Over the years, the use of precast concrete has taken giant

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1 Precast Concrete Bridge Deck - Design, Transportation and Erection: A Designer s Overview Debabrata Mukherjee 1, Manju Balaji 2 1 Group Engineer, Atkins 2 Engineer, Atkins 76 Over the years, the use of precast concrete has taken giant strides in the construction industry. It has also paved way for the design engineers to think of where and how the use of precast concrete may be advantageous. The choice of material is not the only criteria during a desk study. Method of construction, design codes or guidelines to be followed, accessibility to construction site, ease of transporting material and finished items, all play a significant part in deciding the final choice of a bridge arrangement. In this regards, the choice of precast concrete as material, proves many a time beneficial to the designer, builder and client. Choosing the material, that best suits a particular site or arrangement streamlines the options to the designer. The choice of code/ guideline to follow, is particular for a country of work. However, the other aspects of site accessibility and ease of transportation of material is very specific to each site and may not be generalised. Availability of standard precast elements is still limited in many countries worldwide. So stake-holders should also take into account this aspect, when the final decision is made. Precast Concrete: Advantages For sustainable growth in any industry quality should be the prime focus. In construction industry, quality therefore needs to be controlled and monitored. This is much more easily achieved with precast concrete. Precast concrete does have many advantages compared to site-cast concrete: - The precast units are manufactured in a controlled casting environment and therefore it is easier to manage the quality of mix, placement, and curing. - At construction site, precast elements are generally installed immediately; there is no waiting period. - With the ability to control the concreting process, from consolidation to curing, a very durable concrete can be obtained. - Accelerated curing may be achieved, by heating the precast parts. This increases strength gain, reducing the time between casting a part and putting it into service. - The same precast moulds may be used multiple times and this maximises repetitions. A contractor can get plenty of value from a single mould and a set-up - A precast manufacturer can buy materials for multiple projects, thereby offer quantity products at lower costs. - Less labour is required compared to in-situ cast. - Much less exposure to health and safety issues as work is mostly in controlled environment and exposure to site is only during erection. It is intended through this paper, to cover the design criteria s of precast pre-tensioned girders, following widely practiced international standards for bridge construction. From our past working experience, two classic pre-tensioned precast girder examples are identified for this write-up. The first one is for a highways structure underpass deck, following British Standard design and the second a bridge structure over a railway line following Eurocode. Additionally, the corresponding AASHTO LRFD design criteria are also reviewed. Design For the first example, the concrete grade used was C55, i.e. a concrete with characteristic compressive cube strength of 55MPa. While designing the deck using U girders following British Standard (BS), it was required to limit the compressive stress to 0.4fcu (which was 22MPa for this case), as per clause of BS No tensile stress is allowed for Combination 1, however a tensile stress of (which was 3.3MPa for this case) was allowed for Combination 2 to Combination 5 under service loads as per clause of BS The span for the proposed bridge was 21.9m, and therefore was well suited, as it was within the range as specified in Q-Girder T3 Type construction of bridges with spans from 16m up to 24 m as per ASHGHAL (Public Works Authority, Qatar) guidance. For the second example, the concrete grade used was C50/60, i.e. a concrete with characteristic compressive cylinder strength of 50MPa. As this was following Eurocode, accordingly it was important to consider the construction sequences and build-up of the stresses for frequent combination and limit the allowable stresses.

2 Figure 1 Underpass Deck Girders (Q Type) span of 21.95m Limiting compressive stress as per Eurocode was 0.6fck(t) (which was 30MPa for this case) as per clause of BS EN It was also required to work out pre-camber to the girders for calculated deflections as per clause A1.4.3 of BS EN Instead of checking tensile stress limits as given in BS codes, it is recommended in Eurocode, that the stresses at decompression should be checked under the quasi-permanent combination as per clause of BS EN The decompression limit requires, that all parts of the bonded tendons or duct should lie at least 25 mm within concrete in compression. The span for the proposed bridge was 25.5m, and therefore was well suited, as it was within the range as specified in Y & YE Girder construction of bridges with spans from 14m up to 31 m [Ref. 1]. For comparison purpose, the limiting stresses according to AASHTO LRFD 2012(6th Edition) are: The compressive stress limits in pre-tensioned prestress concrete at service limit state after losses due to sum of effective prestress and permanent loads shall limit to 0.45f c (ksi) as given in table of the above design specification. The tensile stress limits, in pre-tensioned prestress concrete at service limit state after losses, for components with bonded prestressing tendons or reinforcement that are subject to not worse than moderate corrosion condition shall limit to 0.19 sqrt(f c) (ksi) as given in table of the above design specification. No tensile stress is allowed for pretensioned prestress Figure 2 Construction sequence: Bridge Deck with Y and YE Girders span of 25m 77

