphs 742 and 743. The maximum pressure of eccentricity must be less than or equal to the soil bearing capacity, and the minimum pressure of eccentricity must be greater than zero for all loading cases.
Figure 723. SoilPressure Points
Shear and Moment
750. Determine the shear force and moment as shown below. Each section is computed separately.
751. Sections AA and BB. Determine the shear force and the moment for Sections AA and BB (Figure 722) using the equation below. For Sections AA and BB, consider only that portion of the surcharge force plus the backfill force that acts at or above the respective section.
752. Section DD. The shear force at Section DD (Figure 722) is the total of the forces exerted at the centroid of the rectangle ABLK and the triangle KLM in Figure 723. Compute for shear force and moment as follows:
753. Section EE. The shear force at Section EE (Figure 722), resulting in tension at the bottom of the slab, is the weight of the soil plus the surcharge above the heel and the weight of the heel minus the forces exerted at the centroid by the rectangle CDHG and the triangle GHJ. Compute for shear force and moment as follows:
Critical Sections
754. The design procedure for critical sections corresponds to the procedure used for flat slabs with reinforcement in one direction. Base the design on a 1footwide section. At each critical section, check for moment, shear, and bond (between steel and concrete). Assume that there are 10 stringers and that the ultimate compressive stress is 3 ksi, the allowable concrete stress is 1.35 ksi, and the allowable steel stress is 24 ksi. Compute as follows:
 Required section depth.
 Required steel area. The total depth of Section EE (Figure 722)
is the required section depth plus 3 inches. This depth provides protection for the reinforcing steel. Once the steel area is known, choose the bar sizes from Appendix D.
 Actual shear stress. If the actual shear stress is critical, increase the required section depth to decrease the shear stress. Ensure that the actual shear stress does not exceed the allowable shear stress (0.09 ksi).
 Bond stress. Check the bond stress of the reinforcing bars against the allowable bond stress (0.03 ksi). Compute as follows:
Sliding and Overturning
755. Use the methods described in paragraphs 729 and 730 to check for sliding and overturning. If either sliding or overturning presents a problem, increase the heel width to increase the total weight of soil over the heel. Also use the measures described in paragraph 731 to prevent sliding and overturning. If the safety factors for both sliding and overturning are much larger than required and the soil bearing capacity is more than adequate, decrease the size of the originally designed abutment to provide a more economical design.
Soil Shear Failure
756. Apply the measures described in paragraph 744 to reinforced concrete abutments. This will prevent soil shear failure.
RETAINING WALLS
757. Design retaining walls the same as abutments, but do not apply superstructure dead and live loads to the top of the retaining walls. Retaining walls may be concrete (mass or reinforced), timber, or steel sheet piles.
DEADMAN DESIGN
758. Use a deadman as a means of preventing sliding and overturning. Place the deadman outside the natural angle of repose of the soil. A setback equal to 150 percent of the abutment height is sufficient. Anchor the cables or steel tie rods by casting them into the body of the concrete abutment as shown in Figure 724. If the bridge is constructed of piles or posts, bolt the cables or tie rods to the abutment as shown in Figure 725. Position the cables or tie rods horizontally between the abutment and the deadman. The cover over the cables should be three times the deadman's depth dimension or the depth of the covered stem plus 1 foot, whichever is greater.
Figure 724. Deadman Connection, Cover, and Setback
Figure 725. Deadman Installation on a Pile or Post Abutment
Sliding
759. Use equation 723 to check for sliding on a concrete abutment. If the safety factor is less than 1.5, design a deadman as follows:
 Concrete or footingtype abutment.
 Pile abutment.
 Compute the allowable shear capacity per pile as follows:
 Compute the total horizontal force resisting shear as follows:
 Compute the horizontal force that the deadman must resist, in equations 744 and 745, substituting H_{av} for ∑HM .
 Compute the allowable shear capacity per pile as follows:
Overturning
760. Use equation 724 to compute the safety factor for overturning. If it is less than two, design the abutment as follows:
761. Design the deadman to resist the horizontal shear force, including a safety factor of two. Design the deadman to resist the larger force required for sliding and overturning. Compute as follows:
Deadman Specifications
762. Length and Depth. The deadman length normally equals the abutment length. Compute the required structural depth of the deadman as follows:
763. Number of Cables. Compute the number of cables using the equation below. If using steel rods instead of cables, use the same equation but substitute the rod strength (rod area x 29 ksi) for the allowable rod strength.
