1、Section 7 SUBSTRUCTURES Part A GENERAL REQUIREMENTS AND MATERIALS 7.1 GENERAL 7.1.1 Definition A substructure is any structural, load-supporting com- ponent generally referred to by the terms abutment, pier, retaining wall, foundation or other similar terminology. 7.1.2 Loads Where appropriate, pier
2、s and abutments shall be de- signed to withstand dead load, erection loads, live loads on the roadway, wind loads on the superstructure, forces due to stream currents, floating ice and drift, temperature and shrinkage effects, lateral earth and water pressures, scour and collision and earthquake loa
3、dings. 7.1.3 Settlement The anticipated settlement of piers and abutments should be estimated by appropriate analysis, and the ef- fects of differential settlement shall be accounted for in the design of the superstructure. 7.1.4 Foundation and Retaining Wail Design Refer to Section 4 for the design
4、 of spread footing, driven pile and drilled shaft foundations and Section 5 for the design of retaining wails. 7.2 NOTATIONS The following notations shall apply for the design of pier and abutment substructure units: B = Width of foundation (ft) e = Eccentricity of load from foundation centroid in H
5、 = Height of abutment (ft) K = Coefficient of earth pressure (dim); (See Article the indicated direction (ft) 7.5.4.) Ka = Active earth pressure coefficient (dim); (See Arti- VI = Vertical soil stress (ksf); (See Article 7.5.4.) V2 = Vertical stress due to footing load (ksf); (See Arti- uH = Supplem
6、entary earth pressure (ksf); (See Article cle 7.7.4.) cle 7.5.4.) 7.5.4.) The notations for dimension units include the follow- ing: dim=dimensionless; ft = foot; and ksf = kip/ft2. The dimensional units provided with each notation are pre- sented for illustration only to demonstrate a dimensionally
7、 correct combination of units for the design procedures presented herein. If other units are used, the dimensional correctness of the equations should be confirmed. Part B SERVICE LOAD DESIGN METHOD ALLOWABLE STRESS DESIGN 7.3 PIERS 7.3.1 Pier Types 7.3.1.1 Solid Wall Piers Solid wall piers are desi
8、gned as columns for forces and moments acting about the weak axis and as piers for those acting about the strong axis. They may be pinned, fixed or free at the top, and are conventionally fixed at the base. Short, stubby types are often pinned at the base to eimi- nate the high moments which would d
9、evelop due to fixity. Earlier, more massive designs, were considered gravity types. 7.3.1.2 Double Wall Piers More recent designs consist of double walls, spaced in the direction of traffic, to provide support at the continu- 183 184 HIGHWAY BRIDGES 7.3.1.2 ous soffit of concrete box superstructure
10、sections. These walls are integral with the superstructure and must also be designed for the superstructure moments which develop from live loads and erection conditions. 7.3.1.3 Bent Piers Bent type piers consist of two or more transversely spaced columns of various solid cross sections, and these
11、types are designed for frame action relative to forces act- ing about the strong axis of the pier. They are usually fixed at the base of the pier and are either integral with the su- perstructure or with a pier cap at the top. The columns may be supported on a spread- or pile-supported footing, or a
12、 solid wall shaft, or they may be extensions of the piles or shaft above the ground line. 7.3.1.4 Single-Column Piers Single-column piers, often referred to as “T” or “Ham- merhead” piers, are usually supported at the base by a spread- or pile-supported footing, and may be either inte- gral with, or
13、 provide independent support for, the super- structure. Their cross section can be of various shapes and the column can be prismatic or flared to form the pier cap or to blend with the sectional configuration of the super- structure cross section. This type pier can avoid the com- plexities of skewe
14、d supports if integrally framed into the superstructure and their appearance reduces the massive- ness often associated with superstructures. 7.3.2 Pier Protection 7.3.2.1 Collision Where the possibility of collision exists from highway or river traffic, an appropriate risk analysis should be made t
15、o determine the degree of impact resistance to be provided andor the appropriate protection system. 7.3.2.2 Collision Walls Collision walls extending 6 feet above top of rail are required between columns for railroad overpasses, and similar walls extending 2.35 feet above ground should be considered
16、 for grade separation structures unless other protection is provided. 7.3.2.3 Scour The scour potential must be determined and the de- sign must be developed to minimize failure from this condition. 7.3.2.4 Facing Where appropriate, the pier nose should be designed to effectively break up or deflect
17、 floating ice or drift. In these situations, pier life can be extended by facing the nosing with steel plates or angles, and by facing the pier with granite. 7.4 TUBULAR PIERS 7.4.1 Materials Tubular piers of hollow core section may be of steel, reinforced concrete or prestressed concrete, of such c
