Behavior and Design of Link Slabs for Jointless Bridge Decks

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Jointless Bridge Decks
   ehavior and Design of Link Slabs for Jointless ridge Decks Alp Caner Ph.D. Structural Engineer Parsons Brinckerhoff Quad e and Doug l as N ew York  New York Paul Zia Ph.D. P E Distinguished University Professor of Civil Engineering Eme ritu s) North Ca rolina State University Raleigh North Carolina 68 Maintenance of bridge deck joints is a costly problem Debris accumulation in the joints can restrain deck expansion caus in g und es ir able forces in the deck and damage to the structure. Water leaking through the joints is a major ca use for the deterioration of bridge girder bearings and supporting structures Therefore  elimination of deck joints at the supports of multispan bridges wi ll reduce the cost of const ru ction and maintenance. This paper presents the results of a test program to in vestigate the behavior of link slabs connecting two adjacent simple-span girder s and proposes a simple method for designing the link sl ab. To illustrate the proposed design method three design examples are included. M any highway bridges are designed as multiple simple span composite structures that utilize either steel or prestressed con crete girders and a cast in place concrete deck spanning from one pier (or bent) to another. At each end of the simple-span deck, a joint is provided for deck movement due to temperature, shrinkage, and creep effects. Bridge deck joints are a persistent and costly maintenance problem. Water leaking through the joints is a major cause for the deterioration of bridge girder bearings and supporting structures. Debris accumulation in the joints restrains deck expansion and causes damage to the bridge. Joints and bearings are expensive to install and maintain. Therefore, the cost of construction and maintenance for a bridge can be greatly reduced if the number of deck joints in multi-span bridges can be minimized. It should be noted that when the deck joints are removed and replaced by a jointless deck, fine cracks can be expected to develop in the jointless deck and in many cases water may still leak through the fine cracks . However, the situation s preferable to that of jointed decks. During the last several years, bridge engineers have designed different types of jointless bridge decks. Integral bridges are jointless and designed as single- or multiple-span continuous bridges with capped pile stub type abutments. Burke - 3 has discussed the attributes and limitations of integral bridges. The use of prestressed con- crete pile s in integr al abutment PCI JOURN L  bridges has been discussed by Kamel et al. Jointless bridge decks with continu ous girders are commonly used in many states. Wasserman 5 and Loveall 6 have described their extensive experience with such bridges in Tennessee. Their experience included both bridge rehabilitation and new construction. A comprehensive study of jointless decks with continuous girders was conducted by Oesterle et aU In their study, the precast, prestressed girders were made continuous to resist live load by the use of continuity steel and end diaphragms at the bridge piers. Their design recommendations have been used by many state highway departments; however, the required deck reinforcement tends to be excessive. In actual practice, most bridge engi neers have used a smaller amount of reinforcing steel based on their own judgment and experience. In addition, the end diaphragm is a difficult con struction detail to execute in the field. Bridge deck joints can also be elimi nated by making the deck continuous while keeping the girders as simplespans. The section of the deck connecting the two adjacent simple-span girders is called the link slab. 8 Jointless bridge decks supported by simple-span girders have been used both in the United States and abroad. 9ã   Two such examples are shown in Fig. 1. It is noted that end diaphragms sep arated by two layers of smooth roofing paper are used by the Florida DOT but no diaphragms are used by the Texas DOT. By eliminating the end di- aphragms, this construction detail is greatly simplified. In 1981, Zuk 12 studied the concept of jointless bridge decks built on multiple simply-supported girders. He analyzed the effects of expansion and contraction of the jointless deck and considered the interactive forces between the girders and the deck. Although the concept seemed promising, it was not used in any actual applications. In the late 1980s, Gastal and Zia 13 described the results of a finite element method of analysis for jointless bridge decks supported by simple- span girders. The analysis accounted for the nonlinear material properties, cracking of concrete, creep, shrinkage, May June 1 998 Prestressed Concrete Panel Interior Bent Center Line a) Texas · ----- r_ _____ r 8 4 Deck Slab with li Bus 6 o.   . top t bottom End or 1 Presu-essed I Beam = F I. I I · I r I enter line : : 11C i aphraJm I I I epuate Di ap Beams with t 0 3 lbslsq. hragms and wo layers of d. smooth roofin1 paper b) Florida ig . 1. Typical jointless bridge deck. Note: 1 in.= 25.4 mm; 1 lb per sq yd = 0.543 kg/m 2. temperature effects and various loading conditions. For Jack of experimental data, the computer solutions were validated by comparisons with the re sults of several different tests of simplysupported beams (without a jointless bridge deck) that were reported in the literature. El- SaftyB modified Gastal s finite element program 14 by incorporating an optional analysis for partial debonding of the deck from the supporting beams. He also introduced the assumption of constant strain through the depth of the link slab, whereas Gasta l assumed a linearly varying strain through the depth of the link slab. Richardson 15 also studied there- moval of expansion joints from bridges using continuous and partially debonded decks and developed a sim plified design procedure. Computer programs were developed to predict the crack width and spacing in the deck and to calculate the vertical de flection of the structure. These analytical studies notwith- standing, no experimental validation of the concepts of analysis and design for jointless bridge decks supported by simple-span girders can be found in the literature. This paper presents the results of a test program to investigate the behavior of the jointless bridge deck, and proposes a simple design method for the link slab. 16ã17 Three numerical design examples are included to demonstrate the proposed design method. TEST PROGRAM The test program included two large test specimens of composite construc tion one being a continuous rein- forced concrete deck slab cast on two simple-span steel beams, and the other being a similar slab cast on two simple span precast reinforced concrete 69  beam s. Fig. 2 shows the details of the two test specimens. The material and geometrical properties of both specimens are given in Table 1 Fig . 3 shows a general view of the test setup for the concrete bridge under the ultimate load test. Detailed descriptions of instrumentation can be found in Refs. 16 and 17 . It should be noted that even though this test program used steel and pre cast reinforced concrete beams, the concept being evaluated should be directly applicable to bridge deck rehabilitation with existing precast, prestressed concrete girders. The concept is also applicable to new bridge construction using precast, prestressed concrete girders if the effects of creep and shrinkage are adequately ac counted for. Steel Bridge The first specimen represented a steel bridge with a jointless composite concrete deck. Two simply-supported W 12x26 steel beams, each 20 .5 ft (6.25 m) long, were aligned with a 2 in. (50.8 mm) gap between the adjacent ends of the two beams. These two steel beams were then joined with a continuous concrete deck reinforced with three #6 epoxy coated longitudinal bars, thus creating a 41 ft 2 in. (12.55 m) long two-span structure with a jointless deck supported by two simple-span beams (see Fig. 2). A row of 0. 75 in. (19 mm) shear connectors spaced at 17 in. 432 mm) were welded to the top of the steel beams to develop composite action with the reinforced concrete deck, except in the deck debonded zone as described below. The concrete deck was 2 ft (610 mm) wide, 4 in. (102 mm) thick, and 41 ft 2 in. (12.55 m) long. At the two adjacent ends of the steel beams, the concrete deck was debonded from each steel beam for a distance of 12 in. (305 mm), which was 5 percent of the span of each beam. The 5 percent debonding length was based on the results of theoretical studies that showed that the load-deflection behavior of the struc ture would not be affected by a debonding length of up to 5 percent of the span length . The purpose of debonding is to reduce the stiffness of 70 1 p 26 Debonding 1 p 4 + + ~ plaJr/.{///.(71r W/~9 5'-8 20'-0 40'-8 (a) Elevation of Test Specimen [i 2 (b) Cross-section of Steel Bridge . c 0 ãããã0 4 ã ã '· . . ã 0 0 (3) #6 Bars Wl2x26 (4) #8 (c) Cross-section of Concrete Bridge Fig. 2 Details of test specimens. Note 1 in.= 25.4 mm . Table 1. Material and geometrical properties of steel and concrete bridges. Properties Steel bridge Concrete bridge Compressive strength of concrete deck 4200 psi 5670 psi Compressive strength of girder - 4580 psi Girder yield strength 52,000 psi - Girder modulus of elasticity 30,500,000 psi - Girder reinforcement - (4) #8 Girder reinforcement yield strength - 62,000 psi Girder reinforcement modulus of elasticity - 29,550,000 psi Girder cross-sectional area (gross) 7.65 sq in. 96 sq in. Girder moment of inertia (gross) 204 in. ã 1152 in 4 Deck width 24 in . 