764. Cable or Rod Spacing. Compute the proper spacing and deadman width as follows:
765. Cable Protection. After installation, coat the cables or rods with tar to protect them from rust. Treat the deadman for protection against decay.
INTERMEDIATESUPPORT DESIGN
VERTICAL PIER LOADS
766. Vertical pier loads include dead and live loads, pier weight, soil forces, and buoyancy forces. The dead load acts vertically through the centerline of the support.
Dead Load
767. Compute the dead load of the pier as follows:
Live Load
768. Compute the live load of the pier as follows:
769. Note that the two span lengths resting on the pier are added, and the liveload shear is found for the combined span lengths. Impact is not included in this load. The live load acts vertically through the centerline of the pier.
Pier Weight
770. The weight of timber or steelframed piers is negligible. However, include the weight of a concrete pier in the substructure design. Determine the concretepier weight from known or assumed dimensions. Figure 726 shows a typical concrete pier (with dimensions). The forces of the stem and footing weights act vertically through the pier centerline.
Figure 726. Typical Concrete Pier
771. Stem Weight. Compute the stem weight as follows:
772. Footing Weight. Compute the footing weight using equation 758. Substitute the footing dimensions for the stem dimensions.
Soil Forces
773. If there is a layer of soil over the footing, include the weight of the soil in the vertical pier load. Compute the soil weight over the pier (as shown in Figure 726) as follows:
774. If the stem is steel Hsections, the area of the steel is negligible. If the stem is timber, compute the soil weight as follows:
Buoyancy Forces
775. If the pier is partially submerged (Figure 726), the water creates buoyancy forces on the submerged parts of the pier. The buoyancy force on any part is simply the volume of that part below water multiplied by the unit weight of water. Buoyancy forces for steel or timber piers are negligible. Compute for the buoyancy force acting on the pier stem as follows:
776. Similarly, the buoyancy forces acting on the footing and soil are computed as follows:
Total Vertical Pier Load
777. The total vertical pier load is the sum of all vertical loads minus the buoyancy forces. For the pier shown in Figure 726, the total vertical load is as follows:
LONGITUDINAL LOADS
778. The only longitudinal load considered in pier design is the vehicular longitudinal force. Paragraph 712 describes how to determine the number of vehicles per lane.
LATERAL LOADS
Wind Load
779. Determine the wind force as follows:
 On vehicles. At normal convoy spacing of 100 feet, the wind load on vehicles is negligible.
 On the substructure. The effects of wind on the substructure are taken into account when determining the wind load on the superstructure.
 On the superstructure. Short bridges are designed with no allowance for wind. For bridges with combined span lengths of 100 feet or more, compute the wind force acting at each pier using the equation below. The safety factor (1.5) accounts for wind acting on the substructure, curbs, and handrails and the fact that wind acts with reduced force on stringers behind the windward stringer. Wind force acts at the top of the windward stringer as shown in Figure 727.
Figure 727. Wind Force on the Superstructure
Water Force on the Substructure
780. Water force acts on a pier at onefourth the water height, measured downward from the water surface (Figure 728). To determine the water force acting on a pier, first compute the area of the pier or posts upon which the water force acts.
 Compute the area of a concrete pier on which the water force acts as follows:
 Compute the area of a pile or post on which the water force acts as follows:
 Compute the water force acting on a pier as follows:
Figure 728. Water Force on the Substructure
Ice Forces
781. Consider the forces from ice as follows:
 Crushing. Ice crushing on concrete piers is negligible, since the compressive strength of concrete is much greater than the crushing force of ice. Timber piers, however, must be protected if the temperature falls below 0 degrees Fahrenheit (F). To protect the piers, attach steel angles to break up the ice (Figure 729A).
 Thrust. Ice thrust occurs at normal water level during winter months. Estimate this force using the equation below. For piers with dolphins (Figure 729B), reduce the ice thrust by 50 percent.
 Pileup. Ice pileup occurs during the spring thaw when river ice breaks up and moves downstream in floes. These floes create large impact loads on anything in their path. In streams where ice pileup could occur, bridge piers should either be located out of the water or protected by dolphins.
Figure 729. Pier Protection Devices
INTERMEDIATESUPPORT SELECTION
782. Intermediate supports may be constructed of timber, steel, concrete, or a combination of materials. They may be supported by footings or piles. Figures 730 through 736 show various types of intermediate supports required for different conditions. Table 74 gives a general guide for selecting intermediate supports.
