Integral abutments, where appropriate, should be considered for most bridge projects in order to eliminate joints and bearings thereby simplifying construction and reducing maintenance problems. Integral abutments are defined as those abutments that are rigidly attached to both the superstructure and thesupporting piles so that all thermal movements and girder end rotations are transferred from the superstructure through the abutment to the piles.
Design Limitations for the Use of Integral Abutments
Length: The total bridge length shall not exceed the following limits unless a more in-depth analysis is made. Concrete Structures – 650 feet Steel Structures – 350 feet For bridge skews of 25 degreesor less, no skew effects need to be considered. For skews greater than 25 degrees the forces tending to rotate the structure shall be accounted for in the design. Abutments shall be supported on a single row of steel H-piles, steel smooth hollow pipe pile or steel-encased concrete piles utilizing smooth steel tubes. Piles should be embedded into the abutment concrete at least 2 feet. The preferredorientation of the piles is for bending about the strong axis. On skewed bridges the pile flanges should remain parallel with the abutment.
Abutment Thickness: The minimum thickness of the abutment wall should be 3 feet in order to provide enough width to encase the piles and girders. Wing Walls: The wing walls should be cantilevered off of the abutments and shall beconstructed parallel with the girders in order to minimize the soil pressure against the wings.
Pile Design Procedures
The number of piles in the abutment shall be based on the vertical load requirements. The total dead load reaction of the structure at the abutment shall be distributed to all piles equally. The live load reactions at each pile shall be determined by assuming the piles in theabutment act as a group:
P Pex + N I
where: R = single pile live load reaction (KIPS) P = total live load reaction at the abutment, without impact (KIPS) N = number of piles in the abutment e = the eccentricity of the total live load relative to the center of the pile group (FT) x = the distance from a given pile to the center of the pile group (FT) I = the moment of inertia of the pilegroup (FT2) For vertical loads piles shall be designed in accordance with Article 10.7 of the AASHTO LRFD Bridge Design Specifications for Strength I. Lateral loads on the piles may need to be considered on bridges skewed more than 25º when required to resist the soil pressure forces tending to rotate the structure. Piles must be ductile enough to accommodate both thermal movements and dead load andlive load rotations of the superstructure. The ductility of the piles may be checked using the following equations: For Steel H-pile:
3CiMpL ⎡ Δ MpL ⎤ 2⎢ − ⎥ + θw ≤ 4EI ⎣ L 6EI ⎦
19 fy bf − 5.68 , 6 E 2 tf
0 < C i < 1 .0
Article 184.108.40.206 Page 2 of 5 4/2008
For hollow and concrete-filled pipe piles:
CiMpL ⎡ Δ MpL ⎤ 2⎢ − ⎥ + θw ≤ 2.08EI ⎣ L 6EI ⎦
where: Δ= L=
Ci = 3.5 −1.25
fy D , E t
0 < Ci < 1.0
one half the factored thermal movement range at the abutment (IN) twice the length from the bottom of the abutment to the first point of zero moment in the pile determined taking into account the effect of the soil on pile behavior and assuming a lateral deflection of Δ (IN) Mp = plastic moment of the H-pile about the axis of bending or the plastic moment ofthe steel pipe pile without considering the concrete filling (KIP-IN) E = modulus of elasticity of the steel (KSI) I= H-pile moment of inertia about axis of bending, the moment of inertia of the hollow pipe, or moment of inertia of the concrete-filled pipe considering both the concrete and steel (IN4) θw = maximum range of the factored angle of rotation of the superstructure at the abutment...