SERVICIOS FINANCIEROS

11.6.4.1 General-All bridge piers and compression members should be designed by strength design procedures with appropriate consideration given to serviceability at a working stress level. The primary serviceability criteria

Fig. 11.6.3.2-Special moment connections of super-structure to pier

J

(c)

1

Ktop . o (Expansion Typo

Connection)

Section A - A (f) Joint Detail

Fig. 11.6.3.3(a) to (f)-Monolithic and hinged connections

should be a crack control evaluation. The recommendations given in Section 8.4 should be followed.

The capacity analysis should be based on a concrete ulti-mate compressive strain of 0.003 and should consider stress and strain compatibility for both direct load and moment.

The strength for pure compression, balanced conditions, and pier bending should be as given in Chapter 7.

11.6.4.2 Slenderness-Slenderness considerations in-volve control of the slenderness ratio kl/r and computation of secondary moments at the ultimate state.

In most cases, kl/r should be limited to a maximum of 100.

The effective length value k is computed as discussed in Sec-tion 11.6.4.3, and the radius of gyraSec-tion is computed as 0.25 times the diameter of a round column and 0.30 times the depth of a rectangular column. If kl/4 exceeds 100, a precise secondary analysis should be made that includes the influ-ence of axial loads and variable moment of inertia on mem-ber stiffness and fixed end moments. This analysis should also include the effects of creep due to long term loads, along with the effect of deflections on moments and forces. For columns with a kl/r value less than 22, secondary effects might be ignored.

Secondary effects should be computed using a secondary analysis as previously suggested, or by using an approximate

method for developing secondary moment effects. Several approximate methods are appropriate. The moment magnifi-er given in ACI 318 and explained in ACI 318R is the most straightforward approach and usually is the most practical procedure for making this analysis. As an alternative, it would be acceptable to use the P-Delta method, as described in References 11-35, 11-36, and 11-37, which may result in reduced amounts of reinforcement in the pier over the previ-ous simplified analysis.

11.6.4.3 Effective length factors--The effective length factor k is used in computing both the slenderness ratio kl/r and the critical buckling load The factor is given to con-vert any column or pier into an equivalent pin-ended column bent in single curvature. The variation in effective length factor can be considerable, depending upon end conditions and the braced or unbraced condition.

The effective length factor can be selected from the charts shown in ACI 318R. The chart for unbraced frames should be used. It may also be determined by the following equa-tions for braced

k = 0.7 + 0.05 (G, + 1.0 (11-l)

k = 0.85 + 0.05 1.0 (11-2) Use the smaller of the two values.

Use the following equations for unbraced

For 2k = (11-3)

For _ = (11-4)

where

Columns

= Members resisting column bending at A end of column

In computing effective length factors for monolithic con-nections, it is important to properly evaluate the degree of fixity in the foundation. The following values can be used:

1.5 Footing anchored on rock 3.0 Footing not anchored on rock 5.0 Footing on soil

1.0 Footing on multiple rows of piles

In determining these effective length factors, the designer should use some judgment as to whether there is a braced or an unbraced condition. It is reasonable to assume that most bridge analysis should use the unbraced condition.

11.6.4.4 Biaxial bending-All pier section analysis should be done using strength design procedures. For biaxial bending, either an actual stress and strain compatibility

anal-ysis should be made or an approximate analanal-ysis that will simulate such.

As an approximate analysis when 0.1 the recip-rocal load equation

= + + (11-5)

should be used. This equation generally gives conservative results and is practical to use.

In reality, most bridge piers will be subjected to ultimate loads where < 0.1 The elliptic equation

+ 1 .O (11-6)

gives a reasonably good simulation of the stress and strain compatibility analysis.

11.6.4.5 Irregular shapes-The capacity analysis for ir-regular and unsymmetrical shapes is somewhat similar to the capacity analysis for biaxial bending. However, simple ap-proximate solutions are not readily available. A stress and strain compatibility analysis is suitable for determining the capacity of these unusual shapes. This analysis is most suit-ably carried out by using a computer.

In the analysis of many irregular shapes, consideration should be given to using an increased concrete strain of 0.004 at the ultimate load provided, however, the strength of the section where the strain is greater than 0.003 is not in-cluded in the computation of the required strength of the member.

In shapes where thin walls exist, it is important that these thin portions of the section be analyzed for shear capacity and requirements. This is particularly important when con-sidering large dynamic loads due to seismic action.

