Page 227 - Bridge and Highway Structure Rehabilitation and Repair

202 SECTION 2 STRENGTHENING AND REPAIR WORK

8. Transverse stiffness of diaphragms and spacing.

9. Intensity of load (AASHTO 72 kips HS 20 deflection vehicle with dynamic allowance of
30 percecnt).
10. L/d ratio where d is beam depth (for shallow, medium depth, and deep beams).
11. L/D ratio where D is beam depth 4 haunch depth 4 deck slab depth (for composite sec-
tion).
• Shallow beams with high L/D ratios are likely to result in higher deflections and cause

vibrations as compared to the following two types.
• Medium depth beams are commonly used in practice and are usually based on AASHTO
LRFD optional defl ection criteria.

• Deep beams (with small L/D ratios) have lower deflections. The depth of beam approaches
a small height wall. Stress strain behavior is nonlinear and the beam cannot be modeled
as a line girder. Deep beam/wall effects need to be considered using the fi nite element
method.
In design, dead load defl ection is practically eliminated by providing initial camber in the
beam. Live load deflection can be reduced to some extent, for example, by providing high tensile

strands in prestressed concrete beams and pre-tensioning or post-tensioning the strands. The
practice of prestressing a steel beam is less common.
5.3.8 Effect of Beam Materials on Defl ections
AASHTO criteria of L/800 or L/1000 is generally applicable to beams that are made of
composite with deck slabs using shear connectors. Modulus of elasticity of the material affects
deflection. For deflection control, the response of a variety of concrete, steel, and timber beams

needs to be considered such as:
1. Reinforced concrete beam.
2. Prestressed concrete I girders and box beams with normal strength.
3. Prestressed concrete I girders and box beams with HPC. Composite beams with HPC decks
(fc1 values ranging from 4000 psi to 8000 psi are permitted by most states).
4. Prestressed concrete I girders and box beams with ultra high performance concrete.
5. Grade 50W steel beams (both rolled sections and fabricated welded sections).
6. HPS 70W steel beam (fabricated welded sections).
7. Fabricated welded sections using hybrid HPS 70W/50W and hybrid HPS 100W/70W.
8. Sawn timber beams.
9. Glue-laminated timber beams.
5.3.9 Secondary Effects of Defl ections
Vibrations can result from live load deflections on highway, pedestrian, and equestrian

bridges. It is assumed that vibrations will depend upon differential (varying) live load defl ec-
tions due to fast moving vehicles. There may be harmful effects of rapid variations in live load
deflections giving rise to vibrations and fatigue.

1. Deflection resulting in bridge vibration: Control of undesirable psychological effects on

passengers and pedestrians is apparently one of the primary reasons for AASHTO defl ection
limits (Table 5.2). However, prior research indicates that it is not just live load defl ection
but vertical acceleration and bridge dynamic characteristics (e.g., frequency) that control
vibration and human perception. Acceleration is the most important parameter affecting
psychological discomfort.

2. Use of the finite element model representing deck and girder stiffness in two directions:

Analysis of bridge systems with applications of 2-D and 3-D finite element models is re-
quired to study statics and dynamics response of highway bridges. The finite element analysis