Transcript Steel
Steel History No role till 19 century – in a structural cap. First all metal structure - cast iron bridge built in the late 18 century in England. Still in use today. Cast iron was first – but is very brittle. 1850’s still came available inexpensively with introduction of the Bessemer process. – Bessemer process – air blown into vess4el of molten iron to burn of the impurities 1868 – open hearth method was developed which further reduced the cost. Material Steel Steel is any range of alloys of iron and carbon – that contains less than 2 % carbon. – Ordinary structural steel called mild steel contains less then 3/10 of 1% carbon – Ordinary cast iron contains 3 to 4% carbon – To much carbon makes a hard but brittle metal – While to little produces a soft weak material – Thus mild steel is iron whose properties have been optimized for structural purposes Iron is produced in a blast furnace charged with alternating layers of iron ore, coke-coal without its volatile combustibility leaving only carbon, and limestone. Uwe R. Zimmer photo Process Coke burned by large quantity of air forced in from the bottom – produces a gas of carbon monoxide – which reacts with the ore to reduce it to elemental iron. Limestone forms a slag which reduces the impurities of the iron Molten iron is drawn off the bottom of the furnace – One ton of iron requires Today most steel is produced by basic oxygen process – other methods can be used – – 1 ¾ tons of iron ore ¾ ton of coke ¼ ton of limestone 4 tons of air In which a water cooled lance is lowered into a constancy of molten iron. Stream of pure oxygen at very high pressure is blown from the lance into the metal to burn off carbon and other impurities. Molten steel from the furnace is cast into molds to produce ingots Production of structural shapes Rolling process begins at the blooming mill – Then brought to structural mill with temp still above 2200 f Here it passes through a succession of rollers – – The hot ingots is reduced in size by squeezing it between rollers That press the metal into progressively more refined shapes and sizes Typically wide flange shapes passes through set of rollers around 40 times Then it is allowed to cool After cooling a straightening machine takes out minor crookedness And a saw cuts the lengths of steel into shorter pieces. Sizes and Shapes By varying the space of the rollers you can obtain a varied of shapes and sizes Wide flange uses for most column and beams American Standard I beam – less efficient structurally then wide flange – because roller arrangement that produces them is incapable of increasing the amount of steel to the flanges without also adding steel to the web – where it does little to increase the load carrying capacity. Symbols Wide flange W21 x 83 – – American Standard S18 x 70 – Sizes range from 4 to 18” in 2 in. increments 18 to 36” in 3” increments Sizes 3,4,5,6,7,8, 10,12, 15,18,20,24 Wide flange measurements – W12 x 24 used as a beam or girder – W12 x 336 intended for columns L4 x 3 x 3/8 Uses as lintels spanning doors and windows in masonry construction Steel buildings cut into short pieces and used to connect wide flange shapes Diagonal to braces steel frames Members of steel trusses Channels – – Depth actual 17 in. Steel angles – extremely versatile – – – – – Depth actual 12.22 inches flanges 6.49 in. C9 x 13.4 Used as truss members and bracing Tees, plates and bars – WT13 x 47 Open Web Steel Joists One of the many structural steel products fabricated A mass produced truss used in closely spaced arrays to support floors and roof decks Comes in three sizes – K series span 60 ft Depth 8 to 30 inches – LH series (long span) 96 ft Depth 18 to 48 inches – DLH series (deep long span) 144 ft Depth 52 to 72 inches Cold work steel Sheets of steel can be bent into c shaped, z shaped sections Channel stud Cee stud Double Stud Cee Joist Double Joist Joining Steel Members Rivets – a fastener consisting of a cylinder body and a formed head – Heated to a white color – Inserted through the holes in the member – With a pneumatic hammer to produce a second head opposite the first – As the rivet cools its shrinks and clamping the joined pieces together tightly Joining Steel Members Bolts – Two categories Carbon steel bolts – Also known as unfinished or common bolts – Similar to ordinary machine bolts that can be purchase at hardware store – Not very strong Highest strength bolts – Heat treated to develop necessary strength – Their connecting ability either is a shear connect – which stems from their shear resistance – Or from being tighten to the point that the members join are kept from slipping by the friction between the producing a fiction connection Joining Steel Members Welding – Offers a unique and valuable capability – Can join members of steel together as if they were one Joining Steel Members Shear Connections and Moment Connections – In order to understand the respective roles of shear connections and moment connections necessary to understand the means by which a building may be made stable against the lateral forces of wind and earthquakes Three basic mechanisms – – – Diagonal bracing Sheer panels Moment connections Sheer connection Diagonal bracing works by creating stable trim angles within the unstable geometry of a steel building The connections within a diagonally brace frame do not transfer moments They behave like pins which is another way of calling them shear connections Shear panels – rectangle panels made of steel or concrete – shear connections Moment connection Metal Decking Sheets of steel that have been corrugated to increase its stiffness Span determined by thickness Fireproofing Building fires are not hot enough to melt steel – but many are able to weaken it to cause structural failure Fireproofing – encasing steel in brick or concrete – but the weight added is considerably to the load and cost Metal lath and plaster Boards or slabs of gypsum or other fire resistance material – can serve as a finished surface Spray on materials #1 today –general consists of fibers.