In areas where a detailed seismic analysis is required, the columns theoretically should be designed for the maximum compressive load due to gravity and seismic loading acting simultaneously on the column. However, there is little guid-ance available for computing the vertical seismic load. In general, it would not significantly change the results if the vertical seismic load was ignored.

11.6.4.6 Tie requirements-It has been generally ac-cepted that up to a point near the crushing strain of concrete, lateral reinforcement does little to enhance the structural per-formance of a column. Beyond this point, however, the tied column is liable to exhibit a brittle failure due to the fact that ties are normally widely spaced, and after any concrete dete-rioration, there is inadequate support for the main vertical re-inforcing against buckling and for the concrete core against crushing. Spiral columns usually have spirals at a much clos-er spacing, and these pclos-erform bettclos-er when the concrete starts to deteriorate. This normal close spacing of spirals is ade-quate to restrain the vertical reinforcing against buckling.

Spiral columns also have the added benefit that the spiral reinforcing creates a hoop tension confining concrete, and thereby increasing the strength and ductility of the concrete core. Closely spaced ties, whose ends are adequately an-chored inside the concrete core, will increase the strength

and ductility of tied columns, but this increase will not be as significant as the same volume of closely spaced spirals. It is not feasible to design a pier to withstand all damage from se-vere seismic action. Therefore, current design practice is to provide for ductility at potential plastic hinge areas, such as in the piers, through increased hoops and ties or spirals.

It is recommended that the normal requirements for hoops and ties be #4 hoops at 1 ft (300 mm) vertical spacing with ties, not to exceed 2 ft (600 mm) transversely. As previously suggested, these spacings should be reduced in potential plastic hinge regions. Potential plastic hinge zones occur at the bottom of piers for simply supported girder-type struc-tures and at the top and the bottom of piers for framed-type structures. Ties should have hooks anchored in the compres-sive zone (confined core).

The New Zealand Concrete Design Code gives guidance for determining lateral reinforcing in the potential plastic hinge regions. However, the recommendations of

are rapidly gaining acceptance in the U.S. and should be used where feasible. This code indicates that the length of the po-tential plastic hinge region should not be less than a column diameter for a circular column, or the longer column cross section dimension for rectangular columns, or one-sixth of the clear height of the column but not less than 18 in. (450 mm) and, if applicable, this length should be increased to cover the entire distance where the moment exceeds 0.8 times the end moment. At no time should it be necessary for the potential hinge length to extend over more than one-half of the pier height. In situations where the maximum design load on the pier exceeds the anticipated length taken from the previous criteria should be increased by 50 percent.

In potential plastic hinge regions, it is recommended that the maximum center-to-center spacing of transverse rein-forcing not be less than the larger of one-fourth of the small-er column dimension (column diametsmall-er for circular columns), or 8 in. (200 mm).

In circular columns, should not be less than the greater of

or

= 0.45

[ - 1] &‘I

= (11-8)

In rectangular columns, the total area of hoops and supple-mentary cross ties in each of the principal directions of the cross section within the spacing should not be less than the greater of

or

A,, =

- 1]

= 0. (11-10)

Section T h r o u g h P i e r (a)

Placing 2nd Pier

C o m p l e t d Pier with Cast-in-Place Cap

(e)

Fig. 11.6.5(a) to (e)-Post-tensioned pier construction

In rectangular shafts, it is recommended that the center-to-center spacing of tied bars not exceed the larger of one-third of the column cross section dimension in that direction or 8 in. (200 mm).

It is apparent that many piers that have wide cross sections may have sufficient strength in the transverse direction of the bridge to sustain seismic loads in the same direction without yielding. This elastic response to a severe earthquake would require a capacity to resist a lateral seismic load which is 4 to 6 times the usual code lateral seismic load. If such large strength is available (both in bending and in shear), then it would be only necessary to provide special detailing of rein-forcement for horizontal loading in the longitudinal direction of the bridge.

11.6.5 Post-tensioned piers-For very tall or highly load-ed piers, a nonprestressload-ed reinforcload-ed pier may not be practi-cal. In such cases, post-tensioned piers may be used, and these are usually segmental construction.

