Bascule Bridge Design and Construction

Bascule Bridge Design and Construction â€Å"DOUBLE LEAF BASCULE BRIDGE† 1. OBJECTIVES: Become familiar with the wood work. Become familiar with lathe machine and drilling machine. Build a simple pulley. 2. INTRODUCTION: A bascule bridge (commonly referred to as a drawbridge) is a moveable bridge with a counterweight that continuously balances a span, or leaf, throughout its upward swing to provide clearance for boat traffic. It may be single or double leafed. 3. METHOD: Cut out an arch with the scroll saw from the 5-by-7-by-2-inch block of pine wood. Make the arch about 3 inches wide and 4 inches high. Be sure to cut in the longest direction, so that the bottom of the arch becomes the bottom of the tower, and there is 3 inches left above the top of the arch. Measure and mark with the pencil every 1/2 inch across the top of the tower, above the arch. At every other mark, cut a notch 1/2 inch deep and 1/2 inch wide with the scroll saw. These are the tower battlements. Center the tower on the 5-by-10-by-1/2-inch block and nail the two pieces together through the bottom to form the tower base. Lay down the two 4-by-1/2-by-1/2-inch blocks parallel to each other 3 inches apart on your work surface. Lay the eight 4-by-1/2-by-1/8-inch strips across the two blocks. Attach the strips to the blocks with finishing nails. This is your drawbridge. Turn over the drawbridge and tap in two finishing nails, one into each block, as close to the ends as you can without splitting the wood. Leave the nails sticking out past the edges of the drawbridge, about 1/4 inch. This will be the pivot end of your gate. Tap in two more finishing nails, one into each block, about 1/2 inch from the end opposite the pivot end of the drawbridge. Leave the nails sticking past the edges about 1/4 inch. Lay the drawbridge, block side down, on the tower base in front of the tower. Place an upholstery staple over the protruding nails on the pivot end and gently tap the staples into the tower base. This should hold the drawbridge in place while allowing it to rotate freely into open and shut positions. Adjust the staples and nails if needed. Tap two finishing nails into the tower, one on either side at the top of the arch. Make sure they are spaced 4 inches apart, as wide as the drawbridge. Angle the nails at about 45 degrees, and leave half of each nail sticking up. Attach one end of each chain to the nails at the top of the arch. Attach the other ends to the drawbridge. Stain the wood with the paintbrush, then allow to dry. 4. DISCUSSION: SAMPLE DIAGRAM: TERMS AND EXPLANATION: PRACTICAL INFORMATION ABOUT BASCULE BRIDGES: Almost all double leaf bascule bridges consist of two cantilever spans projected toward each other, connected at their tips by a suitable shear lock. Other types of double leaf bascule are comparatively rare, such as those which form arch bridges in the closed position, and are not the topic of this discussion. Double leaf bascule bridges are possibly the least practical, from a maintenance and operation standpoint, of all commonly used types of modern era movable bridges. They use two separate moving leaves when one would do, with all the associated expense in construction, operation, and maintenance of two totally independent movable bridge leaves. They also join these two moving leaves together for the support of live load, compounding the difficulties. There are advantages to double leaf bascules: they can open and close somewhat more quickly than any other type of movable bridge; a double leaf bascule is less affected by wind loads than a single leaf bascule spanning the same channel width; they use slightly less structural steel than other types of movable bridges with the same load rating spanning the same width of navigation channel; double leaf bascules are less susceptible to collision with vessels navigating past them than other movable bridge types, and they are generally consid ered more aesthetically pleasing than other types of movable bridges. One might ask, however, whether these advantages are worth putting up with the additional complications, particularly in regard to stabilizing the structures under live load. Double leaf bascule bridges, more so than most other movable bridge types, frequently have problems with seating. These problems arise from several sources. The bridge may be carrying live loads larger than those designed for, overstressing the support system. The bridge stabilizing devices may have suffered deterioration so that they cannot contain the forces imposed on them. The bridge stabilizing devices may be improperly adjusted so that they do not perform their intended function. The entities which contribute to stability of a double leaf bascule include: live load shoes which form stops for each moving leaf as it attains its seated position; center or shear locks forming a vertical tie between the two leaves of a double leaf bascule bridge when in the closed position; live load anchors which are capable of exerting a downward force at the rear of bridge counterweight; tail locks Which form a shear connection at or near the rear of the bridge counterweight, and