Authors: Vikas Gautam, Atul Yadav, Kapil Bhutani, Arun Bhardwaj
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Seismic Response Analysis of the given structural system is considered to be a subset of stability and configurationally analysis of the system. It is subjected under the calculations obtained in the form of structural response of the buildings produced by the earthquakes. It is found to be the essential and prime component of the designing phenomenon of the structures, or assessment of structural configuration and as a retrofit for the seismic prone regions of the India. The most severe types of earthquakes are essentially emerged closely nearer to the boundaries of the tectonic plates that are covered in the form of globe surface. These plates tries to move with respect one another in the form of relative motion but are resisted by doing so in terms of generation of friction until or unless the various stresses produced between the plates under epicenter reach to such a extent that the movement between the plates gets generated suddenly, which is termed as the earthquake. The local shock produces waves inside the ground which further propagates towards the surface of earth by the movement creation at the structural bases. The intensity of waves decreases with the propagation of distance as we move away from the point of epicenter. Hence, it results into the formation and existence of such regions on the earth surface having very high or less risk of seismic assessment, which indirectly depends upon the proximity to the main tectonic plates boundaries. Apart from the major earthquakes that generally exists near the boundaries of tectonic plates, there are some other earthquakes, that may have their origin and formation at the interior parts of tectonic plates nearer to the fault lines. Such kind of earthquakes are Called as Intra-plate earthquakes. These earthquakes consist of lesser energy, but may be destructive to the area nearer to the point of epicenter The earthquake imparts the action towards the structure in the form of shaking of ground and its motion in the form of vertical and horizontal components. The horizontal component consists of very high strength and the structures are usually designed to resist gravitational forces than horizontal forces, and as a results, it is the most severe component. The vertical components of the earthquake force are found to be 50% of its value in horizontal direction except in the region of point of epicenter because in the region of point of epicenter, both the components are of same orders.
A study has been conducted based on the guidelines over the dowels of the epoxy grouted in earthquake strengthening projects [Wylli Jr. 1988]. Frequently, it is mandatory to strengthened existing concrete structures for improved seismic performance, either after a damaging earthquake or in preparation for a future extent. This work includes the attachment of new steel or concrete member to the pre-existing structures. The epoxy grouted dowels are found to be ideal for the above said task due to its high strength and installation ease. The short term load implementation on the dowels in the form of seismic loading concludes in the form of creep concern and as the dowels are grouted in the concrete, it is also required to impart an insulation to prevent the epoxy from the exposure heat such as fires.
As per Amr S Elnashai (2000), the past earthquakes have been resulted into a specific conclusion that the internal earthquake response and resistance to the steel structures must not be taken lightly. These structures require a special attention towards their design during earthquake. As per the study, it was earlier reported that the damage due to earthquake is strictly existed on the connections of the structures; related efforts and impacts in the many countries have resulted into some of the serious modifications in their designing and quality control measure practices. The other development that affected the steel structure designing is the performance evaluation in the displacement design method and the procedures to generalize the concepts of limit state methods into the framework of performance-based design method. This present paper presents the recent innovations; research, findings related to the material used, their section, connections between members, etc. are reviewed and analyzed with the various codal provisions of the different countries including India. It also deals with the implementations of investigations related to the various steel frames along-with the bare slabs and composite slabs.
After calculation of seismic forces that acts at various levels along the height of the structure, the various lateral forces and bending moments in all the different members may easily be calculated by the use of the standard computer application. The structure should be designed in such a way it must resist all the overturning impacts and lateral storey drifts due to lateral seismic forces. To ensure this type of consideration, the various forces and bending moments in the members are calculated by the use of P-? effect. The IS: 1893 – 2002 estimates the value of lateral storey drift in the structural storey due to specified lateral loads, having the 1.0 as the partial load factor and it must not be more than 0.4 % of the height of the storey.
