【文章內(nèi)容簡介】 hreedimensional concepts, or by two dimensional analogy, where possible, whichever method is used, it must be capable of accounting for the effects of shear lag. The presence of shear lag, detected first in aircraft structures, is a serious limitation in the stiffness of framed tubes. The concept has limited recent applications of framed tubes to the shear of 60 stories. Designers have developed various techniques for reducing the effects of shear lag, most noticeably the use of belt trusses. This system finds application in buildings perhaps 40stories and higher. However, except for possible aesthetic considerations, belt trusses interfere with nearly every building function associated with the outside wall。 the trusses are placed often at mechanical floors, mush to the disapproval of the designers of the mechanical systems. Nevertheless, as a costeffective structural system, the belt truss works well and will likely find continued approval from designers. Numerous studies have sought to optimize the location of these trusses, with the optimum location very dependent on the number of trusses provided. Experience would indicate, however, that the location of these trusses is provided by the optimization of mechanical systems and by aesthetic considerations, as the economics of the structural system is not highly sensitive to belt truss location. TubeinTube Structures The tubular framing system mobilizes every column in the exterior wall in resisting overturning and shearing forces. The term?tubeintube?is largely selfexplanatory in that a second ring of columns, the ring surrounding the central service core of the building, is used as an inner framed or braced tube. The purpose of the second tube is to increase resistance to over turning and to increase lateral stiffness. The tubes need not be of the same character。 that is, one tube could be framed, while the other could be braced. In considering this system, is important to understand clearly the difference between the shear and the flexural ponents of deflection, the terms being taken from beam analogy. In a framed tube, the shear ponent of deflection is associated with the bending deformation of columns and girders (, the webs of the framed tube) while the flexural ponent is associated with the axial shortening and lengthening of columns (, the flanges of the framed tube). In a braced tube, the shear ponent of deflection is associated with the axial deformation of diagonals while the flexural ponent of deflection is associated with the axial shortening and lengthening of columns. Following beam analogy, if plane surfaces remain plane (, the floor slabs),then axial stresses in the columns of the outer tube, being farther form the neutral axis, will be substantially larger than the axial stresses in the inner tube. However, in the tubeintube design, when optimized, the axial stresses in the inner ring of columns may be as high, or even higher, than the axial stresses in the outer ring. This seeming anomaly is associated with differences in the shearing ponent of stiffness between the two systems. This is easiest to understand where the inner tube is conceived as a braced (, shearstiff) tube while the outer tube is conceived as a framed (, shearflexible) tube. Core Interactive Structures Core interactive structures are a special case of a tubeintube wherein the two tubes are coupled together with some form of threedimensional space frame. Indeed, the system is used often wherein the shear stiffness of the outer tube is zero. The United States Steel Building, Pittsburgh, illustrates the system very well. Here, the inner tube is a braced frame, the outer tube has no shear stiffness, and the two systems are coupled if they were considered as system