Capped steels are made much like rimmed steels but with less rimming action. Capped steels have less segregation. They are used to make sheet, strip, skelp, tinplate, wire, and bars. Semikilled steel is deoxidized less than killed steel. Most deoxidation is accomplished with additions of a deoxidizer to the ladle. Semikilled steels are used in structural shapes and plates. Killed steels usually are deoxidized by additions to both furnace and ladle. Generally, silicon compounds are added to the furnace to lower the oxygen content of the liquid metal and stop oxidation of carbon (block the heat). This also permits addition of alloying elements that are susceptible to oxidation. Silicon or other deoxidizers, such as aluminum, vanadium, and titanium, may be added to the ladle to complete deoxidation. Aluminum, vanadium, and titanium have the additional beneficial effect of inhibiting grain growth when the steel is normalized. (In the hot-rolled conditions, such steels have about the same ferrite grain size as semikilled steels.) Killed steels deoxidized with aluminum and silicon (made to fine-grain practice) often are specified for construction applications because of better notch toughness and lower transition temperatures than semikilled steels of the same composition. While plates and shapes for construction applications can be obtained from processes other than casting and rolling of ingots, such as continuous casting, most plates and shapes are made by hot-rolling ingots (Art. 4.46). But usually, the final products are not rolled directly from ingots. First, the ingots are generally reduced in cross section by rolling into billets, slabs, and blooms. These forms permit correction of defects before finish rolling, shearing into convenient lengths for final rolling, reheating for further rolling, and transfer to other mills, if desired, for that

A moderate rise in temperature of structural steel, say up to 500F, is beneficial in that the strength is about 10% greater than the normal value. Above 500F, strength falls off, until at 700F it is nearly equal to the normal temperature strength. At a temperature of 1000F, the compressive strength of steel is about the same as the maximum allowable working stress in columns. Unprotected steel members have a rating of about 15 min, based on fire tests of columns with cross-sectional areas of about 10 in2. Heavier column, possessing greater mass for dissipation of heat, afford greater resistance20 min perhaps. Columns with reentrant space between flanges filled with concrete, but otherwise exposed, have likewise been tested. Where the total area of the solid cross section approximates 36 in2, the resistance is 30 min, and where the area is 64 in2, the resistance is 1 hr. The average coefficient of expansion for structural steel between the temperatures of 100 and 1200F is given by the formula C  0.0000061  0.0000000019t (7.81) in which C  coefficient of expansion per F and t  temperature, F. Below 100F, the average coefficient of expansion is taken as 0.0000065. The modulus of elasticity of structural steel, about 29,000 ksi at room temperature, decreases linearly to 25,000 ksi at 900F. Then, it drops at an increasing rate at higher temperatures. Steel members, such as spandrel beams and columns, on the exterior of a building may sometimes be left exposed or may be protected in an economical manner from fire damage, whereas interior steel members of the same building may be required to be protected with more expensive insulating materials, as discussed in Art. 7.51. Standard fire tests for determining fire-endurance ratings of exterior steel members are not available. But from many tests, data have been obtained that provide a basis for analytical, thermodynamic methods for fire-safe design. (See for example, Fire- Safe Structural SteelA Design Guide, American Iron and Steel Institute, 1101 17th St., N.W., Washington, DC 20036.) The tests indicate that an exterior steel spandrel beam with its interior side protected by fire-resistant construction need only have its flanges fire protected. This may be simply done by application of fireproofing, such as sprayed-on mineral fibers, to the upper surface of the top flange and the under surface of the bottom flange. In addition, incombustible flame-impingement shields should enclose the flanges to deflect flames that may be emitted through windows. The shields, for example, may be made of 1/4-in-thick weathering steel. This construction prevents the temperature of the spandrel beam from reaching a critical level. Exposed-steel columns on the outside of a building may be made fire safe by placement at adequate distances from the windows. Such columns may also be located closer to the building when placed on the side of windows at such distances FIGURE 7.64 Fire protection of steel columns by encasement with (a) concrete, (b) plaster on gypsum lath, (c) plaster on metal lath, (d ) furring and gypsumboard, (e) gypsumboard without furring, and ( ) gypsum block and plaster. that the steel is protected by the building walls against flame impingement. Thermodynamic analysis can indicate whether or not the chosen locations are fire safe. Structured steel may be protected with any of many materialsbrick, stone, concrete, gypsumboard, gypsum block, sprayed-on mineral fibers, and various fireresistant

All bars must be bent without heating, except as permitted by the engineer. Bars partly embedded in hardened concrete may not be bent without permission of the engineer. No welding of crossing bars (tack welding) is permitted without the approval of the engineer. For unusual bends, heating may be permitted because bars bend more easily when heated. If not embedded in thin sections of concrete, heating the bars to a maximum temperature of 1500F facilitates bending, usually without damage to the bars or splitting of the concrete. If partly embedded bars are to be bent, heating controlled within these limits, plus the provision of a round fulcrum for the bend to avoid a sharp kink in the bar, are essential. Tack welding creates a metallurgical notch effect, seriously weakening the bars. If different size bars are tacked together, the notch effect is aggravated in the larger bar. Tack welding therefore should never be permitted at a point where bars are to be fully stressed, and never for the assembly of ties or spirals to column verticals or stirrups to main beam bars. When large, preassembled reinforcement units are desired, the engineer can plan the tack welding necessary as a supplement to wire ties at points of low stress or to added bars not required in the design. Bar supports are commercially available in three general types of material: wire, precast concrete, and all-plastic. Descriptions of the various types of bar supports, as well as recommended maximum spacings and details for use, are given in the CRSI Manual for Standard Practice. Wire bar supports are generally available in the United States in three classes of rust prevention: plastic-protected, stainless-steel-protected, and no protection (plain). Precast-concrete bar supports are normally supplied in three styles; plain block, block with embedded wires, and block with a hole for the leg of a vertical bar for top- and bottom-bar support. Various types and sizes of all-plastic bar supports and sideform spacers are available. Consideration should be given to the effects of thermal changes, inasmuch as the coefficient of thermal expansion of the plastic can differ significantly from that of concrete. Investigation of this property is advisable before use of all-plastic supports in concrete that will be exposed to high variations in temperature. Bar supports for use with epoxy-coated rebars should be made of dielectric material. Alternatively, wire bar supports should be coated with dielectric material, such as plastic or epoxy. This involves approval of rebar material for conformance to the physical properties required, such as ASTM specifications for the strength grade specified; approval of the bar details and placing drawings; approval of fabrication to meet the approved details within the prescribed tolerances; and approval of rebar placing. Approvals of rebar material may be made on the basis of mill tests performed by the manufacturer for each heat from which the bars used originated. If samples are to be taken for independent strength tests, measurements of deformations, bending tests, and minimum weight, the routine samples may be best secured at the mill or the fabrication shop before fabrication. Occasionally, samples for check tests are taken in the field; but in this case, provision should be made for extra lengths of bars to be shipped and for schedules for the completion of such tests before the material is required for placing. Sampling at the point of fabrication, before fabrication,