Paint selection and surface preparation are a matter of economics. For example, while blast-cleaned surfaces are concealed to be the best paint foundation for lasting results, the high cost is not always justified. Nevertheless, the Council specifies a minimum surface preparation by a blast cleaning process for such paints as alkyd, phenolic, vinyl, coal tar, epoxy, and zinc-rich. As an aid for defining and evaluating the various surface preparations, taking into account the initial condition of the surface, an international visual standard is available and may be used. A booklet of realistic color photographs for this purpose can be obtained from the Council or ASTM. The applicable standard and acceptance criteria are given in Quality Criteria and Inspection Standards, American Institute of Steel Construction. The Council stresses the relationship between the prime coat (shop paint) and the finish coats. A primer that is proper for a particular type of field paint could be an unsatisfactory base for another type of field paint. Since there are numerous paint formulations, refer to Council publications when faced with a painting condition more demanding than ordinary. In the absence of specific contract requirements for painting, the practice described in the AISC Specification for Structural Steel for Buildings may be followed. This method may be considered nominal. The steel is brushed, by hand or power, to remove loose mill scale, loose rust, weld slag, flux deposit, dirt, and foreign matter. Oil and grease spots are solvent cleaned. The shop coat is a commercial- quality paint applied by brushing, dipping, roller coating, flow coating, or spraying to a 2-mil thickness. It affords only short-time protection. Therefore, finished steel that may be in ground storage for long periods or otherwise exposed to excessively corrosive conditions may exhibit some paint failure by the time it is erected, a condition beyond the control of the fabricator. Where such conditions can be anticipated, as for example, an overseas shipment, the engineer should select the most effective paint system. There is some question as to justification for protecting steelwork embedded in masonry or in contact with exterior masonry walls built according to good workmanship standards but not impervious to moisture. For example, in many instances, the masonry backing for a 4-in brick wall is omitted to make way for column flanges. Very definitely, a 4-in wall will not prevent penetration of water. In many cases, also, though a gap is provided between a wall and steelwork, mortar drip- pings fall into the space and form bridges over which water may pass, to attack the steel. The net effect is premature failure of both wall and steel. Walls have been shatteredsheared through the brickby the powerful expansion of rust formations. The preventatives are: (1) coating the steel with suitable paint and (2) good

2. Determine the required panel grade and thickness and the nailing schedule from Table 10.23. Check the anchor bolts in the sill plate for shear. 3. Check wall framing on each end of the shear wall and design a foundation anchor or hold-down, if required (see Fig. 10.10). 10.12.11 Panel Roof Sheathing Table 10.24 lists maximum uniform roof live loads for APA Rated Sheathing Exposure 1, and Structural I Rated Sheathing, Exposure 1 or Exterior. Uniform-load deflection limits are 1/180 of the span under live load plus dead load, and 1/240 under live load only. Panels are assumed continuous over two or more spans with the long dimension or strength axis across supports (Fig. 10.11). Special conditions, such as heavy concentrated loads, may require constructions in excess of these minimums, or allowable live loads may have to be decreased for dead loads greater than 10 psf, for example, for tile roofs. Good performance of built-up, single-ply, or modified bitumen roofing applied on low-slope roofs requires a stiffer deck than does prepared roofing applied on pitched roofs. Although Span-Rated panels used as roof sheathing at maximum span are adequate structurally, an upgraded system is recommended for low-slope roofs. Table 10.25 lists maximum spans for low-slope roof decks. Live loads can be determined from Table 10.24, and minimum fastener requirements are given in

Y (x)  sin (5.258) n L The constants c1 and c2 in Eq. (5.254) are determined by the initial conditions of the disturbance. Thus, the total deflection, by superposition of modes, is  nx y   A (t) sin (5.259) n L n1 where An(t) is determined by the load (see Art. 5.18.4). Equations (5.254) to (5.256) apply to spans with any type of end restraints. Figure 5.109 shows the characteristic shape and gives constants for determination FIGURE 5.109 Coefficients for computing natural circular frequencies  and natural periods of vibration T, s, of prismatic beams. w  weight of beam, lb / lin ft; L  span, ft; E  modulus of elasticity of the beam material, psi; I  moment of inertia of the beam cross section, in4. of natural circular frequency  and natural period T for the first four modes of cantilever simply supported, fixed-end, and fixed-hinged beams. To obtain , select the appropriate constant from Fig. 5.109 and multiply it by EI /wL4. where L  span of beam, ft. To get T, divide the appropriate constant by EI /wL4. To determine the characteristic shapes and natural periods for beams with variable cross section and mass, use the Rayleigh method. Convert the beam into a lumped-mass system by dividing the span into elements and assuming the mass of each element to be concentrated at its center. Also, compute all quantities, such as deflection and bending moment, at the center of each element. Start with an assumed characteristic shape and apply Eq. (5.255). Methods are available for dynamic analysis of continuous beams. (R. Clough and J. Penzien, Dynamics of Structures, McGraw-Hill Book Company, New York; D. G. Fertis and E. C. Zobel, Transverse Vibration Theory, The Ronald Press Company, New York.) But even for beams with constant cross section, these procedures are very lengthy. Generally, approximate solutions are preferable. (J. M. Biggs, Introduction to Structural Dynamics, McGraw-Hill Book Company, New York; N. M. Newmark and E. Rosenblueth, Fundamentals of Earthquake Engineering, Prentice-Hall, Englewood Cliffs, N.J.) 5.18.3 Impact and Sudden Loads Under impact, there is an abrupt exchange or absorption of energy and drastic change in velocity. Stresses caused in the colliding members may be several times larger than stresses produced by the same weights applied statically. An approximation of impact stresses in the elastic range can be made by neglecting the inertia of the body struck and the effect of wave propagation and assuming that the kinetic energy is converted completely into strain energy in that body. Consider a prismatic bar subjected to an axial impact load in tension. The energy absorbed per unit of volume when the bar is stressed to the proportional limit is called the modulus of resilience. It is given by /2E, where y is the yield 2y stress and E the modulus of elasticity, both in psi. Below the proportional limit, the unit stress, psi, due to an axial load U, in-lb,