Figure 8-9. - Details of a superstructure of a timber trestle.
LONGITUDINAL BRACING and TRANS- VERSE BRACING are the two types commonly used to support the substructure in heavy timber construction, as shown in figure 8-8. Longitudinal bracing is used to provide stability in the direction of the bridge center line. Transverse bracing, sometimes called lateral bracing, provides stability at right angles to the center line.
Sometimes a third type of bracing, called a DIAPHRAGM, is used. This bracing is used between the stringers to prevent BUCKLING (deflecting laterally under load).
The SUPERSTRUCTURE of a bridge consists of the stringers, flooring (decking and treads), curbing, walks, handrails, and other items that form the part of the bridge above the substructure. Figure 8-9 is an illustration of a superstructure.
As shown in figure 8-9, those structural members that rest on and span the distance between the intermediate supports or abutments are called STRINGERS. The stringers are the main load-carrying members of the superstructure. They receive the load from the flooring and transmit it to the substructure. Although the figure shows both steel and timber stringers, in practice, only one type is normally used.
The flooring system includes the deck; the wearing surface, or tread, that protects the deck; and the curb and handrail system. The plank deck is the simplest to design and construct, and it provides considerable savings in time compared to other types of decks. Plank decking is normally placed perpendicular to the bridge center line (direction of traffic) for ease and speed of construction. A better arrangement, however, is provided when the decking is placed at about a 30- to 60-degree skew to the center line. Provide space of approximately one-quarter inch between the planks to allow for swelling, to provide water drainage, and to permit air circulation. The minimum thickness of decking is 3 inches in all cases; however, when the required thickness of plank decking exceeds 6 inches, then use a laminated type of decking.
One of the inherently hazardous parts of construction operations is excavating. The main hazards of excavation work areas follows:
1. Collapse or failure of excavation walls burying workers and equipment
2. Materials, tools, and equipment falling into holes and striking workers below
3. Hazards involving public utilities, such as electricity, water, gas, or natural gases and oxygen deficient atmosphere
4. Wet, muddy conditions, causing slips, trips, or falls, complicated by limited spaces in which personnel work
Take precautions to make sure the excavation banks do not collapse and cause injury or death to persons work- ing in the excavation. The method used to protect excava- tion banks from collapsing depends on the type of soil in the area, the depth of the excavation, the type of founda- tion being built, and the space around the excavation.
Before beginning the excavation, the Builder must secure all possible information regarding any,Continue Reading