Showing papers in "Journal of the Structural Division in 1980"

Strength and ductility of tied concrete columns

Abstract: Short 12-in. (305 mm) square concrete columns with rectangular ties as confinement steel are tested under monotonic axial compression to failure. Land-deformation curves are presented. Contribution of concrete to load carrying capacity and the significance of cover spalling are examined. Effects of distribution of longitudinal steel around the column perimeter and the resulting tie configuration, the amount and characteristics of tie steel, tie spacing, and the amount of longitudinal steel affecting the behavior of the confined column core are examined. Concrete cores of columns confined with ties and well distributed longitudinal steel exhibit significant gain in strength and ductility. Tie spacing and the characteristics and the amount of tie steel significantly affect the strength and ductility of confined core. The amount of column steel does not appear to have significant effect on confinement.

Fatigue under Wide Band Random Stresses

Abstract: A method for predicting high cycle fatigue under stationary gaussian wide band stress processes is developed. The rainflow cycle counting method is used as the basis for predicting fatigue damage. Stress simulations from various spectral density models were analyzed using the rainflow method. Statistical analysis of these results provided a closed form expression for fatigue damage. A principal application of the method is to predict fatigue in the welded joints of offshore structures, an example of which is provided. A long term nonstationary sea state is modelled as a sequence of stationary sea states. Thus the example also illustrates how fatigue under certain types of nonstationary stress processes can be analyzed.

Earthquake Rocking Response of Rigid Bodies

Abstract: Using a computer program, the rocking and overturning response of rectangular blocks of various sizes and aspect ratios is studied under several strong motion earthquakes. The effect of coefficient of restitution and of vertically prestressing the blocks to the floor is also studied. This investigation was undertaken primarily to study the response of solid concrete block stacks used as radiation shields in particle accelerator laboratories. Results from this study indicate that rocking should be prevented in such systems on account of the possibility of overturning once rocking commences, unless a tie-down design is used. The paper also points out the sensitivity of overturning to small changes in base geometry and coefficient of restitution as well as to the form of the ground motion. This suggests that it may be difficult to use data from observations on standing and overturned rigid bodies after an earthquake to provide much useful information on the intensity of ground motion.

Rough cracks in reinforced concrete

Abstract: Equations derived are which relate the normal and shear stresses transmitted across a crack due to aggregate interlock to the opening and slip displacements. Average strains due to a system of parallel continuous cracks are superimposed on the strains due to the solid concrete between the cracks. The response of cracked concrete reinforced by a regular net of bars is then calculated using an incremental loading procedure. The stress-displacement relations exhibit a singularity at the initial state of zero displacements and the initial crack opening must begin at zero slip. Responses to various types of loading are calculated and the variation of the secant friction coefficient, crack dilatancy due to slip and the effect of crack opening are studied. The theory allows designing for a maximum crack width if the crack spacing is known. The previously proposed slip-free concept yields limit loads which are found to correspond to the states just before the start of a rapid increase in crack opening, while the classical frictional approach yields limit loads that are achieved only after very wide cracks develop.

Hurricane Wind Speeds in the United States

Abstract: A Monte Carlo simulation technique is used to obtain estimates of hurricane wind speeds along the Gulf and East Coasts of the United States. The paper describes the sources of data, the probabilistic models for climatological characteristics of hurricanes, and the physical models for the hurricane wind speed field used in the estimations. Estimated values of fastest-mile hurricane wind speeds at 10 m above ground in open terrain at the coastline and at 200 km inland are given for various mean recurrence intervals. The estimated hurricane wind speeds were found to be best fitted by Weibull distributions with tail length parameters γ≥4. Estimates are given of various errors inherent in the estimated values of the hurricane wind speeds. Owing to uncertainties with respect to the applicability of the physical models used in this work to locations north of Cape Hatteras, estimated hurricane wind speeds given for these locations should be viewed with caution.

Load Combinations in Codified Structural Design

Abstract: Alternative formats for load combination requirements in structural design codes are examined. Basic alternative models of macro load-time dependence for live, snow, wind, and earthquake loads are reviewed and a sensitivity analysis performed to establish a set of models sufficient for combination studies. On the basis of normalized design loads and a spectrum of influence coefficients, load combination factors in the alternative basic code formats are established. Extensive numerical results are presented for possible use in code development, It is concluded that a companion action approach is preferable to a probability reduction factor approach and a set of potential design values is suggested.

Inelastic Response Spectra for Aseismic Building Design

Abstract: The variability of inelastic response spectra is investigated by time-history analysis using sets of artificial ground motions with different strong-motion durations. It is concluded that the Newmark-Hall inelastic response spectra of elasto-plastic systems are unconservative for 5% damping, and conservative for 2% damping. New inelastic response spectra are proposed for different ductility ratios and damping coefficients. A 10-story steel moment-resisting frame is designed and analyzed to assess the validity of the design procedure based upon modal analysis and inelastic response spectra. The results indicate that at least for this or similar frames, the method is satisfactory. However, the procedure leads to slightly unconservative design for upper-story exterior columns. The study also suggests that the maximum lateral story displacement predicted directly by the SRSS modal analysis using the inelastic displacement response spectrum is too conservative. The conservatism is more pronounced in the upper stories.