Figure 730. TimberCrib Pier
Figure 731. TimberTrestle Bent
Figure 732. TimberTrestle Pier
Figure 733. Pile Pier
Figure 734. Pile Bent
Figure 735. SteelFramed Tower
Figure 736. Concrete Pier
TIMBERCRIBPIER DESIGN
783. Timbercrib piers are assembled in logcabin fashion (Figure 730). For stability, the bottom of the crib is wider than the top. The base width is at least onethird the pier height. Drift bolts hold the timbers together at the corners. Fill the crib with rocks for ballast, if desired. For use in water, partially construct the crib on the shore, float it to position, and then sink it by filling it with rocks. Make the top of the pier level and solid to form a substantial support. Bridge stringers may rest directly on top of the crib. If more height is needed, construct a short timbertrestle bent or pier on top of the crib.
TIMBERTRESTLEBENT AND PIER DESIGN
784. Timbertrestle bents and piers (Figures 731 and 732) are not suitable for use in soft soil or swift or deep watercourses. Construct them in dry, shallow gaps with firm soil. When longitudinal spacing between bents exceeds 25 feet, bracing becomes cumbersome. To provide for greater support and longitudinal stability for longer spans, use timbertrestle piers. The design of timbertrestle piers is the same as the bent design except for the cap and corbel system (paragraph 7120). Use the procedures discussed below to design a timbertrestle bent.
Loads
785. Use the procedures in paragraph 766 to determine dead and live loads. Compute the total design load as follows:


Size and Number of Members
786. Posts. The absolute minimum post size is 6 x 6 inches, and the minimum number of posts per bent is four.
 Compute the allowable bearing capacity per post as follows:
 Limit the length of the post to ensure that it will not fail by buckling. Check as follows:
 Recompute the number of posts required as follows:
787. Caps and Sills. The absolute minimum size for caps and sills is 6 x 8 inches. The larger dimension is usually vertical. The cap and sill must also be at least as wide as the posts ( Figure 737). Determine the bearing failure, the post spacing, and the depth as follows:
 Bearing failure. Check the cap and sill for bearing failure. If bearing failure is a problem, increase the number of posts.
 Post spacing. Compute post spacing the same as stringer spacing.
 Depth.
Figure 737. Limiting Dimensions for a TimberTrestle Bent
788. Footings. The number of footings must equal or exceed the number of posts. A footing design for bents is identical to a footing design for abutments (paragraph 722).
789. Bracing. Provide adequate longitudinal bracing between bents to ensure longitudinal stability of the bridge. The minimum bracing size is 2 x 12 inches.
PILES
790. A pile is a slender structural member that is forced into the ground to support vertical, horizontal, or inclined loads. Since one pile may not have the capacity to carry a certain load, several piles may have to be grouped together for the pile foundation. Use piles when placing a foundation on soft soil, in deep water, or in swift watercourses that are likely to scour. The factors discussed below will affect pile design.
Scour
791. Scour is the gradual removal of earth surrounding a pier or abutment by water action (Figure 738). If a bridge has long spans, it needs intermediate piers, which obstruct the water flow and increase stream and scouring action. (When the stream velocity is great, or as much as 2 fps, the bottom may be disturbed.) For example, when a pile is inserted into a flowing watercourse, the turbulence and eddy currents created by the pier cause scour. If the axis of a pier is not parallel to the direction of flow, excessive scour will result. Minimize scour by ensuring proper alignment of piers in watercourses.
Figure 738. Types of Scour
792. Local Scour. The scour depth varies with many factors. As a rule, estimate local scour depth as follows:


793. Flood Scour. Consider flood scour in the substructure design because it is the cause of many bridge failures. If possible, determine the depth of flood scour at the site (Figure 738B) . If it cannot be determined, estimate the depth of flood scour as follows:


794. Total Scour. Total scour is the combination of local and flood scour. When the water surface drops after a flood, sediment is redeposited in the scoured area and the bottom returns to its original level. Compute the total scour as follows:


795. Scour Prevention. Protect foundations from scour by
 Locating bents or piers parallel to the direction of water flow.
 Placing sandbags around bents, piers, or abutments on the upstream sides.
 Placing riprap around bents, piers, or abutments.
 Driving a row of closely spaced pile fenders (dolphins) perpendicular to the water flow on the upstream side of the bent, pier, or abutment. Recognize that fenders may compound the problem of scour because they further restrict water flow. Fenders are most effective when their widths are small relative to the watercourses width.
PileSupport Types
796. Pilesupport types include end bearing, friction, and batter. Each is discussed below.
797. EndBearing Piles. Endbearing piles (Figure 739A) are firmly seated on rock or hard strata. The entire support of the piles is provided by the hard strata so that the load carried is limited by the strength and unbraced length of the pile material.