Precast segmental piers are also used in special cases where particular construction requirements are specified. Ei-ther the environment is fragile enough that the contractor will have minimum access dictated by the locale or the con-struction schedule, or the concon-struction season is so short that precasting of the piers is necessary to obtain a strict construc-tion schedule. A prime example of this is mountainous re-gions, where the environmental constraints are severe and the summer season is short. In such a case, the pier segments can be precast during the winter and erected the following spring. By precasting, intricate and interesting shapes can be economical since the forms are reused. Each segment is match cast to the previous segment to insure exact seating when erected. Erection of a segmental pier, once the founda-tion is cast, is simple. Using strands, both the foundafounda-tion and

the shaft will require post-tensioning ducts placed prior to pouringconcrete [Fig. 11.6.5(a) and (b)]. Erection continues by placing segment on segment [Fig. 11.6.5(c) and (d)] until the desired height is obtained. Keys should be placed in each section to aid in alignment. With all of the segments in place, the pier is ready for stressing and then grouting, followed by casting the pier cap [Fig. 11.6.5(c) and (d)]. Joints between precast segments may be grouted with cement or epoxy-in-jected. Joint edges should be sealed against water

penetra-tion.

The weight of each individual segment should be kept at approximately 40 tons (350 kN) to obviate the need for spe-cial transport trucks or spespe-cial overload permits. This weight restriction will also help in erection, since most bridge cranes can handle up to 40 tons (350 kN) for piers less than 100-ft (30-m) high.

Generally, tensile stresses are not allowed under AASH-to group loading conditions for the post-tensioning design, tendons or bars may be used for the post-tensioning system.

If bars are used, an anchorage system in the footing is re-quired.

11.6.6 Detailing

11.6.6.1 Splices-Construction joints in piers, doweled connections to footings, and reinforcing length limitations often require splicing of the main vertical reinforcing within the pier height. Reinforcing splices can be made by lap splic-ing bars #11 and smaller, ussplic-ing mechanical splices, or by welding. It is recommended that splices of adjacent bars at the same vertical location be avoided, and vertical locations for splices be at least 3 ft (1 m) apart.

For lap splices where only alternate bars are spliced at the same location, a Class “B” splice length, as defined in Sec-tion 13.2, should be considered adequate. Proper clear spac-ing between bars should be maintained in lap splice areas, and mechanical splices should be detailed according to the manufacturer’s recommendations.

When welded splices are unavoidable, it is recommended the splices be prepared in accordance with AWS D 1.4. Care should be taken during welding operations to protect adja-cent bars from damage. Shielding devices are normally pro-vided for protection from splatter and errant contact with a welding rod. It is also recommended that when Grade 60 bars are specified as welded, the reinforcement should be in accordance with ASTM A 706.

11.6.6.2 Development requirements-When a pier shaft has a moment resisting connection with a footing or with a monolithic cap, full development of the vertical reinforcing beyond the interface should be provided. This development length may consist of a combination of the equivalent em-bedment length of a hook or a mechanical anchorage, plus additional embedment length of the vertical reinforcing.

This required additional embedment length will often dictate the required size of footing. Pedestals may also be used in lieu of increasing the footing depth.

If more vertical reinforcing is provided than actually re-quired by a capacity analysis, then the rere-quired development length and the resultant required footing depth may be re-duced to provide only the capacity required.

11.6.6.3 Dynamic earthquake requirements-For rou-tine design, it is generally not cost-effective to design a bridge structure to resist very large inertia loads resulting from elastic response to severe earthquake action. Instead, the design should be for smaller earthquake loads with the structure being detailed to provide ductility in the piers. Duc-tility becomes important when plastic hinge zones form, since it gives significant energy dissipation. The value known as the ductility factor, is used to measure ductile ac-tion and is defined as the ratio of the maximum displacement under the design earthquake to the theoretical yield displace-ment. An acceptable maximum design value of is 6.

As indicated in Section 11.6.4.6, adequate lateral pier re-inforcement is most helpful in producing ductility. A general design procedure suggested for ductile structures that in-cludes plastic hinge areas in piers should be:

a. Design plastic hinge sections to have a dependable flexural strength which is at least equal to the required flexural strength. The dependable strength should be times the ideal strength, based on the specified material strengths of the steel and concrete.

b. Design all sections, other than plastic hinge areas, for flexure and shear. This should be based on an analysis using plastic hinge flexural capacities, based on over-strength materials and including an allowance for strength increase due to steel strain hardening. Design the plastic hinge areas for shear.

11.7-Pier

11.7.1 Fender systems-In the case of bridge piers located