Welding of structural steels is governed by the American Welding Society Structural Welding Code, AWS D1.1, the American Institute of Steel Construction Specification for the Design, Fabrication and Erection of Structural Steel for Buildings, or a local building code. AWS D1.1 specifies tests to be used in qualifying welders and types of welds. The AISC Specification and many building codes require, in general, that only qualified welds be used and that they be made only by qualified welders. Structural steels may be welded by shielded metal arc, submerged arc, gas metal arc, flux-cored arc, electroslag, electrogas, or stud-welding processes. Shielded-metal-arc welding fuses parts to be joined by the heat of an electric arc struck between a coated metal electrode and the material being joined, or base metal. The electrode supplies filler material for making the weld, gas for shielding the molten metal from the air, and flux for refining this metal. Submerged-arc welding fuses the parts to be joined by the heat of an electric arc struck between a bare metal electrode and base metal. The weld is shielded from the air by flux. The electrode or a supplementary welding rod supplies metal filler for making the weld. Gas-metal-arc welding produces fusion by the heat of an electric arc struck between a filler-metal electrode and base metal, while the molten metal is shielded by a gas or mixture of gas and flux. For structural steels, the gas may be argon, argon with oxygen, or carbon dioxide. Electroslag welding uses a molten slag to melt filler metal and surfaces of the base metal and thus make a weld. The slag, electrically conductive, is maintained molten by its resistance to an electric current that flows between an electrode and the base metal. The process is suitable only for welding in the vertical position. Moving, water-cooled shoes are used to contain and shape the weld surface. The slag shields the molten metal. Electrogas welding is similar to the electroslag process. The electrogas process, however, maintains an electric arc continuously, uses an inert gas for shielding, and the electrode provides flux. Stud welding is used to fuse metal studs or similar parts to other steel parts by the heat of an electric arc. A welding gun is usually used to establish and control the arc, and to apply pressure to the parts to be joined. At the end to be welded, the stud is equipped with a ceramic ferrule, which contains flux and which also partly shields the weld when molten. Preheating before welding reduces the risk of brittle failure. Initially, its main effect is to lower the temperature gradient between the weld and adjoining base metal. This makes cracking during cooling less likely and gives entrapped hydrogen, a possible source of embrittlement, a chance to escape. A later effect of preheating is improved ductility and notch toughness of base and weld metals and lower transition temperature of weld. When, however, welding processes that deposit weld metal low in hydrogen are used and suitable moisture control is maintained, the need for preheat can be eliminated. Such processes include use of lowhydrogen electrodes and inert-arc and submerged-arc welding. Rapid cooling of a weld can have an adverse effect. One reason that arc strikes that do not deposit weld metal are dangerous is that the heated metal cools very fast. This causes severe embrittlement. Such arc strikes should be completely removed. The material should be preheated, to prevent local hardening, and weld metal should be deposited to fill the depression. Pronounced segregation in base metal may cause welds to crack under certain fabricating conditions. These include use of high-heat-input electrodes, such as the 1/4-in E6020, and deposition of large beads at slow speeds, as in automatic welding. Cracking due to segregation, however, is rare with the degree of segregation normally occurring in hot-rolled carbon-steel plates. Welds sometimes are peened to prevent cracking or distortion, though there are better ways of achieving these objectives. Specifications often prohibit peening of the first and last weld passes. Peening of the first pass may crack or punch through the weld. Peening of the last pass makes inspection for cracks difficult. But peening is undesirable because it considerably reduces toughness and impact properties of the weld metal. (The adverse effects, however, are eliminated by a covering weld layer.) The effectiveness of peening in preventing cracking is open to question. And for preventing distortion, special welding sequences and procedures are simpler and

Design Requirements for Seismic Design Category A. Structures assigned to Seismic Design Category A need only comply with the following: Structure shall be provided with a complete lateral-force-resisting system designed to resist the minimum lateral force, of 1% floor gravity load. The gravity load should include the total dead load and other loads listed below. In areas used for storage, a minimum of 25% of the reduced floor live load (floor live load in public garages and open parking structures need not be included) Where an allowance for partition load is included in the floor load design, the actual partition weight or a minimum weight of 10 psf of floor area (whichever is greater) Total operating weight of permanent equipment 20% of flat roof snow load where flat roof snow load exceeds 30 psf The direction of application of seismic forces used in design shall be that which will produce the most critical load effect in each component. The design seismic forces are permitted to be applied separately in each of two orthogonal directions and orthogonal effects are permitted to be neglected. The effect of this lateral force shall be taken as E in the load combinations. Special seismic load combinations that include Em need not to be considered. TABLE 5.9 Design Coefficients and Factors for Basic Seismic-Force-Resisting Systems Basic seismic-force-resisting system

For engineered construction, some measure of quality control is always employed. In general, all measurements of materials including the cement and water should be by weight. The ACI 318 Building Code provides a sliding scale of overdesign for concrete mixes that is inversely proportional to the degree of quality control provided. In the sense used here, such overdesign is the difference between the specified and the actual average strength as measured by tests.
