Stavros A. Anagnostopoulos

Optimum Partial Strengthening for Improved Seismic Performance of Old Reinforced Concrete Buildings with Open Ground Story

The paper looks into the problem of partial strengthening of old reinforced concrete buildings with open ground story (pilotis). The strengthening is restricted to the ground story, to keep the cost to a minimum and the building in use during the intervention. Strengthening of the building is achieved using (a) steel bracing in selected bays and (b) reinforced concrete jacketing of ground story columns. Since strengthening the ground story will typically increase the loading to the story or stories above, it is necessary to determine the level of required strengthening that will produce optimum results, i.e. will cause the maximum possible reduction to the building’s vulnerability. In essence, the intervention aims at removing the “soft story” weakness for the said class of buildings and increase its overall seismic resistance. Non-linear static and dynamic analyses are performed in order to determine the seismic behavior of the existing building and the strengthening solutions. Results indicate that increasing both stiffness and strength of the open ground story could effectively improve the behavior of the existing building, removing its inherent weakness due to the soft ground story. However, there exists an upper bound of strengthening, which if exceeded shifts the problem to the upper stories, thus canceling the benefits from the intervention.

Improved Earthquake-Resistant Design of Irregular Steel Buildings

In the past several years, the seismic behavior of eccentric buildings has been studied with detailed models of the plastic hinge type, first for reinforced concrete buildings and then for steel, braced frame type buildings, all of them designed in accordance with the appropriate new Eurocodes. In all cases, it was found that the distribution of ductility demands is not as uniform throughout the structure as one might have expected and desired for a well-designed structure. Such an uneven distribution indicates suboptimal material use and a potential for premature failure of certain members. In this chapter, a design modification that has been proposed earlier and improved substantially the inelastic earthquake behavior of buildings with biaxial eccentricity but with rectangular layouts is applied to eccentric, L-shaped buildings. Both a torsionally stiff and a torsionally flexible building are examined, and it is found that the modification gives also good results for such buildings, especially the torsionally stiff one. The improvement of the behavior of the torsionally flexible building may be considered marginal, but this is probably associated with characteristics of the specific building. In any case, modern codes suggest avoidance of torsionally flexible buildings whose seismic behavior is more difficult to control, and for this reason, stricter design requirements are specified.

Assessment of Seismic Capacity of Existing Buildings – Effects of Uncertainties

The seismic capacity of three buildings used as telecommunication centers in Greece is assessed based on design drawings that do not reflect accurately the “as built structure” and on limited information about the soil conditions. The buildings were designed under the old Greek codes for Earthquake Resistant Reinforced Concrete structures, already known to be quite inadequate both in the characterization of the seismic action and in providing sufficient strength and ductility to the structural members and the building as a whole. Steel telecommunication towers with several disk antennas had been added to the building roofs at later times. With limited knowledge about the type of soil at the building sites, the effect of soil flexibility was first examined in a parametric way and subsequently the assessment of the buildings’ capacities was made on the basis of both elastic dynamic and inelastic static (pushover) analyses. For the latter analyses, the main uncertainty that was hereby addressed was the selection of a suitable horizontal load distribution. It was observed that different distribution may lead to different results regarding the buildings’ capacity. However, in all cases examined, it was found that a drastic intervention would be needed to bring all three buildings up to current standards of increased seismic safety required by the new code for high importance telecommunication buildings that should stay operational during a strong, design level, earthquake.