III. RESPONSE SPECTRUM ANALYSIS
Response spectrum analysis is one of the most popular methods in the field of seismic analysis. The diagram which is used to perform it is design spectrum. The ennoblement of multi storey buildings is used by response spectrum analysis by a basic assumption. The assumption used is that the mass is combined at the roof diaphragm levels and at the floor levels. The diaphragms are assumed as infinitely rigid and the column axially inextensible but laterally flexible. The dynamic response of the spectrum is determined in the form of lateral displacements of the combined mass with the degrees of dynamic freedom (or modes of vibration n) being equal to the number of masses. The un-damped analysis of the building can be done following standard methods of mechanics using appropriate masses and elastic stiffness of the structural system, and the natural period (T) and mode shapes of the modes in vibration can be obtained. The mode shapes can be determined by distribution of mass and the stiffness of buildings
Superposition of the vibrations of each individual combined mass gives the ground motion which is applied at the base of multi mass system, the deflected shape and combination of all mode shapes. Dynamic response of multi-degree of freedom system is determined by modal analysis procedure. IS 1893 recommends modal analysis is discussed herewith.
Each individual mode of vibration has its unique period of vibration (with its own shape called mode shape formed by locus of points of the deflected masses.)
Response is determined by using different modal combination methods such as square-root-of-sum- of-squares method (SRSS) or the complete quadratic method (CQC) which are used when natural periods of the different modes are well separated (when they differ by 10% of the lower frequency and the damping ratio does not exceed 5%. IS 1893 recommends the CQC method that can be considered as the modal coupling method of design and analysis of members.
It has been observed that the a rapid fall in both the values i.e., Base Shear and Mass Participation Factor takes place from Mode 1 to Mode 2. This sudden decrease then follows gradual decrease and then increase in both the values. It is interesting to note that the both values gets zero at mode 4. These variations have been shown in the fig. 1 and 2 for base shear and mass participation factor respectively.
A. P - ? Analysis
The P-Δ effect refers to the additional moment produced by the vertical loads and the lateral deflection of the column or other elements of the building resting lateral forces.
IV. DESIGN OF MEMBERS
The Steel beams and columns are designed based on the IS: 800 – 2007. Based on the codes, the members can be designed for the following two criteria:
A. Design Strength Criteria
All the compression members are designed on the basis of this criterion. The buckling strength of these members is mainly calculated which mainly gets affected by residual stresses, accidental load eccentricities, etc.
V. DESIGN OF CONNECTIONS
If we consider node 16, the connection is to be designed between the sections ISWB600A, ISHB 350 and I80012B55012. The design process has been explained as below:
A. Connection of Flange To Steel Plate
Hence, The Design is OK.
As per the seismic resistant design philosophy, the various structural members should not possess any damage during the minor ground shaking, while repairable and even irreparable damage can be possible in case of moderate and severe shaking respectively. To fulfil these conditions, sometimes steel structures may be used in the seismic prone area. This chapter deals with the steel frame analysis by the use of methods namely Lateral Force Method and Response Spectrum (RSA) Method (A type of Dynamic Method).
From the analysis, it is observed that the most critical load combination is 1.7(EQ+DL) and its corresponding drift variation and maximum moment diagrams are obtained from analysis which is shown in the figure 7 and figure 8 respectively. The analysis also provides the value of lateral storey drift at each storey. It is also noted that all the values of drifts lie within permissible limits of deflection as per IS: 800 – 2007.
VI. RESULTS OF RESPONSE SPECTRUM ANALYSIS (RSA)
From the analysis, it is observed that the most critical load combination is 1.3(DL+LL+EQ) and its corresponding maximum moment and shear variation diagrams are obtained from analysis which is shown in the figure 9. The analysis also provides the value of lateral storey drift at each storey. It is also noted that all the values of drifts lie within permissible limits of deflection as per IS: 800 – 2007.
A. Comparison Of Absolute Storey Drift In Both Methods
The absolute storey drift is determined for each storey using both the methods. Table 9 shows all the values of storey drift at each storey and figure 12 indicates its variation. It is very interesting to note that the storey drift at first storey i.e., at 3m height of storey is almost same but the variation in storey drift is observed to be higher in Lateral force method as compared to that in response spectrum method of analysis. The storey drift value at last storey i.e., at 18m storey height, the response spectrum method provides comparatively 60% lesser drift as compared with that of the linear response method of analysis.
