The Built Environment and Loading Conditions 16

A long-span suspension bridge is often located in a unique environment condition. Design loads for a suspension bridge mainly includes dead load, traffic load, wind load, seismic load, and temperature load. Others like erection load, impact load, and support movement may also be considered in some cases. A brief description of loading conditions is given in this section.

2.3.1 Dead Load

Dead load typically dominates the forces on the main components of the bridge. It includes weights of all components of the structure, appurtenances, and utilities attached thereto, earth cover, wearing surface, future overlays, and planned widenings (AASHTO, 2005).

2.3.2 Railway Load

When a long suspension bridge carries railways, the bridge is subjected to moving loads of railway vehicles, which include vertical forces of railway vehicles, longitudinal forces from acceleration or deceleration of vehicles, lateral forces caused by irregularities at the wheel-to-rail interface, and centrifugal forces due to track curvature. Railway vehicles vary greatly with respect to weight, number of axles, and axle spacing. This variability requires a representative live load model for design that provides a safe and reliable estimation of characteristics of railway vehicles within the design life of the bridge (Unsworth, 2010). For both bridge safety and vehicle comfort assessment, the interaction between the railway vehicles and long-span suspension bridge is important (Frýba, 1996).

In design of railroad bridges, the Cooper E80 load (AREMA, 1997) is the most common design live load. It consists of a series of point loads simulating locomotive wheels, followed by a uniformly distributed load of 8 kip per linear foot, equivalent to 14.6 kN/m. Monitoring of railway effects will be described in Chapter 5.

2.3.3 Highway Load

When a long-span suspension bridge carries a highway, the bridge is therefore subjected to a variety of non-stationary loads due to motorcycles, cars, buses, trucks, and heavy goods vehicles. Highway loadings are rather complicated. The effect of highway loadings on a suspension bridge is a function of several

parameters, such as the axle loads, axle configuration, gross vehicle weight, number of vehicles, speed of vehicles, and the bridge configuration.

The bridge responses under highway loadings can be analysed using the moving load model (Timoshenko et al., 1974), the moving mass model (Blejwas et

al., 1979), and the advanced vehicle-bridge interaction models with consideration

of the road roughness (Yang and Lin, 1995; Cheung et al., 1999).

In American Association of State Highway and Transportation Officials

(AASHTO) specification (AASHTO, 2005), Highway Load ‘93’ or HL93 is adopted as the vehicular live loading of highway bridges, which is a combination of design truck or design tandem and design lane load. The AASHTO design truck is shown in Figure 2.4. The axle spacing between the two 145 kN loads can be varied between 4.3 m and 9.0 m to create a critical condition for the design of each location in the structure. The AASHTO design tandem consists of two 110 kN axles spaced at 1.2 m apart. The transverse spacing of wheels shall be taken as 1.8 m. The design lane load is equal to a load of 9.3 kN/m uniformly distributed over a 3 m width. In Eurocode 1 (2003), the normal highway load model comprises a tandem axle system acting in conjunction with a uniformly distributed load.

Traffic running on bridges produces a stress spectrum which may cause fatigue. Fatigue load models of vertical forces are defined in Eurocode 1 (2003) and AASHTO specification (AASHTO, 2005). Monitoring of highway effects will be described in Chapter 6.

Figure 2.4 Design truck in AASHTO load and resistance factor design

2.3.4 Temperature Effects

Bridges are subjected to daily and seasonal environmental temperature effects (or thermal effects) induced by solar radiation and ambient air temperature. Variation

35 000 N 145 000 N 145 000 N

4300mm 4300 to 9000mm

600mm General

300mm Deck Overhang 1800mm

in temperature of bridge components will cause movements and, usually, thermal stress due to indeterminacy and non-uniform distribution of temperature.

The temperature effects on a structure are dependent on the temperature distribution, structural configuration and boundary conditions, and material mechanical properties of the structure. The local climatic conditions, structural orientation, structural configuration and material thermal properties will affect the structural temperature distribution, which may be divided into three components: • a uniform temperature component;

• a linearly varying temperature gradient component; and • a nonlinear temperature gradient component.

A uniform temperature change will result in a change in length for an unrestrained structure. The linearly varying temperature gradient component will produce a curvature of the element. The nonlinear temperature gradient component results in self-equilibrated stresses which have no net load effect on the element. The uniform temperature ranges are determined in design specifications according to the climatic conditions and material types of the bridge. Monitoring of temperature effects will be described in Chapter 7.

2.3.5 Wind Load

Wind load is particularly important for long-span suspension bridges. Strong wind may induce instability and excessive vibration in bridges. Wind effects on a long- span suspension bridge are mainly due to static forces induced by mean winds, buffeting excitation, flutter instabilities, and vortex shedding excitation. These types of instability and vibration may occur alone or in combination (Cai and Montens, 2000).

