Portal Lightweight Steel Support Structure And Dynamic Performance Analysis

The steel output of China has been successively at the top of the world for many years, but the proportion of that for the construction industry is still very small. Therefore, developing steel structure for the construction industry has become a new architectural requirement. To meet the need of economic development, the Ministry of Construction of China included "steel structure of light-weight buildings with gabled frames" in the "new technique and new product exploitation project" as part of the "quick house-building system" in 1994. Then the Technical Specification for Steel Structure of Light-weight Buildings with Gabled Frames was promulgated and revised, which has markedly stimulated the development of portal frames in China. At present, steel structure of light-weight buildings with gabled frames has become one of the most widely-used structure forms.Since the Technical Specificaion for Steel Structure of Light-weight Buildings with Gabled Frames was worked out very late, it is imperfect and unclear on some problems, such as the calculation method for the internal force of the bracing system and the anti-seismic analysis of steel structure of light-weight buildings with gabled frames and so on. This paper is mainly aiming at the bracing system, the dynamic characteristics and the anti-seismic analysis ofteel structure of light-weight buildings with gabled frames of single span and single story without crane. Some reasonable suggestions are put forward.As for the internal force of the bracing system of teel structure of light-weight buildings with gabled frames, it is often assumed that the braces are subjected to tension only and the simplified calculation method is adopted. The method assumes that every passage brace supports the same amount of longitudinal horizontal force. This is different from the transmitting force between the braces in the bracing system. Therefore, space finite element model and plane finite element model are set up by using the software ANSYS in this paper. Through the comparative analysis of the internal force of the braces obtained by these three methods, the problems lying in the simplified method are discovered and the plane simplified method is suggested (see the following chart) . By using the structure mechanics, the formulae for the greatest internal force of every passage bracing system are deduced for the gabled frame with two-passage bracing system and that with three-passage bracing system. Through the comparison of the results with those from the space finite element model, it is indicated that the results of the plane simplified method are reasonable for the gabled frames with two-passage bracing system and that the results of the structure with three-passage bracing system are amended. As for the struture with three-passage bracing system, the greatest internal force of the first passage must be magnified 15% while that of the second must be reduced 15%. It is discovered that the results obtained in this way are very close to those from the space finite element model and more reasonable.(a) The plane simplified for the structure with two-passage bracing system(b) The plane simplified for the structure with two passages bracing systemModel analyses for the space finite element models with purlin and without purlin are made, which demonstrate that the first whole vibration mode is transverse vibrancy and that purlin can prevent part vibrancy and simulate the actual vibrancy of the structure more accurately. The natural periods calculated by the single-mass system, PKPM and ANSYS differ only slightly. So the single-mass system is practicable and the formula for the natural period is as follows:Where Ib Ic-equivalent moment of inertia of the beam and the column;H H1 span and height of cornice;Geq equivalent gravity load;g acceleration of gravity;E modulus of elasticity of steel, E=2.06x~10~11 N/m2.An analysis of the regularity of the natural period manifests: the natural period rises in a linear way with the increase of the load; it also rises with the increase of the number of the passages of the bracing system.The formula for the ratio of the shear of transverse wind to that of horizontal earthquake is given to learn about the different influences they have on the system.max eqwhere Fwk, Fek- shear of transverse wind and horizontal earthquake action;L, H, H1- length, span and height of the buildings;TgT- design characteristic period of ground motion, nature period;The following conclusions are drawn from many engineering examples: CD when the basic wind pressure is less than 0.5kN/m2, the seismic fortification intensity is 8 to 9 and the type of the site is III to IV, the shear of the horizontal earthquake is greater than that of the transverse wind; (2) when seismic fortification intensity is 7, the effect of the shear of the horizontal earthquake is still too great. Hence the seismic design of portal frames should be paid more attention to.In practice, angle strut is often applied to the bracing system, but how to calculate the internal force of the angle strut is unclear. It is normally assumed to be the tension. In this paper, the finite element models of the structure with angle struts taking for the tension-only brace and the finite element models of the structure with angle struts subjected to tension and compression and the finite element models of the structure with the rod braces are set up. The internal force of the braces and frames and the horizontal displacement of the structure from the different models are calculated. A comparison of the results shows that the angle strut designed according to the limitation of the slender ratio can not only bear tension and compression but also enhance the load-carrying capacity of the bracing system and the gabled frames and the laterally stiffness of the structure.