Figure 739. Types of Pile Support
798. Friction Piles. Friction piles (Figure 739B) derive their entire support from friction between the piles and the surrounding soil. The load a friction pile will carry depends on the properties of the soil and the strength of the pile material.
799. Batter Piles. Batter piles are driven into the ground at an angle (Figure 740). The maximum slope at which a pile may be driven is 1:1 (horizontal to vertical), due to the limitations of the driving equipment. The normal slope for batter piles is 1:12. If the slope is within 1:12, the vertical loadcarrying capacity will not have to be reduced. Using batter piles complicates the design of a pile foundation; therefore, design pile foundations for vertical piles. Then install batter piles as the outside piles for additional safety and stability.
Figure 740. Batter Piles
PILE FOUNDATIONS
7100. Pile foundations may be constructed of timber, steel, or concrete. Since concrete piles require special handling and equipment, they are not used in military construction. Design principles for timber and steel piles are the same except for the exceptions discussed below.
Allowable Load
7101. For an endbearing pile, the maximum allowable load is the smaller value of the allowable load or the buckling load of the pile. For a friction pile, the maximum allowable load is the smallest value of the allowable load, the buckling load, the soil friction capacity, or the piledriving capacity. Check the bearing and buckling loads in both endbearing and friction piles. Also, check the soil and piledriving capacities in friction piles.
7102. Bearing Capacity. The first step in designing a pile foundation is to determine the maximum allowable load that a single pile will carry. Compute the allowable bearing capacity of a pile as follows:


7103. Buckling Load. The allowable buckling load for a single pile depends on the unbraced length of the pile, the size of pile, and the type of soil.
7104. The fixed point (FP) is the point below which the pile is assumed to be completely rigid, so that any bending or buckling in the pile will take place above the FP (Figure 741). The FP distance varies with the soil type (5 feet for sand and 10 feet for clay). Interpolate the FP for intermediate soils. Measure the FP distance from the point of lowest scour to ensure that the design accounts for the worst possible conditions.
Figure 741. Maximum Unbraced Length
7105. For a single row of piles unbraced in the longitudinal direction, the unbraced length is 70 percent of the distance from the FP to the top of the pile (Figure 741A). For a single row of piles with adequate longitudinal bracing, the unbraced length is onehalf the distance from the FP to the lowest bracing. For piles arranged in two or more rows with adequate bracing between, the unbraced length is onehalf the distance from the FP to the lowest bracing (Figure 741B).
 Timber Piles. If the unbraced length divided by the pile diameter is less than or equal to 27, buckling is not a problem and no further checks are necessary. If the length divided by the pile diameter is greater than 27, find the allowable buckling load as follows:
 Steel Piles (A36). Compute the allowable buckling load as follows:
7106. Soil Capacity. For friction piles, use load tests or static or dynamic formulas to find the allowable capacity of the soil surrounding the piles. Load tests are timeconsuming and are justified only on large, permanent bridges. If the soil type is known, the soil capacity can be estimated using a static formula. To use this static formula, first find the area of the pile in contact with the soil.
 Compute the area for timber as follows:
 Compute the area for steel as follows:
 Compute the pile capacity (once the area of pile in contact with the soil is known) as follows:
7107. If the soil conditions are not known, estimate the allowable soil capacity by driving a test pile and applying a dynamic formula to the results. Also use the dynamic formulas to check capacities estimated by the static formula. Dynamic formulas are only approximations; therefore, use them only if load tests are unavailable.
7108. The basic assumption behind dynamic formulas is that driving resistance equals the static resistance of the pile to loads after driving is completed. However, this assumption is not always correct, and the more impervious the soil, the greater the discrepancy between the actual and computed loads. As a pile is driven, it squeezes water from the soil. Until the water drains from the surface of the pile, the full friction between the pile and the surrounding soil cannot develop. To lessen the discrepancies, let the pile rest for at least 24 hours. Redrive for at least 10 blows with a drop hammer or 30 blows with a pneumatic or diesel hammer. Use the penetration per blow after the pile has rested to estimate the allowable pile capacity.
7109. Driving Capacity. Compute the driving capacity as follows:
 Timber piles driven by a drop hammer.
 Steel piles driven by a drop hammer.
 Timber piles driven by a singleacting steam, pneumatic, or openend diesel hammer.
 Steel piles driven by a singleacting steam, pneumatic, or openend diesel hammer.
