CHARACTERISTICS OF VARIOUS TYPES OF SUPERSTRUCTURES :
In this simplest type of bridge superstructure, the deck slab also serves as the principal load carrying element. Slab bridges are easiest to construct and are frequently used for comparatively smaller spans. The form is very efficient at distributing point loads because of its two way spanning ability and high torsional strength. It is relatively easy to construct and this is reflected in its construction cost. The principal disadvantage is a its high self-weight which can be counteracted to some extent, by providing suitable variation in thickness or by providing voids. It may be reinforced concrete or of prestressed concrete. The concrete slab, which may be solid, voided, or ribbed, is supported directly on the substructures. Slab bridges require more concrete and steel than girder bridges of the same spans, but construction cost is usually lower and their formwork is simpler and less expensive.The limit of span of slab bridges depends on the magnitude of load and the relative costs of formwork, materials and labour. The small headroom under slab bridges can also have some bearing on economy through cheaper formwork.
Reinforced concrete bridges decks should be designed in accordance with recognised elastic analysis, assisted by certain empirical analysis. The analysis should take into consideration and configuration of the slab, the supporting conditions, and the effects of cracking on the slab stiffness.
Types of Construction
Cast in Place Slab Decks Concrete slab decks may be cast monolithically together with the supporting elements. Alternatively, they may be cast separately but effectively bonded to achieve composite action under future loads.
Precast Deck System Precast concrete deck systems may be constructed in several different ways. These include :
Precast panel extending the full width of the bridges attached to supporting members, without composite action
Precast panels extending the full width of the bridge and effectively bonded to the supporting members to develop composite action.
Shallow depth for low clearance.
Clean attractive appearance.
Good load distribution system.
Good torsional stiffness.
High dead weight.
Inefficient use of material at mid - depth.
Maintenance of voids may be a problem.
Cannot be repaired if prestressed.
Slab bridges may be divided into three classes :
slabs cast in situ,
slabs built-up of precast elements and
composite slabs, in which precast elements are used in combination with in situ concrete filling.
Cast in situ slab bridges may be adopted from 6 to 30 m spans. Solid reinforced concrete slab of constant depth is normally used for spans unto 10 m. For spans upto about 8 m, solid reinforced concrete slabs, with depths, upto 60 cm, may be adopted. For larger spans, upto 15 m, haunching or variable depth is adopted to reduce dead load.
Voided slab bridges are adopted to reduce the self weight of the bridge. Voided RC slabs, with depths about 1.2m may be adopted for spans of about 10 to 15-20 m. However, for spans of 15 to 30 m. voided prestressed concrete slabs, of depths upto about 1.50 m, are cheaper. The voids are usually circular or rectangular. The depth of voids is generally restricted to sixty per cent of the depth of the slab so that the slab continues to behave like a single plate. If this limit of void depth is exceeded, the slab may behave more like a cellular deck. The voids may either run for the full span length or alternatively these may be provided in the central span length only so that solid section is available near the supports where shear is large.
A solid slab of uniform depth is preferred in highly skewed crossings, particularly if significant curvature and variation in width of the deck is involved. Continuous construction can be used with advantage if the possibility of uplift at abutments is expected. Slab system is usually adopted when the erection of formwork presents no great difficulty. Cables or reinforcements are then placed in position, and after concrete is set, cables are stressed and anchored. Some cables are usually bent up, to reduce shear stresses and also for a convenient distribution of end anchorage's. The advantages of this form is that the structure is monolithic and the stress distribution can be calculated. Disadvantages, however, are that the prestressing of a slab is generally uneconomical, the form of construction is often heavy, and the span lengths are limed.The use of precast elements is of advantage when there is difficulty in supporting the formwork. A quick erection can be possible and very economical if large number of units is required. Precast and prestressed units forming a part of the total deck can be temporarily used as formwork for in situ concrete. Additional cables are then often used and subsequently stressed on the composite section. Transverse prestressing is then often used to achieve transverse rigidity and unison of the whole section.
The main problems in the design of slab bridges are - the choice of maximum economical span, minimum depth, the choice of maximum economical span, minimum depth, the choice whether to precast or cast in situ, the type of cross section to be used, transverse load distribution, deflection and vibrations, and finally the choice of the method of erection.
In RC slab bridges span depth ratio of 15 for simple, spans and 20 to 25 for continuous spans are usually adopted for both solid and voided slabs. For cast in situ prestrssed concrete voided slab bridges, this ratio is nearly 30. In precast prestressed voided slabs, the ratio ranged between 25 and 30.
In many countries standard precast prestressed beams are employed for short and medium span bridges. These standard beams are closely positioned across the width of the bridge and in situ concrete is poured to give transverse connection in order to create slab type deck. This form of deck is described as Pseudo slabs. Such type of two stage casting is also referred to as continuous construction and the standard beams are termed as Continuous beams. Many forms of prsetressed, precast beams are used in pseudo slab decks. Pretensioned cast in situ Pseudo slabs are usually adopted when the erection of formwork presents no difficulty. The advantage of this form of constructions that the structure is monolithic and the stress distribution in the slab can easily be evaluated. On the other hand, the precast girders with in situ concrete filled up, are preferred when there is difficulty in support the formwork. However, in general, the prestressing of slab is uneconomical and the formwork is often heavy.
Reinforced Concrete Slabs :
Appearance : Neat and simple; desirable for low short spans
Construction : Details and formwork simplest
Traffic : May be impeded by false work if cast in place, due to reduced clearance. Guide rail should protect false work openings for traffic lanes.
Construction time : Shortest of any cast in place construction.
Maintenance : Very little except at hinges. Future widening may be difficult.
Cast in place, Post Tensioned :
Structural : Voided slabs up to about 30 m, where a high span depth ratio is required; either simple or continuous spans, high torsional resistance, making it very suitable for curved alignment, especially on single columns.
Appearance; : Neat and simple; desirable; for low short spans.
Construction : More complicated than conventional reinforced concrete. Sequence of stressing and grouting should be supervised by specialists.
Traffic : May be impeded by falsework due to reduced clearances. Guide rail should protect falsework openings for traffic lanes.
Construction time : About same as conventional reinforced slabs.
Maintenance : Very little except at hinges.
Precast, Pretensioned or Post-Tensioned :
Structural : Voided slabs for spans 10 to 30m.
Appearance : Neat and simple; desirable for low short spans.
Construction : Details and formwork very simple; plant fabrication methods are favourable and applicable; field erection may be fast.
Traffic : No false work required; units placed by cranes; no prolonged impediment to traffic.
Construction time : Time for erection of precast elements at site a minimum.
Maintenance : Very little except at hinges.