Current Practice Sheet No. 4
PRESTRESSED CONCRETE B RIDGE B B E A MS
Howard Taylor, Tarmac Precast Concrete Ltd, Concrete Society President and Past-President of the Concrete Bridge Development Group
The fourth in a series of Current Practice Sheets prepared by the Concrete Bridge Development Group
Facts about prestressed concrete bridge beams •
industry standards have been developed developed over 50 50 years
•
proven durability
•
suitability for solid slab, beam and slab, slab, and voided decks
•
possible to simply simply support, continuous continuous and use integral construction techniques
•
used in decking for ports, podia, etc.
Introduction Pre-tensioned prestressed concrete bridge beams have been used in the UK as a major form of deck construction construction for the past 50 years (see Figure 1). Pre-tensioned concrete has the advantage that the stressed tendons are embedded in the beam at the time of manufacture, and so there is no requirement for grouting. This has resulted in a very robust form of construction with excellent durability.
Figure 1: A typical prestressed bridge beam being craned into position over the M11 at Stansted, Essex
In the 1960s, the beam types were accepted as national standards, the Y-beam range being the latest to be developed. These beams can be provided by a number of suppliers and can be specified easily without nomination. Three forms of deck are used in the span range, each finding its economic position.
handling of bridge beams on site provides guidance to planning supervisors, design engineers and contractors on this phase of the procurement and construction(1).
Beam Bea m and deck t ypes Economical Depth range span range (mm) (m)
The trend towards inspection of all surfaces, increased covers and integral construction has led to the introduction of the Y-beam range, which may be used for solid slab and beam-and-slab construction.
Beam
Bridge beams have many uses other than in bridge construction, for example, in building structures, car parks and jetties.
TY-beam
Solid slab
4–17.5
400–850
Inverted T-beam
Solid slab
5–17
380–815
TY-beam
Beam & slab
7.5–17.5
550–850
Y-beam
Beam & slab
14–31
700–1400
SY-beam
Beam & slab
27–45
1500–2000
16–30
720–1360
14–34
800–1600
Design Bridge beams may be designed by the engineer or by the manufacturer. In the latter case, it is sufficient to provide only the moments, shears and torques from the structural analysis and any special layout and detailing requirements.
Details Details of preferred prestressing strand positions, link shape, bearing areas (if any), and parapet fixings are provided in manufacturers’ literature. Manufacturers are also able to assist in deck design and will carry out beam design and detailing if requested. The use of standard details builds in good practice and produces economical economical designs.
Form of deck
Beam & slab / M-beam
Construction The manufacturer is usually responsible for the supply and delivery of the beams to site, while the contractor generally organises erection and deck construction. The Prestressed Concrete Association publication covering the
Section
U-beam
voided
Voided
Forms of deck construction
Parapets Parapets may be cast in-situ onto the deck by the contractor or precast and supplied separately. In either case, fixings or sockets for projecting bars may be provided as part of the bridge beam. In some circumstances, the in-situ parapet can be cast onto the bridge beam in the works, thereby removing a difficult formwork, steel-fixing and concreting problem from site.
Other product s
Solid slab deck using TY-beams, applicable to small spans where the cost of the infill concrete is less than the cost of the form-work and top slab in beam and slab decks. All surfaces may be inspected.
Bridge beams can be used as the basis of simple footbridges, with the walkway and upstand for railings cast in the factory. U-beams are particularly useful in this regard as they then produce an attractive outline. Bridge beam manufacturers have developed a standard prestressed sign gantry. This has legs specially designed for impact resistance which can carry a prestressed beam with the maximum permitted signage across a motorway from a single lane to dual four lanes in a single span.
Ac kn ow led gem ent Information on bridge beam properties may be obtained from the Prestressed Concrete Association, 60 Charles Street, Leicester, LE11FB. Tel: +44 (0)116 253 6161; Fax: +44 (0)116 251 4568; e-mail:
[email protected].
References 1.
PRESTRESSED CONCRETE ASSOCIATION. Handling of bridge beams on site (available as pdf download from http://www.britishprecast.org/pca/pca3.htm)
Further reading Beam-and-slab construction using SY-beams – an economical form of construction for large spans.
British Standards Institution prEN 1994-1-1:2002 Design of composite steel and concrete structures. Part 1.1 General rules and rules for buildings. In draft. prEN 1994-2:2003 Design of composite steel and concrete structures. Part 2. Rules for bridges. In draft. BS 5400-2:1978 Steel, concrete and composite bridges. Specification for loads BS 5400-4:1990 Steel, concrete and composite bridges. Code of practice for design
Highways Agency Design Manual for Roads and Bridges
BD 37/88 Loads for Highway Bridges BD 57/95 Design for durability BA 57/95 Design for durability BD 24/92 Design of concrete bridges and structures – use of BS 5400-4:1990
Others NICHOLSON, B. Simple bridge design using prestressed beams, British Precast, Leicester, 1997, 94pp.
Voided slab construction using U-beams. Some engineers prefer the look of the soffit with this construction technique. Inner voids cannot be readily inspected.
TAYLOR, H. The precast concrete bridge beam– the first 50 years, The Structural Engineer, Vol. 76, No. 21, November 1998. TAYLOR, H. Current Practice Sheet 130: Integral bridges, CONCRETE , Vol. 36, No. 10, November/December 2002, p.31
