Nikos G. Pnevmatikos

Elastic–plastic response spectra for exponential blast loading

Abstract The design of structures subjected to loads due to explosions is often treated by means of elastic–plastic response spectra. Such spectra that are currently available in the literature were computed on the basis of triangular shape of blast pressure with respect to time. In the present paper, response spectra based on an exponential distribution of blast pressure, which is in better agreement to experimental data, are proposed. To that effect, analytical expressions of the solutions of the pertinent equations of motion have been obtained via symbolic manipulation software, and have been used to carry out an extensive parametric study. A comparison of the spectra obtained by the proposed approach to the existing ones, reveals that the commonly used assumption of triangular blast load evolution with time can sometimes be slightly unconservative, particularly for flexible structural systems, but can also be significantly overconservative for stiffer structures.

EXPERIMENTAL VERIFICATION OF DAMAGE LOCATION TECHNIQUES FOR FRAME STRUCTURES ASSEMBLED USING BOLTED CONNECTIONS

This work is focused on experimental verification of existing techniques for localization of a loosened bolted connection. To this end, a laboratory-scale 2-meter-long steel frame is used. The structure consists of 11 steel beams forming a four-bay frame, which is subjected to impact loads using a modal hammer. The accelerations are measured at 20 different locations on the frame, including joints and beam elements. Two states of the structure are considered: a healthy and a damaged one. The damage is introduced by means of loosening two out of three bolts at one of the frame connections. Experimental modal analysis reveals that the loosened bolts in the connection cause a shift only in some of the frame’s natural frequencies, while the others remain insensitive to the damage.

Damage detection of framed structures subjected to earthquake excitation using discrete wavelet analysis

This paper describes an application of discrete wavelet analysis for damage detection of a framed structure subjected to strong earthquake excitation. The response simulation data on each floor were obtained by non-linear dynamic analysis. Damage to the frame was introduced due to the non-linear behaviour of the columns and beams. In order for the structural members to reach the yield point or go slightly beyond yielding, the earthquake excitation was scaled up with the appropriate factor. Since the dynamic behaviour of an inelastic structure during an earthquake is a non-stationary process, discrete wavelet analysis was performed in order to analyze the simulation response data for each floor. It was shown that structural damage on a floor, and the time when this occurred can be clearly detected by spikes in the wavelet details of the response, acquired from the corresponding floor. Damage can be detected by observing the spikes directly from the wavelet details or following a statistical procedure. An automatic numerical procedure that can clearly distinguish between the spikes associated with damage and the spikes due to non-stationary excitation is proposed. The effects of noise were taken into account by adding a white Gaussian noise to the simulation response data. Damage to the element can also be detected again from the noised signal, if the level of details and the order of wavelets in the wavelet analysis of the response signal are increased. Numerical results show the effectiveness of the discrete wavelet approach to damage detection of framed structures.

Seismic Design of Steel Frames Equipped by Control Devices

The design philosophy of EC8 is to ensure that in the event of the design earthquake, human lives are protected and no collapse will occur, while extended damages will be observed. This is achieved by ductility and capacity design. This design philosophy drives to an additional cost for repairing damage of structures. On the other hand, it is costly and uneconomic to design structures behaving in elastic range, especially under high level of earthquake excitation. An alter- native direction to this strategy, which is examined in this paper, is to design a controlled structure capable to resist a de- sign earthquake loads, remaining in elastic range and thus without damage. The idea behind this philosophy is that one portion of earthquake loading will be resisted by a control system while the rest will be resisted by the structure. The structure, initially, is analyzed and designed according to the current codes. The elastic and design earthquake forces are first calculated according to the elastic and the design spectrum. The required control forces are calculated as the differ- ence between elastic and design forces. The maximum value of capacity of control devices is then compared with the re- quired control force. If the capacity of the controlled devices is higher than the required control force then the control de- vices are accepted and installed to the structure. Then, the structure is designed according to the design forces. In the case where the maximum available control device capacity is lower than the demanded control force then an additional portion of control forces should be resisted by the building. In that case, an iterative procedure is proposed and a scale factor, � , that reduces the elastic response spectrum to a new design spectrum, is calculated. The structure is redesigned based on the new design spectrum and then the devices are installed to the structure. The proposed procedure imposes that the con- trolled structure will behave elastically for the design earthquake and no damage will occur, consequently no additional repair cost will be needed. An initial cost of buying and installing the control devices is required. In order to ensure that the controlled structure behaves elastically, a dynamic control analysis with saturation and time delay control is per- formed. Following the proposed procedure the numerical results show that the structure remains in elastic and no damage occurs.

Earthquake design for controlled structures

An alternative design philosophy, for structures equipped with control devices, capable to resist an expected earthquake while remaining in the elastic range, is described. The idea is that a portion of the earthquake loading is undertaken by the control system and the remaining by the structure which is designed to resist elastically. The earthquake forces assuming elastic behavior (elastic forces) and elastoplastic behavior (design forces) are first calculated according to the codes. The required control forces are calculated as the difference from elastic to design forces. The maximum value of capacity of control devices is then compared to the required control force. If the capacity of the control devices is larger than the required control force then the control devices are accepted and installed in the structure and the structure is designed according to the design forces. If the capacity is smaller than the required control force then a scale factor, ?, reducing the elastic forces to new design forces is calculated. The structure is redesigned and devices are installed. The proposed procedure ensures that the structure behaves elastically (without damage) for the expected earthquake at no additional cost, excluding that of buying and installing the control devices.