18、ross section to support the forces and moments acting on the elements. 7.4.2 Configuration The configuration can be as described in Article 7.3.1 and, because of their vulnerability to lateral loadings, shall be of sufficient wall thickness to sustain the forces and moments for all loading situation
19、s as are appropriate. Prismatic configurations may be sectionally precast or prestressed as erected. 7.5 ABUTMENTS 7.5.1 Abutment Qpes 7.5.1.1 Stub Abutment Stub abutments are located at or near the top of ap- proach fills, with a backwall depth sufficient to accom- modate the structure depth and be
20、arings which sit on the bearing seat. 7.5.1.2 Partial-Depth Abutment Partial-depth abutments are located approximately at mid-depth of the front slope of the approach embankment. The higher backwall and wingwalls may retain fill mate- rial, or the embankment slope may continue behind the backwall. I
21、n the latter case, a structural approach slab or end span design must bridge the space over the fill slope, and curtain walls are provided to close off the open area. Inspection access should be provided for this situation. 7.5.1.3 Full-Depth Abutment Full-depth abutments are located at the approxim
22、ate front toe of the approach embankment, restricting the opening under the structure. 7.5.1.4 DIVISION I-DESIGN 185 7.5.1.4 Integral Abutment Integral abutments are rigidly attached to the super- structure and are supported on a spread or deep foundations capable of permitting necessary horizontal
23、movements. 7.5.2 Loading Abutments shall be designed to withstand earth pres- sure as specified in Articles 5.5 and 5.6, the weight of the abutment and bridge superstructure, live load on the su- perstructure or approach fill, wind forces and longitudinal forces when the bearings are fixed, and long
24、itudinal forces due to friction or shear resistance of bearings. The design shall be investigated for any combination of these forces which may produce the most severe condition of loading. Integral abutments must be designed for forces generated by thermal movements of the superstructure. 7.5.2.1 S
25、tability Abutments shall be designed for the loading combina- tion specified in Article 3.22. Abutments on spread footings shall be designed to resist overturning (FS 2 2.0) and sliding (FS 2 1.5). Dead and live loads are assumed uniformly distrib- uted over the length of the abutment between ex- pa
26、nsion joints. 0 Allowable foundation pressures and pile capacities shall be determined in accordance with Articles 4.4 and 4.3. The earth pressures exerted by fill in front of the abutment shall be neglected. 0 Earthquake loads shall be considered in accordance with Article 3.21. 0 The earth pressur
27、es exerted by the fill material shall be calculated in accordance with Articles 5.5.2 and 5.6.2. 0 The cross section of stone masonry or plain concrete abutments shall be proportioned to avoid the intro- duction of tensile stress in the material. 7.5.2.2 Reinforcement for Temperature Except in gravi
28、ty abutments, not less than !4 square inch of horizontal reinforcement per foot of height shall be pro- vided near exposed surfaces not otherwise reinforced to re- sist the formation of temperature and shrinkage cracks. 7.5.2.3 Drainage and Backfilling The filling material behind abutments shall be
29、free draining, nonexpansive soil, and shall be drained by weep holes with french drains placed at suitable intervals and elevations. Silts and clays shall not be used for backfill. 7.5.3 Integral Abutments Integral abutments shall be designed to resist the forces generated by thermal movements of th
30、e superstructure against the pressure of the fill behind the abutment. Integral abutments should not be constructed on spread footings founded or keyed into rock. Movement calculations shall consider temperature, creep, .and long-term prestress short- ening in determining potential movements of abut
31、ments. Maximum span lengths, design considerations, details should comply with recommendations outlined in FHWA Technical Advisory T 5 140.13 (1980) except where sub- stantial local experience indicates otherwise. To avoid water intrusion behind the abutment, the ap- proach slab should be connected
32、directly to the abutment (not to wingwalls), and appropriate provisions should be made to provide for drainage of any entrapped water. 7.5.4 Abutments on Mechanically Stabilized Earth Walls Design of bridge abutment footings and connecting back wall, shall be based on bridge loading developed by ser
33、vice load methods and earth pressures on the back wall. Abutment footings shall be proportioned to meet the over- turning and sliding criteria specified in Article 5.5.5 and for maximum uniform bearing pressures using an effec- tive width of foundations (B - 2e). The maximum allow- able bearing pres