B. Comparison of Storey Shear In Both Methods
The storey shear is determined for each storey using both the methods. Table 10 shows all the values of storey shear at each storey and figure 13 indicates its variation. It is very interesting to note that the difference in observed values of storey shear between both the methods is 28.91%. However, the observed value is higher is Lateral force method as compared to response spectrum method. An increase in difference between shear values is observed with increase in storey height. Moreover, there is a slight decrease in the value from third storey to fourth storey. This difference in the values is maximum in fifth and sixth storey. The average difference in storey shear is around 29.7% in each storey.
C. Comparison Between Pre-Design Drift And Post-Design Drift
Table 11 shows the drift values in frame before the design of members and after the design of members. The average difference between drift values for pre-design and post-design condition is 62.1%. In addition to this, it is observed that the drift values for pre-design conditions are very high as compared to post design condition in Lateral force Method. It can also be said that drift in the storey can be easily controlled by proper designing of the members as per the forces applied. Fig. 14 shows the variation between the values of drift pre-design and post-design members.
1)The inter-storey drift is determined by the use of the both the methods i.e., the lateral force method and the response spectrum method. It is observed that the lateral displacement of the structure analyzed by the response spectrum method is comparatively lesser than the lateral displacement of the structure analyzed by the lateral force method. 2)The shear force distribution along the height of structure from the use of response spectrum method of analysis is observed very less as compared to the distribution obtained from the use of lateral force method of analysis. 3)The numerical difference between the results obtained from both the methods is always analyzed based on some assumptions which are prevalent for the lateral force method of analysis which are as follows: a)The mode natural fundamental frequency of the structures imparts the most reliable and significant role to the distribution of base shear completely throughout the height of the structure. b)The whole mass of structure is assumed to be used in the dynamic procedure. The above assumptions are completely valid to the low to medium rise structures in which mass distribution is completely uniform along the height. 4)From the above results, it is concluded that the results obtained from the dynamic methods of analysis are comparatively lesser than the results obtained from the lateral force method of analysis. The reason behind this result lies in terms of its fundamental time period. The fundamental mode of time period is 0.62913 from dynamic analysis method which is more than the fundamental mode of time period from the analysis by the use of lateral force method which is approximately in terms of 0.33. 5)The both the comparative analysis indicates that the weight of the first model is nearly 86 % of its complete seismic weight based on IS: 1893 – 2002. The weight of second modal is only 8.24% of its complete seismic weight and the fundamental natural time period of the structure is in the range of 0.20s. 6)During the post design analysis of the models, the decrease in the value of storey drift and distribution of base shear along the height of structure is observed significantly for the high weighted structure. As a result, the provision of heavier structural members results in the safe design. E.g., ISMB 350 section members had used for designing but these sections have found to be failed and when the section is redesigned in the platform STAAD. Pro V8i, the higher section, ISWB 600 A is concluded. 7)The cost of steel sections is directly proportional to the quantity of the steel used and it is found to be lesser for lateral force method of analysis when compared with response spectrum method of analysis. It is due to the fact that the response spectrum method of analysis is a type of dynamic analysis, is very effectively accurate method which is taken into the considerations with so many parameters such as the shape of the member, mode of frequency, factors of mass participation to determine the fundamental natural time period and vibrations. The response spectrum method of analysis is highly realistic in nature for the design & analysis of steel framed structures and this present work concluded that the linear method of analysis results into the very high cost effectiveness for the seismic design and analysis of steel structure. 8)The requirement of quantity of the steel for the design and analysis of steel structure by the use of lateral force method, a type of static method is observed to be 19.92% lesser than the quantity of steel required by the use of analysis of response spectrum method. 9)The response spectrum method includes the use of heavier section of member, the value of lateral displacement and the value of storey drift are comparatively less than sections designed by the use of lateral force method 10)It is also observed that the coefficient of sensitivity of the inter storey drift does not necessarily vary by the considerable amount for the both methods of analysis. 11)The base shear value of structure frame designed by the use of lateral force method is 48.83 % base shear value of structure frame designed by the use of response spectrum method. 12)The quantity of steel required for sections of members and their connections is higher for design and analysis by Response Spectrum Method as compared to that of Lateral force Method.
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