Taşıyıcı sistemi betonarmeden çeliğe dönüştürülmüş bir yüksek yapının bazı kabuller altında yaklaşık hesabı

Abstract

Yüksek Yapılar günümüzde gelişmiş ve gelişmekte olan ülkelerin büyükşehirlerinin metropol alanlarında yaygın biçimde inşa edilmektedir. Belirli bir kattan sonra, yapının çelik olarak inşası betonarmeden daha ucuza gelmekte ise de, ülkemizde daha ziyade betonarmenin yaygın kullanılması nedeniyle son yıllarda inşa edilmiş ve edilmekte olan tüm yüksek yapıların taşıyıcı sistemleri betonarmedir. Bu çalışmanın amacı, şu ana kadar ülkemizde denenmemiş olan Çelik Yüksek Yapı konusunda bir uygulama yaparak, yapının taşıyıcı sisteminin çelik elemanlarından teşkil edilemesi durumunda taşıyıcı sistemde ortaya çıkan kesitler ve kuvvetler hakkında yaklaşık bir fikir sahibi olmaktır. Ayrıca yatay yüklerin, yapının döşemelerinin düzenli diyafram çalışması yaptığı kabulüne göre düşey elemanlara dağıtılarak sistemin çözülmüş olması da, taşıyıcı sistemin çözümü için değişik bir ele alış biçimi oluşturmuştur.

Although it is cheaper to built the main structures by structural steel up from a certain height, all the carrying structural systems of the high rise buildings which have been - and are steel being - built by the last years are made of reinforced concrete because of the reason that the application and design methods of reinforced concrete are more widespread and better known in our country. The target of this work is to make an application on the concept of high rise steel structures which haven't been tried in our country up to now and to get an approximate opinion about the profiles and inner forces on the structural system which emerge when the carrying system of the structure is designed by steel elements. Moreover, solving the system according to the supposition that the floor plaques of the building show an orderly diaphragm behaviour under the influence of the lateral forces, presents a different kind of study for the calculation of the carrying system. The buildings which can be defined as high rise structures - with a number of storeys more than 12-15 - possess additional problems by comparision with the traditional problems of the lower buildings. One of the most important problem is that the lateral forces like wind, earthquake etc. become as effective as the vertical forces - even more - with the increase of the number of storeys. According to the generally accepted upper limits, the total lateral displacement of the building should be less than 1 /BOO of the total height of the building and the lateral displacement of any storey occuring because of. the deformation of that storey should be less than 1/500 of the height of the storey. In addition to these limitative upper criteria. the lateral forces must be transmitted to the soil by a safety manner, the entire stability of the building should be maintained and the dynamic effects have to be taken into consideration. Whatever the inner structure of the system - deigned to fulfill these conditions - is the general behavior of the building under the effects of the lateral loads is in the form of a vertical console beam rigitly connected to the ground by its base. The problem about, the entire stabilty of building is to prevent the fall of this console system and primarily related with the soil and foundation. But the positive effects of the structural system which facilitates the transmitting of the loads to the ground in an agreeable way is also remarkable. Dynamic effects can be transformed practically to a static calculation by using some statically characterized relative lateral foces which are relevant to the dominant period of the building, damping ratio and soil conditions. Except for some private conditions, this simple method of calculation delivers enough healty results. According to the "Code Concerning Buildings in Disaster Areas'* of the Ministrey of Public Improvements this kind of calculation can only be applied to the buildings with structural regularity and with a height less than 75 meters. Else a dynamic analysis is needed on the basi of superposition of modes with actual or idealised spectrums, model experiments, or if necessary, rigorous dynamic analysis for earthquake ground accelerati ons. High rise buildings - and others as well - should also be designed to resist torsional moments due to an eccentricity in each direction, calculated as the difference between the centres of mass and stiffness of any floor, plus B% of the largest plan dimension of the building, perpendicular to the direction of the lateral loads. The new code instructs the eccentricity to be increase BO?-S in cases of improtant torsional moments. According to the new earthquake resisting code lateral seismic forces will be considered acting separately in the main orthogonal axes X and Y of a building. It must be taken into account that wind and earthquake actions shall not be superposed and only the unfavourable one well be taken into consideration. Also the concept of "structural regularity" is taken into account in the new code by the following way : VI Structures are classified in two categories : a) Buildings with structural regularity b> Other buildings A building can be classified as regular when the conditions expressed below are simultaneously satisfied : - Structures consist of a number of vertical and continous columns and shear-walls connected by horizontal diaphragms rigid in their plane. - Buildings have approximately symmetrical plan configuration with respect to two main orthogonal directions. When re-entrant or recesses exist, their dimension does not exceed 25% of the building length. The stiffness or partition walls should be taken into account when the symmetry is considered. - The difference between the areas of two following storeys must not exceed 25%. - The eccentricity calculated as the difference between the centres of mass and stiffness of any floor is less than B% of the building dimension perpendicular to the lateral forces direction. - The stiffness and mass should change slowly along the building heigth, the difference between two consecutive floors being less than S0%. In a country where seismic activity is frequent, structural steel is expected to be used widely in many civil engineering constructions. Unfortunately that is not the case in Turkey and except indutrial structures and some large span roofs steel is likely non used. Beyond the main reasons of this situation which have been mentioned at the begining of this summary, it is also important to make clear that the advantages steel can provide are not widely known in the mass of the population and also in some decision -maker engineers who are working in the construction industry. In the conception of this work, a high rise structure, which has been actually built with reinforced concrete, is formed and examined with structural steel without disturbing its architectural design, under the following superpositions in order to get - not exact - but only approximate solutions about the inner forces of the system and concerning profiles. VII By the execution of the statical solution, the supposition has been made that the "cast in-place" reinforced concrete floor plaques of the building form an ordely diaphragm and the lateral forces are transmitted to the bracings directly over these floor plaques. In this case the bracings systems become two dimensional and each one is under the effect of its related lateral loads. The stability of the whole system is accepted to be obtained by the rigid cast in-place reinforced concrete floor plaque and the column - beam - bracing connections are arranged as none moment transferring ones. In the following drawing, the architectural plan of the building with the core and stiffness walls can be seen. -.9, S S m VIII In high rise steel structures, the lateral forces are transmitted to the soil by means of vertical bracing systems which are rigitly connected to the ground» in order to limit the horizontal displacements and to avoid getting none economic cross section of steel profiles. Instead of the breacing systems, the so called stiffness walls can also be constructed by cast in-place reinforced concrete. For the floor plaques concerning itself,, a vertical steffness wall - in case a mass of inertia - cooreponds to a gliding hinge support - in realty an elastic support - concerning the motion in its floor plane. It is also possible to design the stiffness walls around the stairs or lift spaces etc. if the height of the structure - and the number of storeys - increase. In such a case this kind of stiffness wall application resembles the formation of a core. In appropriately projected carrying systems with core formation, the lateral forces are transmitted to the soil by means of the core and the columns are influenced only by the vertical loads. By that kind of design it is fairly important to arrange the position(s) of the core(s) in such a way that the torsional motion of the building under the effects of the lateral loads could be prevented. Otherwise the columns would get too much shearing forces.