Buffeting action on a long suspension bridge is caused by fluctuating winds that appear within a wide range of wind speeds. In wind resistance design of a long-span suspension bridge, the buffeting responses are normally dominant to determine the size of structural members. In addition to buffeting action, the self- excited forces induced by wind-structure interaction are also important because the additional energy injected into the oscillating structure by self-excited forces increases the magnitude of vibrations. The buffeting response prediction can be performed in both the frequency domain (Davenport, 1962; Scanlan, 1978) and the time domain (Bucher and Lin, 1988; Chen et al., 2000). Flutter instabilities may occur in several types that occur at very high wind speeds as a result of the dominance of self-excited aerodynamic forces. The flutter instabilities always involve torsional motions, and the most common consideration of the flutter for a long-span suspension bridge is the coupled translational-torsional form of instability. In the design stage, a critical flutter speed of the bridge shall be determined and shall exceed, by a substantial margin, the design wind speed of the bridge site at the deck height (Holmes, 2007). Vortex shedding excitation can induce significant, but limited, amplitude of vibration of a long-span suspension bridge in low wind speed and low turbulence conditions. Scanlan’s model can be used for calculating the vortex-shedding force (Simiu and Scanlan, 1996).

Design codes usually provide wind loads as a function of design wind velocity at a reference height above the ground or sea level. The mean wind velocity profile within the atmospheric boundary layer is approximated by a logarithmic or power law, while the former is preferred in the new codes (AASHTO, 2005; Eurocode 1, 2005). Terrain conditions should be considered in determination of the design wind velocity.

For large-scale cable-supported bridges (including cable-stayed bridges), appropriate wind tunnel tests are usually required to simulate the wind environment, determine the wind characteristics, and examine the responses of the bridge to various winds. Nevertheless, the on-site wind monitoring can provide a more realistic wind environment and behaviours of the bridge. Monitoring of wind effects will be described in Chapter 8.

2.3.6 Seismic Effects

In a region prone to earthquakes, the seismic design is also important. As the fundamental frequency of long-span suspension bridges is generally low, the seismic load is relatively small. During the construction of the Akashi Kaikyo Bridge, the Kobe Earthquake which occurred in 1995 caused a new fault in the seabed below the bridge and the towers moved by 1 metre. Fortunately, no damage was reported to the bridge itself.

According to the AASHTO Load and Resistance Factor Design specification (AASHTO, 2005), earthquake loads are specified as the horizontal force effects and are given by the product of the elastic seismic response coefficient and the equivalent weight of the superstructure, and divided by a response modification factor. The elastic seismic response coefficient is a function of the acceleration coefficient determined from the contour map of the region or nation, period of vibration, and site coefficient. Monitoring of seismic effects will be described in Chapter 9.

2.3.7 Other Effects

Besides the common types of loading described above, some bridges may also be subject to other loading effects, for example, ship collision and ice load, depending on the particular environmental conditions of the bridge. A few of them are outlined as follows.

2.3.7.1 Corrosion

Corrosion is the deterioration of a metal that results from a reaction with the environment. This reaction is an electrochemical oxidation process that usually produces rust. In bridges, corrosion may occur in structural steels, reinforcing bars, and strands in cables. The corrosion of the reinforcing steel is considered to be the primary contributor to the deterioration of highway bridge decks (Mark, 1977).

For protection against corrosion, structural steels must be self-protecting or have a coating system or cathodic protection. Reinforcing bars in concrete components must be protected by epoxy, galvanized coating, concrete cover, or

painting. Prestressing strands in cable ducts are usually grouted against corrosion. Chapter 10 has more on corrosion monitoring.

2.3.7.2 Vessel Collision

According to the AASHTO Load and Resistance Factor Design (AASHTO, 2005), all bridge components in a navigable waterway crossing, located in design water depths not less than 600 mm, must be designed for vessel impact. The vessel collision loads should be determined on the basis of the bridge importance and characteristics of the bridge, vessels and waterway.

In AASHTO’s specifications (AASHTO, 1991; 2005), the head-on ship collision impact force on a pier is taken as a static equivalent force. In Eurocode 1 (2006), a dynamic analysis or an equivalent static analysis is suggested for inland waterways and sea waterways. This topic will be detailed in Chapter 10 as well.

2.3.7.3 Hydraulics

The design of hydraulics involves the hydrology study, hydraulic analysis, drainage design, and bridge scour evaluation. Scour is actually not a force effect, but changes the conditions of the substructure of the bridge and consequently changes the force effects. It was reported that the most frequent causes of bridge failures were attributed to hydraulics (flood and scour) and collisions (Harik et al., 1990; Wardhana and Hadipriono, 2003). Scour monitoring will be described in Chapter 10.

2.3.7.4 Ice Load

Ice load should be considered in cold regions where the sea is covered with ice during winter. Ice loads were the dominant loading condition for the design of the piers of the Confederation Bridge in Canada (Brown et al., 2010). Dynamic forces occur when a moving ice floe strikes a bridge pier. The forces are dependent on the size of floe, and the strength and thickness of the floe. AASHTO (2005) specification suggests the horizontal force on the basis of field measurement, and vertical forces due to ice adhesion.