 Timber piles driven by a doubleacting steam, pneumatic, or closedend diesel hammer.
 Steel piles driven by a doubleacting steam, pneumatic, or closedend diesel hammer.
Overturning
7110. Drive piles at least 8 feet into sand or 20 feet into clay to prevent overturning due to lateral loads. For intermediate soils, interpolate the distance.
Pile Groups
7111. The spacing between rows of piles must equal or exceed three times the pile diameter. The spacing between rows must equal or exceed the spacing between piles. Each row must have at least four piles.
7112. Number of Piles Required. Compute the number of piles as follows:


7113. Group Action. When several friction piles are driven close together, the interaction of pressure bulbs reduces the efficiency of each pile (Figure 742). Use the ConverseLabarre method shown in Figure 743 to compute the effective number of piles in the group and to
Figure 742. Effect of Grouping on Pile Capacity
Figure 743. ConverseLabarre Method
 Estimate the number of bents. If each adjacent span exceeds 25 feet, a pile pier is needed. If the required number of piles is large, spacing considerations may require a threebent pier.
 Use the number of bents to determine the theoretical spacingtodiameter ratio (shown as Sp/Dp). Divide the number of piles required by the number of bents; then determine the theoretical S_{p}/D_{p } for the number of piles required per bent.
 Find the required number of piles along the effectivenumberofpiles axis.
 Project a horizontal line to the right, intersecting the S_{p}/D_{p}.
 Interpolate the value of this ratio, with a minimum allowable value of three. Project a vertical line from the intersection of the effective number of piles and S_{p}/D_{p } to the bottom of the chart. Round the value for the number of piles up to the next higher whole number to get the actual number of piles per bent.
 Recheck the actual S_{p}/D_{p } with the rounded number of piles to ensure that it does not fall below three. Also, find the actual number of effective piles by reading the chart in reverse. This value must be greater than or equal to the required number of piles. The endbearing piles will carry the full allowable pile load without any reduction in efficiency due to group action.
7114. Combined Loading. Wind, water, and ice are lateral forces that create combined loading conditions. Check the pile supports to ensure that the additional loads created by these forces will not cause failure. For example, compute the maximum actual load on an outside pile in a group as follows (Figure 744, Pile A):


Figure 744. Combined Loading
7115. The allowable load on any friction pile in the group is computed as follows:


7116. For endbearing piles, the allowable load on the outside pile is equal to the load per pile. For safety reasons, the actual load on the outside pile should be less than the load on a vertical pile. Compute the maximum actual load on an outside pile as follows:


7117. If the actual load on a single pile is negative, compare it with the allowable buoyancy force. Estimate this force as 40 percent of the soil bearing capacity, where the soil capacity is the allowable soil load (equation 786). For safety, the actual load is less than or equal to the allowable buoyancy force. If the checks indicate that overload or buoyancy forces make the support unsafe, increase the pile spacing or add more piles or rows of piles.
PILEPIER DESIGN
7118. Pilepier design is similar to pilebent design except that two or more rows of piles are used. Also, pile piers require the design of a commoncap and corbel system as discussed below.
PILEBENT DESIGN
7119. A pile bent consists of a single row of piles with a pile cap. Brace bents to one another or to the adjacent abutment to reduce the unbraced length and to provide stability. Use the procedures described in paragraphs 7100 through 7117 to design a pile bent, and use the procedures discussed below to design a pile cap. The consolidated process is as follows:
Step 1. Determine the loads acting on the bent.
Step 2. Determine the design capacity of a single pile (maximum allowable load).
Step 3. Determine the number of piles required.
Step 4. Determine the actual number of piles to be used based on group action.
Step 5. Check the combined loading produced by lateral loads and adjust the number of piles and spacing, if necessary.
Step 6. Design the cap by using the procedures outlined below.
CAP, CORBEL, AND COMMONCAP DESIGN FOR PILE PIERS
7120. To transfer a load from the superstructure to the supports properly, place a cap on each pile or post bent. If there are two or more bents in a single pier, use a commoncap and corbel system. A corbel is simply a short stringer connecting the bents (Figures 745 and 746).
Figure 745. TwoBent Pier With Common Cap
Figure 746. ThreeBent Pier With Common Cap
Cap Design
7121. The minimum size for bent caps is 6 x 8 inches. The cap must also be at least as wide as the piles or posts supporting it. Check the bearing stress between the cap and the posts to ensure that the bearing stress does not exceed the maximum allowable stress for timber (Table C1). Compute as follows:
 Cap design load.
