t  0.4 (7.50) Fyc where t  thickness of column flange, in. The thickness of a stiffener should not be less than one-half the thickness of the beam flange or plate that delivers force P to the column. Stiffener width should not be less than one-third of the flange or plate width. 7.22.2 LRFD for Load-Bearing Stiffeners Six limit states should be considered at locations where a large concentrated force acting on a member introduces high local stresses. These limit states are local flange bending, local web yielding, web crippling, sidesway web buckling, compression buckling of the web, and high shear in column web panels. Detailed requirements for determining the design strength for each of these limit states are contained in the AISC LRFD Specification for Structural Steel for Buildings. When web stiffeners are required to prevent web crippling or compression buckling of the web, they are designed as columns with an effective length of Kl  0.75h, where h is the clear distance between flanges. The effective cross section is the area of the stiffeners plus 25t for interior stiffeners for 12t for stiffeners at the end of a member, where t is the web thickness. For bearing on finished surfaces, such as milled ends and ends of fitted bearing stiffeners, or on the projected area of pins in finished holes, the allowable stress in

Suboptimization of the system may be achieved by first optimizing the footings, for example, designing the lowest-cost footings. Next, the design of both the columns and the footings should be optimized. (Optimization of the columns alone will not yield an optimum structural system, because of the effect of the column weight on the footings.) Finally, roof, columns, and footings together should be optimized. (Optimization of the roof alone will not yield an optimum structural system, because of the effect of its weight on columns and footings. A low-cost roof may be very heavy, requiring costly columns and footings, whereas the cost of a lightweight roof may be so high as to offset any savings from less-expensive columns and footings. An alternative roof may provide optimum results.) Appraisal. In step 5 of systems design, the designers should evaluate the results obtained in step 4, modeling the system and applying the model. The designers should verify that construction and life-cycle costs will be acceptable to the client and that the proposed system satisfies all objectives and constraints. During the preceding steps, value analysis may have been applied to parts of the building. In step 6, however, value analysis should be applied to the whole building system. This process may result in changes only to parts of the system, producing a new system, or several alternatives to the original design may be proposed. In steps 7 and 8, therefore, the new systems, or at least those with good prospects, should be modeled and evaluated. During and after this process, completely different alternatives may be conceived. As a result, steps 4 through 8 should be repeated for the new concepts. Finally, in step 9, the best of the systems studied should be selected. (R. J. Aguilar, Systems Analysis and Design in Engineering, Architecture Construction and Planning, Prentice-Hall, Inc., Englewood Cliffs, N.J.: R. L. Ackoff and M. W. Saseini, Fundamentals of Operations Research, John Wiley & Sons, Inc., New York; K. I. Majid, Optimum Design of Structures, Halsted Press/Wiley, New York; E. J. McCormick, Human Factors in Engineering, McGraw-Hill Publishing Company, New York; F. S. Merritt and J. A. Ambrose, Building Engineering and Systems Design, 2nd Ed., Van Nostrand Reinhold, New York; R. DeNeufville and J. H. Stafford, Systems Analysis for Engineers and Managers, McGraw-Hill Publishing Company, New York; L. Spunt, Optimum Structural Design, Prentice-Hall, Englewood Cliffs, N.J.) Many of the restrictions encountered in building design are imposed by legal regulations. While all must be met, those in building codes are the most significant because they affect almost every part of a building. Building codes are established under the police powers of a state to protect the health, welfare, and safety of communities. A code is administered by a building official of the municipality or state that adopts it by legislation. Development of a local code may be guided by a model code, such as those promulgated by the International Conference of Building Officials, Inc., Building Officials and Code Administrators International, Inc., and Southern Building Code Congress International,

N
(10.32) P sin2   Q cos2  where N, P, and Q are, respectively, the adjusted design value, lb, or stress, psi, at inclination  with the direction of grain, parallel to grain, and perpendicular to grain. Design values are based on the assumption that the wood at the joint is clear and relatively free from checks, shakes, and splits. If knots are present in the longitudinal projection of the net section within a distance from the critical section of half the diameter of the connector, the area of the knot should be subtracted from the area of the critical section. It is assumed that slope of the grain at the joint does not exceed 1 in 10. The stress, whether tension or compression, in the net area, the area remaining at the critical section after subtracting the projected area of the connectors and the bolt from the full cross-sectional area of the member, should not exceed the design value of clear wood in compression parallel to the grain. Tables in the NDS list the least thickness of member that should be used with the various sizes of connectors. The design values listed for the greatest thickness of member with each type and size of connector unit are the maximums to be used for all thicker material. Design values for members with thicknesses between those listed may be obtained by interpolation. The NDS also lists minimum end and edge distances and spacing for timber connectors (Table 10.37). Edge distance is the distance from the edge of a member to the center of the connector closest to that edge and measured perpendicular to the edge. End distance is measured parallel to the grain from the center of the connector to the square-cut end of the member. If the end of the member is not cut normal to the longitudinal axis, the end distance, measured parallel to that axis from any point on the center half of the connector diameter that is perpendicular to the axis, should not be less than the minimum end distance required for a squarecut member. Spacing of connectors is measured between their centers along a line