Steel structures, design failures

In all blast-resistant structures (steel, concrete, or masonry) special attention should be given to the integrity of connections between structural elements up to the point of maximum response. For example, it is important to prevent premature brittle failure of welded connections to avoid stress concentrations or notches at joints in steel structures and to provide ductile reinforcement detailing in concrete/masonry structure connections. For all materials, it is recommended that connections be designed to be stronger than the connected structural members such that the more ductile member will govern the design over the more brittle connection. 

With aflame length of 13.9 m, the jet flame will impinge on the steel structure overhead. Consequently, the steel will see high convective and radiative heat fluxes on the order of 200 kW/m. Since the structure will be exposed to direct flame impingement, the expected failure time would be 3-4 minutes (Table 5-7) or less due to the high heat flux from the jet fire, depending on the type of steel structure and design factor of safety. 

To adequately demonstrate that the failures listed above are very unlikely, stress criteria consistent with ASME III, Div. 1, subsection NG for steel structures and ASME III, Div. 2 for the permanent side reflector and core support graphite govern their design. (Ref. 10) For the fuel and replaceable reflector element graphite, stress criteria currently under development will govern. 

RTR pipe designers also use a stress-strain curve similar to that used by steel pipe designers. However, instead of a yield point, they use what is called an empirical weep point, or the point of first crack (see Fig. 5-30). It is determined by either coupon or hydrostatic tests. The weep point is the point at which the matrix becomes excessively strained so that minute fractures begin to appear in the structural wall. At this point it is probable that in time even a more elastic liner will be damaged and allow water or whatever else the pipe is carrying to ooze or weep through the wall. As is the case with the yield point of steep pipe, reaching the weep point is not necessarily cataclysmic. The pipe can still continue to withstand quite a bit of additional load before it reaches the point of ultimate strain and failure. 

Since blast-resistant design is based on structural response beyond elastic limits into the inelastic range, buildings should be carefully designed in a manner that minimizes stress concentrations, brittle behavior, and abrupt failures. Some of the significant design considerations for steel and reinforced concrete blast-resistant structures are briefly summarized in this section. 

Limit state design methods are used in blast resistant design. These methods provide a comprehensive, reliable and realistic means of predicting failure mechanisms and structural capacities. Limit state design methods for structural steel, cold formed steel, reinforced concrete and reinforced masonry are available. However, as of now, no similar design specification is available for aluminum structures. 

Ductility limits for structural steel members are established such that gross member collapse due to failure of the member itself or its connections is precluded. It is presumed that local and gross member instabilities are prevented by providing adequate bracing and stiffeners. Shear failure modes are also to be precluded by design. Determination of failure mechanisms and corresponding capacities for flexural members and beam-columns arc adequately covered by the LRFD specifications. 

The steel shell that encloses the refractory is exposed to significant forces from the expansion of the refractory as well as the load from the refractory and the charge within the furnace. Similarly, the structures that support the furnace and the foundations must be designed to assure safe operation. A failure of any component can have serious consequences. 

Select low-temperature steels for fracture-critical structural members designed for tensile stress levels greater than a ksl (40 MPa) and specify a minimum Charpy V notch Impact energy absorption of 20 ft-lb (27 J) for base metal, heat-affected zones (HAZs), and welds when the structures are exposed to low-ambient temperatures. Fracture-critical members are those tension members whose failure would have a significant economic impact. 

Tests to measure the bond which can be obtained with the concrete of the structure to be strengthened are best carried out on the structure itself. A possibility is to utilise a pull-off test as developed for the non-destructive testing of concrete(21). A circular steel probe is bonded to the concrete surface and specially designed portable apparatus is then used to pull off the probe, along with a bonded mass of concrete, by applying a direct tensile force. Any defects in bond would be revealed by the occurrence of failures at the adhesive-concrete interface. 

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