 Actual bearing stress. The actual bearing stress must not exceed the allowable bearing stress for the weakest member in the system. If the allowable bearing stress is exceeded, correct the system by adding more piles or posts or by adding timbers on the sides of the supports (Figure 747).
Figure 747. Timbers Added to Reduce Bearing Stress
Corbel Design
7122. The corbel design process is different depending on the pier. The corbel design for two and threebent piers is discussed below.
7123. TwoBent Pier. The design length of each corbel equals the centertocenter spacing between bents. The actual corbel length exceeds the design length by a minimum of one cap width so that the corbel will have contact with the full area of each bent cap. Compute the required effective shear area and the required number of corbels as follows:
7124. Shear normally governs corbel design. If the corbel length divided by the corbel depth is less than or equal to 12, do not consider shear controls and moment. Compute as follows:
 Total shear.
 Shear capacity.
 Number of corbels (shear).
 Spacing.
 Moment. If moment governs the corbel design, compute as follows:
 Number of corbels (moment). First, compute for the required section modulus:
then, compute for the number of corbels:
 Spacing.
7125. ThreeBent Pier. A threebent pier requires two corbel systems (Figure 746), each designed separately. The design length equals the centertocenter bent spacing. The length of the top corbel equals one and onethird times the length of the bottom corbel. The bottom corbels are not continuous; they are simply supported at the center bent. See equations 7104 and 7105 for top corbels. If the ratio of 4 times the design length to 3 times the pile depth is less than or equal to 12, shear governs. If moment governs, see equations 7107 and 7108.
CommonCap Design
7126. The minimum size for a common cap is 6 x 8 inches. Compute the commoncap width and depth as follows:
 Commoncap width.
 Commoncap depth. The depth of any common cap must equal or exceed onefifth the spacing of the corbels supporting it. Compute as follows:
TIMBER AND STEELFRAMED TOWERS
7127. The AFCS contains design drawings for timber and steelframed towers.
CONCRETE PIERS
7128. Concrete piers may be constructed of either mass or reinforced concrete. Massconcrete piers can be built in about the same amount of time as reinforced concrete piers but are not economical because they require such large quantities of concrete. Since concrete piers are normally used in permanent bridges, this FM does not go into detail on their design.
SHALLOW FOUNDATIONS
7129. When shallow bedrock prevents pile installation, use some other type of specially designed shallow foundation. The paragraphs below describe a few shallow piers (other than standard timber and steel piers) that have been successful in the field.
Rock Footings
7130. In dry gaps or shallow streambeds, construct rock footings as shown in Figure 748. Hold the rocks in place by driving steel pickets into the soil. Dump the rocks into the steel enclosure and level them at the proper elevation. To provide a smooth surface for timber or steel piers, place a layer of concrete over the rocks. Settlement may be a problem since the rocks have to be placed directly on mud. In this case, put a surcharge of heavy material on the rock footing to induce settling before leveling and capping. After the footing settles, remove the surcharge and level and cap the rock footing. The total load acting on the footing (including the weight of the footing itself) divided by the planned area of the footing must not exceed the bearing capacity of the local soil.
Figure 748. Rock Footing
Concrete Footings
7131. In shallow watercourses with firm beds, use a concrete footing. Place the forms and anchor them with weights so that they will not float away. Then use a tremie to place concrete in the forms. Place the footing only when the temperature of the water will not fall below 45°F during the curing process.
Steel or Timber Footings on Rock or Concrete
7132. Embed steel or timber posts directly in the concrete or on top of the rock or concrete footings.
SteelCulvert Pipe
7133. Steelculvert pipes can serve as expedient bridge piers. Fill the pipes with rocks, cap them, and place them on edge. At least two rows of pipe are needed, with both lateral and longitudinal bracing to provide stability. This type of pier requires a cap and corbel system. Weld the steel pipe together as rigidly as possible to provide one continuous pipe. Piers of this type should not exceed 12 feet in height and can be placed on a rock or concrete footing. Do not use concrete pipes since sections are short and tend to be unstable when stacked.
FOOTINGS AND GRILLS
7134. The AFCS contains descriptions on the use of footings and grills as special foundations for steel or timber towers. See FM 5277 for more information on grills.
CULVERTS
7135. Reinforced concrete and corrugatedmetal culverts are often used in shallow or intermittent watercourses in place of bridges. See FM 5430001 for information on culvert designs in connection with drainage.
NEWSLETTER

Join the GlobalSecurity.org mailing list 

