This paper investigates the use of structural damping in blast response calculations. In recently published literature, there are many examples of structural damping being used in computational models with little or no experimental or theoretical justification. The use of even small amounts of damping in computational models involving nonlinear plastic response can significantly influence the response calculations. For example, for a given blast loading, a reinforced concrete slab with only 48 kPa maximum capacity and 25 percent of critical damping (a value typically recommended) will deflect the same as (i.e., provide the same level of protection as) a slab with 690 kPa maximum capacity and no damping. Clearly a fictitious damping term that provides as much as 93 percent of the resistance is problematic. Structural damping during plastic response cannot be clearly defined or verified experimentally. Therefore, the use of damping in plastic response calculations should be avoided.

There are a number of existing facilities at petrochemical plants which : house a significant number of personnel as well as expensive control equipment which must provide protection during an explosion accident. Many of these structures are not capable of resisting blast pressure which may occur during an explosion because they were not designed for these loads. As a result, the potential for significant hazards to personnel and equipment exists at many plants. This paper describes a project involving the scenario postulated above. The existing building was constructed of unreinforced masonry yet was subjected to peak reflected blast loads on the order of 70 psi (483 kPa). A poured-in-place, reinforced concrete box was selected for the new structure. Walls were designed to resist reflected blast loads in flexure and to transmit reactions to the roof diaphragm and shear walls. Walls and roof sections were designed using single-degree-of-freedom (SDOF) methods for determination of dynamic response to the transient blast load. Control conduits extending from the existing walls presented several difficulties for construction of the new walls. A confined working area, high water table, and a requirement for equipment to remain operational also posed unique design challenges.

A large experimental program has been carried out in order to better under-stand basic physical mechanisms explaining rate effects on concrete strength. Direct tensile tests and compressive tests were performed on different con cretes. Slab tests using a shock tube were also carried out using the same materials. It has been demonstrated that for intermediate strain rates (about 1O-5 to 1 s ) the strength enhancement can be explained by the presence of free water in the nanopores of concrete. A mathematical expression is pro-posed which accounts for the role of significant parameters. The slab tests confirm the physical analysis developed at the material level. Some specific phenomena (possible occurrence of either shear failure or bending failure, relative smaller strength enhancement than in direct tension) confirm the ne-cessity of models based more on the physical properties of the material for structural analysis in dynamics.

Department of Defense (DOD) facilities which may be subjected to blast effects from accidental explosions are required to satisfy the safety requirements delineated in DOD 6055.9-STD, "DOD Ammunition and Explosives Safety Standards."(l) In the safety standard, Army Technical Manual 5-1300, "Structures to Resist the Effects of Accidental Explosions, "(2) is referenced for specific criteria to be used in the analysis, design, and construction of blast resistant structures. Design procedures for concrete elements are provided in chapter 4 of the manual. According to chapter 4 of TM 5-1300, mechanical splices must be capable of developing the ultimate dynamic strength of the reinforcement without reducing its ductility before they can be used in blast resistant concrete elements. Unfortunately, no mechanical splicing system is currently available which can fully satisfy these requirements. Numerous splicing systems can develop the ultimate dynamic strength of the reinforcement but none can do so without some reduction in ductility. An effort is currently underway to more accurately define the performance of mechanical splices under rapid dynamic loading. It is hoped that the results of this research will permit the use of mechanical splices in blast resistant concrete structures. Preliminary investigations have indicated that some splicing systems may be safely used in low ductility regions. In this paper, available data from dynamic tests i of mechanical splicing systems will first be reviewed. The current research effort will then be outlined, and I

Reliable analitical methods are needed to aid the analysis and design of concrete structures under impact and blast loading. Calculated results from such an analysis are often compared and fitted with physical test results to validate the method of analysis employed. Material models used in the analysis must, account for strain-rate sensitive behavior, and these material models are also based on results from experiments. Hence, reliable development of material models and analytical techniques is contingent upon correct, observation of experimental results. This paper focuses on effects which could alter the test results and influence their subsequent interpretation, such as testing machine characteristics, inertia, time delays in measured signals caused be analogue filters, and vibrational energy. All of these effects can lead to incorrect, measurement of a test response under high strain-rate loading. Examples are given of incorrect measurement, of the compressive stress-strain response of concrete at strain-rates in the order of 0.1 s -1 , where results from such tests have been obtained with hydraulic testing machines. Failure to account. for inadequacies in the testing technique affected conclusions about changes in deformation behavior (such as stiffness and axial strain at peak stress). and also led to an apparent loss of ductility. Results from impact tests on a flexural member demonstrate how vibrational effects from a falling mass can lead to incorrect conclusions about the measured contact. load.