Researches On The Methods Of Limit To Storey Stiffness Ratio Of Reinorced Concrete Frame Structure

It is well known that the structure's seismic behavior depends on the total bearing capacity, deformation and dissipation energy capacity,it also has great relationship with the existence of the partial weakness position.The formation of the structure's weakness position always due to the discontinuity of stiffness and the yield strength ratio.The enlargement of the storey height of a frame structure can lead to the discontinuity of stiffness and bearing capacity, it may cause the stress of the storey increased and the plastic deformation concentrated, or even destroyed or collapsed.In this article,the author focused on the discipline of vertical stiffness sudden change and the influence on seismic behavior which due to the change of storey height in a reinforced concrete frame structure, and also give suggestions on controlling methods of storey lateral stiffness ratio in frame structure.The article analyses and compares the three ways on the calculations of storey lateral stiffness ratio that existed.It designed nine structural models according to the bottom, centeral and upper parts with various storey height.These examples are designed to evaluate the seismic performance of buildings based on the elastic calculation under frequent earthquake, the static ealstoplastic analysis and dynamic elastoplastic time-history analysis under rare earthquake.In consideration of the complexity of the change in first storey height in in real construction cases, we did three tests of ten-storey frame structure shaking table model, the bottom storey height of the prototype of the three models are 5.4m,6.8m,7.8m. the storey height from 2 to 10 are all 3.6m, and was designed under the scale of 1:10. The vertical additional weight of Model 1,2,3 are 500.78kN,511.72kN and 517.82kN respectively, the stress and strain under gravity load is close to the actual structure, the input peak acceleration of earthquake wave is 1:1. In order to have a clear and general knowledge of the characteristic of earthquake response, fracture morphology and failure mechanism under earthquakes in frame structure which has sudden change in lateral stiffness due to the change of storey height, Model 1 and 2 conducted two stages to test the dynamic response under uni-directional seismic input and the collapse under bi-direction seismic input. Due to the the damage of model 3 under the uni-directional earthquake,model 3 only conducted one stage test.Through detailed calculations and analysis, especiallly on the three tests of shaking table model, We get the following creative results.1. Based on the calculation and analysis of the shaking table tests, it proved that we should use storey shear and the inter-storey drift ratio to impose restrictions on storey lateral stiffness ratio of the reinforced concrete frame structure,to avoid emerging soft storey or even the whole storey's damage or collapse under severe earthquake; The adaption of inter-story drift angle ratio and equivalent shear stiffness ratio to control the storey lateral stiffness ratio is not safe in condition of the sudden changes of lateral rigidity in frame structure due to the changes of storey height. It can't prevent the soft storey under strong shock. 2. The first nationwide collapse experiment of the reinforced concrete frame structure shaking table model under bidirection earthquake action shows themodels of earthquake resistance, damage and collapse of reinforced concrete frame structure under strong shocks directly.3. When the results of storey lateral stiffness ratio of the in reinforced concrete frame structure calculated by the method of storey shear and storey displacement ratio are not in the range of the limited value, it is necessary to enhance the bearing capacity of the soft storey to avoid causing serious plastic deformation concentration under rare earthquake action. The present standard stipuated of the earthquake shear of soft storey is multiplied by 1.15. In this article, it indicated that in certain circumstances, multiplied by the amplification factors of 1.15 can not avoid plastic deformation concentration in the condition of rare earthquake. When the yield strength coefficient of structure's soft storey is smaller than that of its neighboring storey mean value 65%, we should multiply the amplification coefficient which is equal or bigger than 1.25 or enhance the yield strength of the soft storey directly to make sure that the yield strength coefficient is not smaller than its neighboring storey mean value 80%.4. Storey yield strength coefficientξ_y reflects the relative relationship between the shearbearing capacity and the elastic seismic shears of the storey under rare earthquake actions. It is the main factor of the effection on the eastoplastic deformation in structure. The structure's elastoplastic story displacement is mainly depends on the storey yield strength coefficient and the distribution of along the building's height. Regarding the structure which it'sξ_y distributesnon-uniformed, itsξ_y is the storey which has the smallestξ_y or relatively smaller always takethe lead to yield and present big elastoplasticity storey displacement under rare earthquake actions, the storey displacements of others are relatively smaller.5. For the reinforced concrete frame structure, when it is designed according to the national present code, the computation of the moment bearing capacity at the end of beam should take the teamwork of the floor of effective flange width scope and the beam into consideration, If we take the control of the beam-to-column stiffness ratio (model 1 is 1: 4.1) and make sure the cross section of the beam is not too big, with the storey lateral stiffness ratio in the limitation ranges.it is possible to be designed into the mechanisms of beam's plastic hingle yielding( the mechanisms of overall yielding).6. For the reinforced concrete frame structure with irregularity of vertical stiffness, when it comes to the elastic design, it is not enough to use the method of response spectrum only, using the elastic time-history method to check the calculation is necessary in some cases.