Santiago Pujol Dissertation

Investigation on the initial response of beams to blast and fluid impact

Oscar Alfredo Ardila-Giraldo, Purdue University

Abstract

Experimental evidence shows that an element that can develop a flexural mechanism of failure under static load may fail in shear under blast load. This study shows that the critical parameter defining the failure mode of an element under blast or impact load is the ratio of shear demand to shear capacity. The matter is not as simple as it may appear because for dynamic load, shear demand depends on inertial forces and shear capacity depends on loading rate. This study provides means to estimate dynamic shear demand. ^ The ratio of shear demand to shear capacity controlled the failure mechanism of small-scale reinforced concrete (RC) beams tested under fluid impact. The test specimens had their ends restrained against rotation, and were subjected to transverse impact at midspan of liquid bodies traveling at speeds of up to 150 m/s. The specimens were observed to fail in shear at nominal shear stresses above approximately 1.6 times their static shear strengths. The threshold for shear failure seemed independent of vibration properties. Control specimens developed flexural mechanisms of failure under static load. The tests showed that the change in failure mode observed in aluminum beams subjected to distributed blast load (Menkes and Opat, 1973) can also occur in small-scale RC beams subjected to concentrated fluid impact. ^ The deformed shape of a beam in the initial phase of response to blast load (0 ≤ t ≤ 0.1T, where T is the vibration period) is dominated by shear deformations near the supports. Deformations progress from the supports towards midspan as the beam acquires a shape similar to the deflected shape under static load at a time longer than 0.1T. The initial deformed shape is dramatically different from the deformed shape under static load. The deformed shape affects shear demand. Two numerical methods to estimate initial shear demand caused by blast load are proposed. The methods account for the mentioned change in the deflected shape. One method is based on the work by Biggs (1964) to compute dynamic reactions, and uses a single-degree-of-freedom (SDOF) system with time-dependent stiffness. The other method involves numerical solution of the differential equations of motion of a beam according to the theory proposed by Timoshenko (1937), which includes the effects of shear deformations and rotational inertia. Both methods assume the response of the beam to be linear and provide estimates of shear demand that are consistent with the experimental observations available to date.^

Degree

Ph.D.

Advisors

Santiago Pujol, Purdue University.

Subject Area

Engineering, Civil

Drift capacity of reinforced concrete columns subjected to displacement reversals

Santiago Pujol, Purdue University

Abstract

In previous tests of columns under displacement reversals in the inelastic range of response, different arbitrary displacement histories have been used. Comparisons of drift-capacity data from columns tested under different displacement histories can only be made if displacement-history effects are ignored. Possibly because of this reason, currently available methods for column drift capacity ignore displacement-history effects. To investigate whether drift capacity is a function of displacement history, sixteen cantilever columns were tested under various displacement patterns. The test results indicate that column drift capacity is sensitive to displacement history. For columns cycled beyond yield, drift capacity decreases as a function of the amplitude and number of cycles the column has experienced. Based on measurements of average unit strains in the transverse reinforcement, a model is proposed for estimating the drift capacity for a given column under any symmetric displacement pattern. ^

Degree

Ph.D.

Advisors

Major Professors: Julio A. Ramirez, Purdue University, Mete A. Sozen, Purdue University.

Subject Area

Engineering, Civil

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