| Seismic base isolation is a unique technology that is being incorporated into the use of seismic vibration attenuation in buildings and bridges and even these days, the concept is being introduced into some factories, where excessive vibrations from some machines can cause damage to the other essential contents of the facility. Seismic isolation, regardless of the placement in a structure, has the aim to be decoupling the structure to be isolated from the vibrations originating from a known or anonymous source.A 4 storey masonry-reinforced office building is advanced in analysis in the area of base isolation, by using the SAP2000 software. The structure is first assessed as a nonisolated model and the structural responses are analyzed, making comparisons to the 3 ground motions used for this research, namely, the El Centro, the Cholame–Shandon and the Delta Artif. The ground motions are analyzed for compatibility to be used with each other, even as each ground motion has its own wave characteristics. Suitable matching is done firstly among the 3 ground motions while using the target response spectrum and then amongst the 3 ground motions combined. The average misfit is found in the El Centro at a value of 1.90% and the maximum misfits, at a value of 9.60% with the Cholame – Shandon, these values however are acceptable. The lead rubber bearings, LRB 400 s and LRB 500 s are used primarily to carry out the base isolation on the structural model and ascertain the effects of the structural response and then lastly, the friction isolators are combined with the LRBs to verify the structural response.The results of the non-isolated building structure indicates that the floor accelerations for each of the ground motions, started off at a peak value, from the 4th floor, then diminished till reaching the ground floor. The same trend was evident in the floor velocities and displacements. The highest acceleration for the maximum accelerations is at 2.82 m/s2 and that for the minimum acceleration is recorded at-2.97 m/s2 all with the El Centro Y. The highest maximum velocities are recorded at 0.189 m/s for the Delta Artif Y and the highest minimum recorded at-0.182 m/s for the El Centro Y. The displacements for both the maximum and minimum values are observed to be below the Chinese code's specification, 26.36 mm. The highest maximum displacement is at a value of 15.01 mm, whereas the highest minimum displacement is at a value of-14.02 mm, with both recorded with the Delta Y. The maximum drift angle in the 1st floor is 1/1174, being recorded with the Delta Artif Y, and the minimum is-1/1000, recorded with the El Centro Y. The story drifts for all the stories are below the limit stipulated by the code(1/100) for the structure to enter into the plasto-elastic phase. As such the structure is safe and has not entered the plastic stage. The response spectral values of these structural responses were lower than the enveloped values for the ground motions. The p-delta effects were considered for some selected braces and columns of the structure against the floor loads, shears and moments.After isolation, it is observed that, the highest value for the percentage reduction in maximum acceleration was recorded at 65.3% with the El Centro Y and that for the minimum acceleration at a value of 67% with the Delta Artif Y but the response spectrum had a higher value at 91.3% reduction in the y direction. The highest percentage of reduction in the maximum velocity is at a value of 34% with the Delta Artif X and a highest for the minimum is at 32.8% with the El Centro Y, whereas the response spectrum recorded the highest reduction in both the maximum and minimum at a value of 57.2%. The highest percentage increment in maximum displacement is 86.8% with the Cholame – Shandon Y and the minimum is 87.4% with the Cholame – Shandon X, whereas the response spectrum gave lower values at 56%. The highest maximum drift angle for the 1st floor is at 1/186, occurring with the Cholame – Shandon Y and the highest minimum is-1/169 with the Cholame – Shandon Y. The story drift angles are below the code's specification, thus the structure is safe.The friction isolator, upon combining with the LRB is found to increase the structural fundamental period by 14.2%, that is when compared with the isolation of the structure with only the LRBs and a percentage increase of 362% with respect to the initial non-isolated structure. The masonry wall stresses were checked for any effect from seismic isolation and it is found that from a maximum of 132 KN/m2 in the non-isolated structure, the stresses reduced incredibly, reaching a value of 3.5 KN/m2, that is a 97.3% reduction in the masonry stresses. Buckling analysis on the structure is conducted and found that the non-isolated structure is safe in buckling failure, however the isolated structure is observed to start to have started to buckle, with no failure repercussions to the structure.For the base shears, the highest values for the maximum percentage reductions are evident in the Delta Artif and the El Centro in the x and y directions at 70.1% and 37.2%, respectively. In the case of the minimum base shear reductions, the Delta Artif and the El Centro in the x and y directions recorded percentages of 70.5% and 30.0%, respectively as the highest values. The floor loads, shears, torsional moments, torsional angles and moments were evaluated before and after base isolation and found to be significant effects. The rare earthquake is simulated and used on the structure and the highest story drift angles for the maximum and the minimum were seen to be 1/283 and-1/205, respectively, being smaller than the provision in the code.The structural model is found fit for use with the seismic base isolation technology, with acceptable results. As such this technology can be used on other masonry-reinforced concrete structures and even on masonry structures to attenuate the structural responses from the seismic vibrations. |
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