3 concrete at service limit state after losses for components with unbonded prestressing tendons as given in table of the above design specification. Transportation and Erection It is important to consider ways of transportation and erection of a precast concrete girder at site. The girders may need to be checked for this temporary stage as well. Often the choice of girder length gets limited due to the possible restraints faced during this. During the construction stages, precast girders are handled in a variety of ways. They must be transported from the precast yard to the construction site and may need to be lifted as many as four times between the casting bed to their final position within the bridge. For precast concrete beams self-weight is significant. way, and hence the stability of the beam will require several different assessments. a. Simply supported arrangement: Stacking of precast beams and before deck or diaphragm concreting The beam at this stage is simply supported at its ends, with no overhangs. The support is assumed to be at the soffit level and is assumed to allow rotation about the major and minor axes, but to prevent axial rotation and deflection. The idealised arrangement (Figure: 5) and at actual at site (Figure: 6) is given here. Figure 5 Simply supported stage for analysis Figure 3 Girders at support location before Diaphragm casting Figure 6 Simply supported stage before deck concreting b. Transport-supported arrangement: Stage during transportation 78 Figure 4 Girders in position before deck concreting Three important temporary stages that are identified during the process of handling a precast girder are lifting, transportation and placement over structure. The stability of the beam at each stage needs to be determined as the beam is supported differently during all these stages. So checking of the beams for each of the support conditions is very important. Support Conditions In each stage of a beam s use, it is supported in a different Beams are commonly transported by road, where they span between a tractor unit and a trailer. The supports at either end of the beam comprise a turntable (to allow cornering) and a roller (to allow for change in slope, such as when driving up a ramp). The indicated arrangement is given in Figure: 7 and actual site transport in Figure: 8. This represents the extreme support condition during transportation. The beam can be modelled as simply supported at one end (where axial rotation is restrained) and resting on a ball or pin at the other end, which prevents deflection but allows rotation. The indicative arrangement is given in Figure: 9. c. Hanging arrangement: Beam lifts for erection and transport When a beam is lifted at site, it typically hangs from two in-

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7 Figure 7 Articulation during Transportation stage Figure 11 Lifting stage at site The idealised arrangement in indicated in Figure 10 and actual site lift in Figure 11. Conclusion 82 Figure 8 Transportation of Girders to site Figure 9 Transportation stage for analysis clined cables on a single crane, while in the pre-casting factory it is invariably lifted using vertical cables supported from two cranes. In either case these cables are attached to lifting yokes at some height above the beam s centroid. The yokes are in turn attached to the beam, not necessarily at its ends. The yokes can be inclined at any angle, but are usually either fixed to be vertical or allowed to rotate in line with the cables. Figure 10 Lifting stage for analysis It is preferred as a designer, to be aware of the above aspects related to design, transport and erection of precast concrete girder. It is the responsibility of all the stake holder to assess these and then take an informed decision regarding the choice of material and construction technique. Overall a precast concrete construction, has an edge over the standard cast in place concrete construction, with certain definite kind of bridge forms. This leads to a clean, safe and healthy construction site. This is also a better practice compared to the conventional cast in place construction as this has reduced site hazards like dust, noise, wastes, pollutants, chemical presence etc. at site. Use of specialised transport vehicles and lifting cranes though may cost a bit more, however scaffolding less construction and clean site may compensate for a better whole life cycle cost to the structure. As the industry is moving towards more mechanised construction, use of more and more precast concrete elements in bridge construction is inevitable. So, it is more and more important to be aware of the different aspects of these and take informed decision and create bespoke solutions to problems faced by the engineering community. Reference - Tarmac - Prestessed beams Tarmac Precast Concrete Technical specification - For Transportation and erection support condition: Lateral stability of long precast concrete beams -T. J. Stratford, BA, BEng, and C. J. Burgoyne, BA, MSc, CEng, MICE - AASHTO LRFD BRIDGE Design Specifications, 6th Edition (US) - BS 5400 Part4: Code of practice for design of concrete bridges - BS EN : Concrete bridges - Design and detailing rules - BS EN : General rules and rules for buildings