24 in. Deck th ickness 4 in. 4 in. Link slab reinforcement (3) #6 (3) #6 Link slab reinforcement yield strength 63,600 psi 72,400 psi Link sl ab reinforcement modulus of elasticity 28,500,000 psi 30,300,000 psi Note: I in . = 25.4 mm; I sq in.= 645.2 mm 2; I psi = 0.006895 MPa. PCl JOURNAL  the link slab so that the stress devel oped in the link slab can be minimized. The continuity of the deck rein forcement in the debonding zone was developed by a lap splice 17 in. ( 432 mm) long to simulate the situation where a damaged joint of a bridge deck would be removed and replaced by a jointless deck. The computed mo ment capacity of the composite section was 247 ft-kips (335 kN-m), based on the actual material properties given in Table 1. C   crete ridge The second specimen represented a concrete bridge with a jointless deck. First, two 20.5 ft (6.25 m long reinforced concrete beams were precast in the laboratory. The beams were 12 in. (305 mm) high and 8 in. (203 mm) wide. Each beam was reinforced with four 8 bars and thirty-one 3 stirrups. A deck of the same size as that of the first specimen was cast on the precast beams when the concrete of the beams had gained enough strength. As before, the jointless deck was also debonded from each beam for a distance of 12 in. (305 mm) at the cen ter of the specimen. Debonding was achieved by omitting the stirrups and by placing two layers of plastic sheet between the beam and the deck. The longitudinal reinforcement in the deck consisted of three 4 bars which, in turn, were lap spliced with three 6 epoxy coated bars at the center of the specimen. The computed moment ca pacity of the composite section was 185ft-kips (251 kN-m), based on the actual material propertie s given m Table l. TEST PRO EDURE The test procedure used for both specimens was similar. The steel bridge was tested with four different support configurations: HRRH, RHRH, RRRR and RHHR, where H stands for hinge and R stands for roller. In each configuration, the first and fourth letters rep resent the two exterior supports. The second and third letters represent the two interior supports. A hinge was provided by using a 1.5 in . (38 mm) diameter steel pin between two 1.5 in. (38 mm) thick bearing plates, each with May-June 1998 Fig. 3 Concre te bridge during ultimate load test. a V -groove. A roller was provided by using the same size pin and plates but without V -grooves. The concrete bridge was tested with the same support conditions, except for the RRRR configuration, which is an unlikely support condition in the field. The goal in testing different sup port conditions was to observe if there were any differences in the behavior of the jointless deck (i.e., link slab) under different support conditions, as previously predicted by El-Safty's computer model. 8 In all cases, tests were carried out to no more than 40 percent of the estimated ultimate load capacity of each test specimen to observe the behavior in the elastic range. The load was applied on each span in increments. For each load increment, data for the steel and concrete strains, loads, crack growth and deflections were collected. The final ultimate load test was per formed with the support configuration of RHHR and a complete set of data on strains, loads, crack widths, and de flections was collected. TEST RESULTS A discussion of the test results will be given first for the steel bridge and then for the concrete bridge. Steel ridge The steel bridge was tested with four different support configurations , namely, HRRH, RHRH, RRRR, and RHHR. Initially, the load was applied up to 17.4 kips (77 .4 kN) on each span to observe the behavior in the elastic range. Within this elastic range, the load-deflection behavior was comparable for all the four test cases, as shown by the measured load-deflection relationships in Table 2. In addition, the load-deflection behavior was almost identical for both spans of the test specimen as required by symmetry. It is noted that the measured slopes of the load-deflection curves are comparable to the theoretical value. The theoretical value is obtained by using the average of the moment of inertia of a fully composite section and the moment of inertia of the steel beam Table 2. Slope of load-defl ec tion curve kips/in.). Support configuration Steel bridge I Experimental Theoretical T HRRH RHRH RRRR RHHR f Note: I kip/m. = 0.175 kN / mm . 55.8 58.7 49.6 54.8 t 526 j 52.6 52.6 52.6 1 Co ncrete bridge Experimental J Theoretical 57.6 1 52.3 55 .0 l 52.3 l 4.8 52.3 - 71
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