【正文】 l loads in highrise buildings and resulting in costeffective columnfree interior space with a high ratioof to gross floor area. Because of the contribution of the stressedskin faade the framed members of the tuberequire less mass and are thus lighter and less expensive. All the typical columns and spandrelbeams are standard rolled shape sminimizing the use and cost of special builtup members. The depth requirement for the perimeter spandrel beams is also reduced and the need for up setbeams above floors which would encroach on valuable space is minimized. The structural system has been used on the 54story One Mellon Bank Center in Pittburgh. Systems in concrete. While tall buildings constructed of steel had an early start development of tall buildings of reinforced concrete progressed at a fast enough rate to provide a petitive chanllenge to structural steel systems for both office and apartment buildings. Framed tube. As discussed above the first framed tube concept for tall buildings was usedfor the 43story DeWitt Chestnut Apartment Building. In this building exterior columns were spaced at centers and interior columns were used as needed to support the 8in .thick 20m flatplate concrete slabs. Tube in tube. Another system in reinforced concrete for office buildings bines the traditional shear wall construction with an exterior framed tube. The system consists of an out erframed tube of very closely spaced columns and an interior rigid shear wall tube enclosing the central service area. The system Fig .2 known as the tubeintube system made it possible to design the world’s present tallest 714ft or 218mlightweight concrete building the 52storyOne Shell Plaza Building in Houston for the unit price of a traditional shear wall structure of only 35 stories. Systems bining both concrete and steel have also been developed an example of which is the posite system developed by skid more Owings ampMerril in which an exteri. An optical micrograph and a phase map of the asreceived base material are shown in Fig. 4. The base material has atypical microstructure of wrought duplex stainless steels consisting of ferrite matrix with austenite islands. OIM analysis revealed that the austenite islands contained a higher number of grain boundaries (mostly twin type boundaries) than the ferrite. OIM analysis also revealed that the ferrite content was about 51%. Average grain sizes of austenite and ferrite phases in the base material were about and m, respectively. Optical microstructures of regions “A,” “B,” “C” and “D” shown in Fig. 3 are indicated in Fig. 5. Region B lies on the weld centre, and region D is located on the border of the stir zone and TMAZ. Regions A and C are located around 2 mm away from the weld centre at the retreating and advancing sides, respectively. Region B has the microstructure consisting of ferrite matrix with the more elongated austenite islands. Austenite islands of region B look finer than those of the base material. Region A has the similar microstructure to region B, while region C seems to contain finer austenite islands than region B. Distribution of the austenite islands is finest in the stir zone at the advancing side, as shown in micrograph of region D. In this region, D, the austenite in the stir zone exhibits an average grain size of lying immediately adjacent to elongated austenite islands in the TMAZ. Phase maps of regions, CEN and in the weld are shown in All regions consist of a ferrite matrix with the austenite islands similar to that of the base material. Distribution of the austenite islands in regions and CEN is similar to that in the base material, but the austenite islands contain more grain boundaries than the base material. The grain size profile ( Fig. 8) showed that the austenite and ferrite grains in the stir zone were smaller than those in the base material. Additionally, the phase maps showed that both the austenite and ferrite phases in the stir zone did not exhibit a heavily deformed microstructure, . many low angle grain boundaries. Both the grain size profile and phase maps suggest that dynamic recrystallisation occurred both in the austenite and ferrite phases during FSW. It is generally known that dynamic recrystallisation easily occurs in the austenitic stainless steels, while ferritic steels hardly experience dynamic recrystallisation because the ferrite phase has a high stacking fault energy. In the case of the duplex stainless steels, however, deformation is localized in the ferrite matrix at high temperatures, because the ferrite phase is relatively weaker than the austenite. Consequently, the recrystallised grains are often formed in ferrite phase more easily than in austenite phase. Some research] suggests that the recrystallised grains in the ferrite phase are formed By continuous dynamic recrystallisation, which is characterized by straininduced progressive rotation of