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Erection Bracing of Low-Rise Structural Steel Buildings phần 3

Trong một kháng cấu trúc không-tự-hỗ trợ sẽ được yêu cầu sau khi hoàn thành công việc erectors và sẽ được cần thiết cho đến khi nonstructural thép các yếu tố khác được đưa ra. Trong quá trình cương cứng của khung hình tự hỗ trợ và không-tự-hỗ trợ, điều kiện sẽ phát sinh yêu cầu kháng được cung cấp bởi các | Per ACI 318 0.70 is the factor for bearing on concrete and the value 2 represents the strength increase due to confinement. The design strength obtained from Eq. 4-14 must be compared to the strength obtained from the failure cones Eq. 4-13. The lower value provides the ultimate strength of the hooked rod to be used in the calculation for the bending moment design strength associated with rod pull out. Mn 2 Tnd Eq. 4-15 4.2.7 Anchor Rod Push Out of th e Bottom of th e Footing Anchor rod push out can occur when the rod is loaded to the point where a cone of concrete below the anchor rod is broken away from the footing. This failure mode is identical to anchor rod pull out but is due to a compressive force in the rod rather than a tension force. This failure mode does not occur when shim stacks are used when piers are present or when an additional nut is placed on the anchor rods just below the top of the footing as shown in Figure 4.17. SECTION A Fig. 4.16 Failure Cones be tack welded to the anchor rods to prevent the rod from turning during tightening operations. For hooked anchor rods an additional check must be made because hooked rods can fail by straightening and pulling out of the concrete. When this occurs the rods appear almost perfectly straight after failure. To prevent this failure mode from occurring the hook must be of sufficient length. The hook pullout resistance can be determined from the following equation ộTn 0.7 2 0.85 f cdhookLhook Eq.4-14 where t Tn Hook Bearing Design Strength kips fc the concrete compressive strength psi dhook the diameter of the anchor rod in. Lhook the length of the hook in. Fig. 4.17 Prevention of Push Out Shown in Figure 4.18 is the individual failure cone for a nutted anchor rod and the equation for Ae. The design strength for this mode of failure is Fig. 4.18 Push Out Cones Pn- 44 yiÃAc Eq. 4-16 where Ộ - .75 fc the concrete compressive strength psi 2003 by American Institute of Steel Construction Inc. All rights .