255 Concrete Girder Bridges

25.5.1 Introduction

For modern bridges, both structural concrete and steel give satisfactory performance. The choice between the two materials depends mainly upon the cost of construction and maintenance. Generally, concrete structures require less maintenance than steel structures, but since the relative cost ofsteel and concrete is different from country to country, and may even vary throughout different parts of the same country, it is impossible to put one definitively above the other in terms of "economy."

In this section, the main features of common types of concrete bridge superstructures are briefly discussed. Concrete bridge substructures will be discussed in Section 25.6.

25.5.2 Reinforced Concrete Bridges

Figure 25.30 shows the typical reinforced concrete sections commonly used in highway bridge superstructures.

A reinforced concrete slab (Figure 25.30a) is the most economical bridge superstructure for spans of up to approximately 40 ft (12.2 m). The slab has simple details, standard formwork, and is neat, simple, and pleasing in appearance. Common spans range from 16 to 44 ft (4.9 to 13.4 m) with structural-depth-to-span ratios of 0.06 for simple spans and 0.045 for continuous spans.

The T-beams (Figure 25.30b) are generally economic for spans of 40 to 60 ft (12.2 to 18.3 m), but does require complicated formwork, particularly for skewed bridges. Structural-depth-to-span ratios are 0.07 for simple spans and 0.065 for continuous spans. The spacing of girders in a T-beam bridge depends on the overall width of the bridge, the slab thickness, and the cost of the formwork and may be taken as 1.5 times the structural depth. The most commonly used spacings are between 6 and 10 ft (1.8 to 3.1 m).

25.5.2.3 Cast-In-Place Box Girder

Box girders like the one shown in Figure 25.30c, are often used to span 50 to 120 ft (15.2 to 36.6 m). Its formwork for skewed structures is simpler than that required for the T-beam. Due to excessive dead load deflections, the use of reinforced concrete box girders over simple spans of 100 ft (30.5 m) or more may uneconomical. The depth-to-span ratios are typically 0.06 for simple spans and 0.055 for continuous spans with the girders spaced at 1.5 times the structural depth. The high torsional resistance of the box girder makes it particularly suitable for curved alignments, such as the ramps onto freeways. Its smooth flowing lines are appealing in metropolitan cities.

FIGURE 25.30 Typical reinforced concrete sections in bridge superstructures: (a) solid slab; (b) T-beam; and (c) box girder.

25.5.2.4 Design Consideration

A reinforced concrete highway bridge should be designed to satisfy the specification or code requirements, such as the AASHTO-LRFD (2004) requirements for all appropriate service, fatigue, strength, and extreme event limit states. In the AASHTO-LRFD (2004), service limit states include cracking and deformation effects, and strength limit states consider the strength and stability of a structure. A bridge structure is usually designed for the strength limit states and is then checked against the appropriate service and extreme event limit states.

25.5.3 Prestressed Concrete Bridges

Prestressed concrete, using high-strength materials, makes it an attractive alternative for long-span bridges. It has been widely used in bridge structures since the 1950s.

Figure 25.31 shows FHWA (1990) standard types of precast prestressed voided slabs and their sectional properties. While a cast-in-place prestressed slab is more expensive than reinforced concrete slab, a precast prestressed slab is economical when many spans are involved. Common spans range from 20 to 50 ft (6.1 to 15.2 m). Structural-depth-to-span ratios are 0.03 for both simple and continuous spans.

25.5.3.2 Precast I-Girder

Figure 25.32 shows AASHTO (FHWA 1990) standard types of I-beams. Figure 25.33 and Figure 25.34 show Caltrans standard T-girder "Bulb-Tee" girders (Caltrans 2001), respectively. These compete with steel girders and generally cost more than reinforced concrete with the same depth-to-span ratios. The formwork is complicated, particularly for skewed structures. These sections are applicable to spans 30 to 120 ft (9.1 to 36.6 m). Structural-depth-to-span ratios are 0.055 for simple spans and 0.05 for continuous spans.

Section dimensions Section properties

Span range,

Width B,

Depth D,

D1, in.

D2, in.

A, in.2

I,, in.4

Sx, in.3

ft(m)

in. (mm)

in. (mm)

(mm)

(mm)

(mm2 106)

(mm4 109)

(mm3 106)

25 (7.6)

48 (1,219)

12 (305)

0(0)

0(0)

576 (0.372)

6,912 (2.877)

1,152 (18.878)

0 0

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