adjustment of th e balance of the moving leaf about its axis of rotation. Bascule is French for see-saw. All modern bascule bridges consist of a large moving mass of superstructure, deck, and counterweight, which can be considered balanced for structural purposes. The span can be considered essentially rigid for balancing purposes, as it rotates between opened and closed positions. This applies whether it is a simple trunnion leaf, or a rolling lift of the Scherzer or Ball type. It also applies to the many variations on the articulated counterweight type, as developed by Strauss and others, with the counterweight pivoting about an axis or arc separate from the bridge leaf. An exception to this rule are bascules with operating struts or ropes such as many heel trunnions, some early Scherzer rolling lifts, and others, which do not add simply to the balancing calculations, as they move in a different path than the superstructure. The operating strut could be heavy enough to have a noticeable effect on the balance, but this usually only happens with single lea f railroad bridges. Heel trunnion and articulated counterweight bascule bridges have the counterweight rotating about an axis separate from the leaf itself. The counterweight is always in a fixed position with regard to gravitational moment relative to the bascule span on these bridges, due to the parallelogram arrangement of the pivot points. Double leaf bascule bridges become unstable because they are poorly designed, poorly constructed, or poorly maintained. They are more susceptible to deficiencies from these causes because they are more delicate than other common types of movable bridges. It is very difficult to correct the faults of a poorly designed bridge, but sometimes possible to correct construction defects. It is very difficult to correct the results of poor maintenance except by replacing the components affected. A properly designed double leaf bascule bridge should be very rigid, particularly in regard to primary live load deflections. The leaves of the double leaf bascule should be firmly supported on very solid live load shoes located adjacent to the pier sea wall, as far as possible from the center of rotation. The balance of the double leaf bascule should be such that a dead load reaction exists on the live load shoes, when the bridge is closed, that is substantially in excess of any possible negative reaction, from live load or other sources. The roadway surfaces of the double leaf bascule should be formed so that there is no misalignment at the joints, either at the heels of the leaves or at the toes. This applies to profile as well as elevation the vertical curve should be continuous from one leaf to the other and from each leaf to its approach. Tail locks should be provided as a backup to the stabilization achieved by balancing. The tail locks should firmly grasp the tail end of each leaf with minimum clearances and hold it in the closed position. This will eliminate the possibility of drive machinery being damaged due to live load deflection. CONCLUSION AND RECCOMENDATION: Double leaf bascule bridges become unstable because they are poorly designed, poorly constructed, or poorly maintained. They are more susceptible to deficiencies from these causes because they are more delicate than other common types of movable bridges. It is very difficult to correct the faults of a poorly designed bridge, but sometimes possible to correct construction defects. It is very difficult to correct the results of poor maintenance except by replacing the components affected. A properly designed double leaf bascule bridge should be very rigid, particularly in regard to primary live load deflections. The leaves of the double leaf bascule should be firmly supported on very solid live load shoes located adjacent to the pier sea wall, as far as possible from the center of rotation. The balance of the double leaf bascule should be such that a dead load reaction exists on the live load shoes, when the bridge is closed, that is substantially in excess of any possible negative reaction, from live load or other sources. The roadway surfaces of the double leaf bascule should be formed so that there is no misalignment at the joints, either at the heels of the leaves or at the toes. This applies to profile as well as elevation the vertical curve should be continuous from one leaf to the other and from each leaf to its approach. rail locks should be provided as a backup to the stabilization achieved by balancing. The tail locks should firmly grasp the tail end of each leaf with minimum clearances and hold it in the closed position. This will eliminate the possibility of drive machinery being damaged lue to live load deflection. Movable bridges have been an important part of our nation’s infrastructure for centuries. They present unique challenges to the structural engineer and require extensive coordination of the  structural, mechanical, and electrical systems to achieve a durable and operationally reliable structure. FINAL PROJECT: REFERENCES: http://en.wikipedia.org/wiki/Bascule_bridge http://heavymovablestructures.org/assets/technical_papers/00525.pdf