34、sure shall be 4.0 ksf. The mechanically stabilized earth wall below the abut- ment footing shall be designed for the additional loads im- posed by the footing pressure and supplemental earth pres- sures resulting from horizontal loads applied at the bridge seat and from the back wall. The footing lo
35、ad is assumed to be uniformly distributed over the effective width of foun- dation (B - 2e) at the base of the footing and is dispersed with depth, using a slope of 2 vertical to 1 horizontal. The supplemental loads are applied as horizontal shears along the bottom of the footing, uniformly diminish
36、ing in magni- tude with depth to a point on the face of the wall equal to a distance of (B - 2e) multiplied by Tan (45 + +/2) as described in Article 5.8.12.1. Horizontal and vertical stresses in abutment reinforced zones are calculated by superposition as shown in Articles 5.8.4.1 and 5.8.12.1. The
37、 effective length used for calculations of internal sta- bility under the abutment footing shall always be the length 186 HIGHWAY BRIDGES 7.5.4 beyond the end of the footing or beyond a distance of 0.3(Hi) from the facing, whichever is less, where Hl is the height of wall plus surcharge. The minimum
38、 distance from the center line of the bearing on the abutment to the outer edge of the facing shall be 3.5 feet. The minimum distance between the back face of the panel and the footing shall be 6 inches. The abutment footing should be placed on a bed of com- pacted coarse aggregate 3 feet thick when
39、 significant frost penetration is anticipated. Abutments shall not be constructed on mechanically stabilized embankments if anticipated differential settle- ments between abutments or between piers and abutments are greater than one-half the limiting differential settle- ments as shown in Figure 7.5
40、.4A. This figure should be used for general guidance only. Detailed analyses will still be required to address differential settlement problems. For structures supporting bridge abutments, the maxi- mum horizontal force shall be used for connection design throughout the height of the structure. The
41、density, length, and cross section of the soil rein- forcements designed for support of the abutment wall shall be carried on the wing walls for a minimum hori- “1 a l2 i zontal distance equal to 50% of the height of the abutment wall. The horizontal forces transmitted to the piles shall be resisted
42、 by the lateral capacity of the pile itself, or the soil reinforcements in the upper part of the wall designed to cany the additional loads transmitted from the piles to the reinforced soil backfill. Where interference between the piles and the soil reinforcement occurs, the reinforce- ments must be
43、 designed around the piles, and the piles treated as backfill obstructions (see Article 5.8.12.4). A clear distance of no less than 0.5 meters (1.5 feet) from the back of the wall facing to the edge of the nearest pile or pile casing shall be provided. Piles should be driven prior to wall constructi
44、on and cased through fill if necessary. Lateral loads transmitted from the piles to the rein- forced backfill may be determined using a P-Y lateral load anal y sis technique. 7.5.5 Abutments on Modular Systems Abutments seats constructed on modular units shall be designed by considering, in addition
45、 to earth pressures, the supplemental horizontal pressures from the abutment seat beam and earth pressures on the back wall. The top module shall be proportioned to be stable, with the required factor J I I I I I I I i O 2s 50 7s loo i25 150 m 200 SmN LWQM (Pn FIGURE 7.5.4A Limiting Values of Differ
46、ential Settlement Based on Field Surveys of Simple and Continuous Span Structures of Various Span Lengths, Moulton, et al. (1985) 7.5.4 DIVISION I-DESIGN - 187 of safety, under the combined actions of normal and sup- plementary earth pressures. Minimum top module width shall be 6 feet. The center li
47、ne of bearing shall be located a minimum of 2 feet from the outside face of the top precast module. The abutment beam seat shall be supported and cast integrally to the top module. The front face thickness of the top module shall be designed for bending forces de- veloped by supplemental earth press
48、ures. Abutment beam- seat loadings shall be carried to foundation level and shall be considered in the design of footings. Differential settle- ment restrictions in Article 7.5.4. shall apply. 7.5.6 Wingwalls 7.5.6.1 Length Wingwalls shall be of sufficient length to retain the roadway embankment to
49、the required extent and to fur- nish protection against erosion. The wingwall lengths shall be computed using the required roadway slopes. 7.5.6.2 Reinforcement Reinforcing bars or suitable rolled sections shall be spaced across the junction between wingwalls and abut- ments to tie them together. Such bars shall extend into the masonry on each side of the joint far enough to develop the strength of the bar as specified for bar reinforcement, and shall vary in length so as to avoid planes of weakness in the concrete at their ends. If bars are not used, an ex- pansion joint shall be provided
