Steel Bridges. Fourth Year. Building and Construction Department. University of Technology. Baghdad. Dr. Wael Shawky

Transcription

1 Steel Bridges Building and Construction Department University of Technology Baghdad Dr. Wael Shawky Page 1 of 113

2 Bridges A bridge is a structure built to span a valley, road, railroad track, river, body of water, or any other physical obstacle, for the purpose of providing passage over the obstacle. Designs of bridges will vary depending on the function of the bridge and the nature of the terrain where the bridge is to be constructed. History of Bridges: The first bridges were made by nature as simple as a log fallen across a stream. The first bridges made by humans were probably spans of wooden logs or planks and eventually stones, using a simple support and crossbeam arrangement. Epic literature of India provides mythological accounts of bridges constructed from India to Lanka by the army of Rama. The Arthashastra of Kautilya mentions the construction of dams and bridges. A Mauryan bridge near Girnar was surveyed by James Princep. The bridge was swept away during a flood, and later repaired by Puspagupta, the chief architect of emperor Chandragupta I. The bridge also fell under the care of the Yavana Tushaspa, and the Satrap Rudra Daman. The use of stronger bridges using plaited bamboo and iron chain was visible in India by about the 4th century. A number of bridges, both for military and commercial purposes, were constructed by the Mughal administration in India. The greatest bridge builders of antiquity were the ancient Romans. The Romans built arch bridges and aqueducts that could stand in conditions that would damage or destroy earlier designs. Some stand today. An example is the Alcántara Bridge, built over the river Tagus, in Spain. The Romans also used cement, which reduced the variation of strength found in natural stone. One type of cement, called pozzolana, consisted of water, lime, sand, and volcanic rock. Brick and mortar bridges were built after the Roman era, as the technology for cement was lost then later rediscovered. Although large Chinese bridges of wooden construction existed at the time of the Warring States, the oldest surviving stone bridge in China is the Zhaozhou Bridge, built from 595 to 605 AD during the Sui Dynasty. This bridge is also historically significant as it is the world's oldest open-spandrel stone segmental arch bridge. European segmental arch bridges date back to at least the Alconétar Bridge (approximately 2nd century AD), while the enormous Roman era Trajan's Bridge (105 AD) featured open-spandrel segmental arches in wooden construction. Rope bridges, a simple type of suspension bridge, were used by the Inca civilization in the Andes mountains of South America, just prior to European colonization in the 1500s. During the 18th century there were many innovations in the design of timber bridges by Hans Ulrich, Johannes Grubenmann, and others. The first book on bridge engineering was written by Hubert Gautier in A major breakthrough in bridge technology came Page 2 of 113

3 with the erection of the The Iron Bridge in Coalbrookdale, England in It used cast iron for the first time as arches to cross the river Severn. With the Industrial Revolution in the 19th century, truss systems of wrought iron were developed for larger bridges, but iron did not have the tensile strength to support large loads. With the advent of steel, which has a high tensile strength, much larger bridges were built, many using the ideas of Gustave Eiffel. Types of bridges There are six main types of bridges: beam bridges, cantilever bridges, arch bridges, suspension bridges, cable-stayed bridges and truss bridges. Beam bridges Beam bridges are horizontal beams supported at each end by piers. The earliest beam bridges were simple logs that sat across streams and similar simple structures. In modern times, beam bridges are large box steel girder bridges. Weight on top of the beam pushes straight down on the piers at either end of the bridge. Cantilever bridges Cantilever bridges are built using cantilevers horizontal beams that are supported on only one end. Most cantilever bridges use two cantilever arms extending from opposite Page 3 of 113

4 sides of the obstacle to be crossed, meeting at the center. The largest cantilever bridge is the 549-metre (1,800 ft) Quebec Bridge in Quebec, Canada. Arch bridges Arch bridges are arch-shaped and have abutments at each end. The earliest known arch bridges were built by the Greeks and include the Arkadiko Bridge. The weight of the bridge is thrust into the abutments at either side. Dubai in the United Arab Emirates is currently building the Sheikh Rashid bin Saeed Crossing which is scheduled for completion in When completed, it will be the largest arch bridge in the world. Page 4 of 113

5 Suspension bridges Suspension bridges are suspended from cables. The earliest suspension bridges were made of ropes or vines covered with pieces of bamboo. In modern bridges, the cables hang from towers that are attached to caissons or cofferdams. The caissons or cofferdams are implanted deep into the floor of a lake or river. The longest suspension bridge in the world is the 12,826 feet (3,909 m) Akashi Kaikyo Bridge in Japan. See simple suspension bridge, stressed ribbon bridge, underspanned suspension bridge, suspended-deck suspension bridge, and self-anchored suspension bridge. Page 5 of 113

6 Cable-stayed bridges Like suspension bridges, cable-stayed bridges are held up by cables. However, in a cablestayed bridge, less cable is required and the towers holding the cables are proportionately shorter. The first known cable-stayed bridge was designed in 1784 by C.T. Loescher. The longest cable-stayed bridge is the Sutong Bridge over the Yangtze River in China. Page 6 of 113

7 Truss bridges Truss bridges are composed of connected elements. They have a solid deck and a lattice of pin-jointed girders for the sides. Early truss bridges were made of wood, and later of wood with iron tensile rods, but modern truss bridges are made completely of metals such as wrought iron and steel or sometimes of reinforced concrete. The Quebec Bridge, mentioned above as a cantilever bridge, is also the world's longest truss bridge. Page 7 of 113

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42 Compression Members Compression members are members subjected to axial compressive stresses such as columns, posts and struts. Only very short columns can be loaded to their yield stress ( ) but usually situation is that buckling occurs prior to developing the full material strength of the member. Page 42 of 113

43 Euler Elastic Load Then 0 which represents second order differential equation. Then the solution of the equation is: By using the boundary conditions: Page 43 of 113

44 For fundamental buckling mode (single curvature deflection), n=1, the Euler critical load for the pin ended column is: And the critical compressive stress is: Page 44 of 113

45 Effective Length (kl) and effective length factor (k): Columns Non Sway Case Sway Case... Lateral bracing is provided by a diagonal bracing or adequate attachment to shear walls adjacent structure having lateral stability K is calculated from the charts on pages 3 5 or AISC 1989 Page 45 of 113

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47 Allowable stress (Fa) On the gross section of axially loaded compression member: 2 a when b 5 3 when Where E=29000ksi 8 The values of Fa are given in tables on the following pages in AISC manual: For Fy=36ksi on page 3 16 And for Fy=50ksi on page 3 17 Local Buckling: To prevent local buckling of the section elements of compression members, the width thickness ratios of the elements are: Page 47 of 113

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49 Built Up Members: " " Page 49 of 113

50 Design Method: Choose any value of between 0 and 200 Find then Find Choose available section from the manual and write down the properties of the section. Compute From the larger of tables, find and by using the allowable axial stress ; Last check the local buckling of the section. Page 50 of 113

51 EX: Select the lightest section of A36 steel for a member of 30 long and carrying an axial compression load as shown in figure. The member is a part of a braced structure Section A A Solution: Since the member is in a braced structure: From page 3 16: Page 51 of 113

52 " " Try W10 x " 2.01" With the larger value of kl/r (89.55) and from table on page 3 16: The section can be used but it is not represent the most economical section for this problem. Try smaller section W10 x " " 0.53 " 1.98 tw Page 52 of 113

53 Check the local buckling: USE W10 x 39 Page 53 of 113

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