c Mixing and delivery of structural concrete may be performed by a wide variety of equipment and procedures: Site mixed, for delivery by chute, pump, truck, conveyor, or rail dump cars. (Mixing procedure for normal-aggregate concretes and lightweight-aggregate concretes to be pumped are usually different, because the greater absorption of some lightweight aggregates must be satisfied before pumping.) Central-plant mixed, for delivery in either open dump trucks or mixer trucks. Central-plant batching (weighing and measuring), for mixing and delivery by truck (dry-batched ready mix). Complete portable mixing plants are available and are commonly used for large building or paving projects distant from established sources of supply. Generally, drum mixers are used. For special purposes, various other types of mixers are required. These special types include countercurrent mixers, in which the blades revolve opposite to the turning of the drum, usually about a vertical axis, for mixing very dry, harsh, nonplastic mixes. Such mixes are required for concrete masonry or heavy-duty floor toppings. Dry-batch mixers are used for dry shotcrete (sprayed concrete), where water and the dry-mixed cement and aggregate are blended between the nozzle of the gun and impact at the point of placing. (Guide for Measuring, Mixing, Transporting, and Placing Concrete, ACI The ACI 318 Building Code requires prior approval by the engineer of admixtures to be used in concrete. Air Entrainment. Air-entraining admixtures (ASTM C260) may be interground as additives with the cement at the mill or added separately at the concrete mixing plant, or both. Where quality control is provided, it is preferable to add such admixtures at the concrete plant so that the resulting air content can be controlled for changes in temperature, sand, or project requirements. Use of entrained air is recommended for all concrete exposed to weathering or deterioration from aggressive chemicals. The ACI 318 Building Code requires air entrainment for all concrete subject to freezing temperatures while wet. Detailed recommendations for air content are available in Standard Practice for Selecting Proportions for Normal, Heavyweight, and Mass Concrete, ACI 211.1, and Standard Practice for Selecting Proportions for Structural Lightweight Concrete, ACI One common misconception relative to air entrainment is the fear that it has a deleterious effect on concrete strength. Air entrainment, however, improves workability. This will usually permit some reduction in water content. For lean, lowstrength mixes, the improved workability permits a relatively large reduction in water content, sand content, and water-cementitious materials ratio, which tends to increase concrete strength. The resulting strength gain offsets the strength-reducing effect of the air itself, and a net increase in concrete strength is achieved. For rich, high-strength mixes, the relative reduction in the ratio of water to cementitious materials, water-cementitious materials ratio, is lower and a small net decrease in strength results, about on the same order of the air content (4 to 7%). The improved durability and reduction of segregation in handling, because of the entrained air, usually make air entrainment desirable, however, in all concrete except extremely high-strength mixtures, such as for lower-story interior columns or heavy-duty interior floor toppings for industrial wear. Accelerators. Calcium chloride for accelerating the rate of strength gain in concrete (ASTM D98) is perhaps the oldest application of admixtures. Old specifications for winter concreting or masonry work commonly required use of a maximum of 1 to 3% CaCl2 by weight of cement for all concrete. Proprietary admixtures now available may include accelerators, but not necessarily CaCl2. The usual objective for use of an accelerator is to reduce curing time by developing 28-day strengths in about 7 days (ASTM C494). In spite of users familiarity with CaCl2, a number of misconceptions about its effect persist. It has been sold (sometimes under proprietary names) as an accelerator, a cement replacement, an antifreeze, a waterproofer, and a hardener. It is simply an accelerator; any improvement in other respects is pure serendipity. Experience, however, indicates corrosion damage from indiscriminate use of chloride- containing material in concrete exposed to stray currents, containing dissimilar metals, containing prestressing steel subject to stress corrosion, or exposed to severe wet freezing or salt water. The ACI 318 Building Code prohibits the use of calcium chloride or admixtures containing chloride from other than impurities from admixture ingredients in prestressed concrete, in concrete containing embedded aluminum, or in concrete cast against stay-in-place galvanized forms. The Code also prohibits the use of calcium chloride as an admixture in concrete that will be exposed to severe or very severe sulfate-containing solutions. For further information, see Chemical Admixtures for Concrete, ACI 212.3R. Retarders. Unless proper precautions are taken, hot-weather concreting may cause flash set, plastic shrinkage, cold joints, or strength loss. Admixtures that provide controlled delay in the set of a concrete mix without reducing the rate of strength gain during subsequent curing offer inexpensive prevention of many hotweather concreting problems. These (proprietary) admixtures are usually combined with water-reducing admixtures that more than offset the loss in curing time due to delayed set (ASTM C494). See Hot Weathering Concreting, ACI 305R, for further details on retarders, methods of cooling concrete materials, and limiting temperatures for hot-weathering concreting. Superplasticizers. These admixtures, which are technically known as high-range water reducers, produce a high-slump concrete without an increase in mixing water. Slumps of up to 10 in. for a period of up to 90 min can be obtained. This greatly facilitates placing concrete around heavy, closely spaced reinforcing steel, or in complicated forms, or both, and reduces the need for vibrating the concrete. It is important that the slump of the concrete be verified at the jobsite prior to the addition of the superplasticizer. This ensures that the specified water-cementitious materials ratio required for watertight impermeable concrete is in fact being achieved. The superplasticizer is then added to increase the slump to the approved