Hysteretic Cycles of Axially Loaded Steel Members

Abstract: Seventeen tube specimens and eight angle specimens were tested under repeated axial loading. Length of the specimens varied from 20 in. to 59 in. and their effective slenderness ratio varied from 30 to 140. The objective of this experimental investigation was to quantify the reduction in maximum compressive loads and increase in member length, to study the influence of mode of buckling and shape of cross section on the hysteresis behavior and energy dissipation through hysteretic cycles. It was concluded that local buckling and shape of the cross section can have significant influence on the hysteresis behavior of axially loaded steel members. A hysteresis model is proposed for tubular members which is in reasonable agreement with the observed inelastic behavior.

Buckling Properties of Monosymmetric I-Beams

Abstract: Approximations are derived for the section properties required for the calculation of the elastic critical loads of monosymmetric I-beams and are found to be related to the ratio of compression flange and section minor axis second moments of area. Approximations are obtained that are applicable to I-sections with unequal and lipped flanges and that are in close agreement with accurate calculations of the monosymmetry section properties made for a wide range of cross sections. An improved design rule is proposed for the elastic critical stress of a monosymmetric I-beam. A comparison is made of the results obtained using the proposed rule and the present rules of the AS 1250, BS 449, and AISC Specification.

Assessment of Empirical Concrete Impact Formulas

Abstract: Data from 145 recent impact tests of reinforced concrete walls with solid and pipe missiles are used to assess the accuracy of several empirical formulas. For solid missiles, penetration depths greater than six-tenths the missile diameter are predicted by the NDRC formula to within —25%, scabbing is predicted equally well by the NDRC, Bechtel, and Stone and Webster formulas, and perforation is predicted best by the CEA-EDF formula. For pipe missiles, the best overall agreement with data is obtained with the NDRC formulas, using the outside diameter in the penetration equation and the equivalent solid diameter in the equations for scabbing and perforation. The limitations of all of the formulas for impact conditions outside their range of applicability are pointed out. For example, the formulas are inapplicable for highly deformable missiles such as wooden poles and autos and give only approximate results for rotating or irregularly shaped missiles such as fragments from rotating machinery.

Web distortion and flexural torsional buckling

Abstract: In a member with a slender, unstiffened web, a combined buckling mode may occur in which the web distorts and reduces the member’s torsional resistance, and consequently its resistance to lateral (or flexural-torsional) buckling. In this paper, an approximate analysis of the effect of web distortion on the elastic flexural-torsional buckling of a simply supported I-section member is presented. This analysis leads to a closed form interaction equation, relating the critical uniform moment and axial load acting on the member. The buckling moments and loads determined from this equation are compared with the results of a more sophisticated finite strip analysis, and are shown to be of high accuracy.

Probability Model for Design Live Loads

Abstract: Extensive floor load data are utilized in a probabilistic design model for building live loads. Results from most major live load surveys are employed to derive representative loading parameters for several common occupancy types. Simulation is used to verify a number of probabilistic models describing the stochastic nature of the live load components. Area dependent sustained and transient load processes comprise the total load. The mode by which the maximum total load (or design load) is reached during the lifetime of a structure is not known with certainty. A probabilistic model is developed that accounts for the relative likelihood of occurrence of each mode, and is used to obtain example design loads corresponding to the mean of the lifetime maximum total load. For most occupancy types considered these loads are in reasonable agreement with present building code values.

Glued-Laminated Beam Strength: A Model

Abstract: A model for the prediction of bending and tensile strength of glued-laminated beams is developed. The model accounts for effect of size, lamination thickness effect, E-rating, and proof-loading of the individual laminations, and it can be used in computer simulations to estimate the distribution of beam strength. Eight types of beams, differing in size as well as in lamination lay-up, were tested. The experimental results are compared with the model’s simulations and the agreement is found to be satisfactory.

Vertical Vibration Analysis of Suspension Bridges

Abstract: A method of analysis is outlined for free vertical vibrations of suspension bridges. The method employs a digital computer and a finite element approach, and uses a linearized theory which restricts the amplitudes of vibrations to be small. The analysis is designed to determine the natural frequencies, modes of vibration, and energy storage capacities of the different members of the bridge. Both symmetric and antisymmetric modes of vibrations are considered. For the symmetric modes (which include additional vibrational cable tension), it is shown that it is imperative to include the elasticity of the cables if correct solutions are to be obtained. A detailed numerical example, which includes the effect of the extensibility of the cables and the elasticity of the towers, is presented and the results are examined by a comparison between measured and computed natural frequencies and mode shapes of a real bridge; the comparison shows good agreement.

Seismic Design Input for Secondary Systems

Abstract: Acceleration floor spectra curves are commonly used as seismic design inputs for light equipment and other secondary systems. Earlier the writer had presented a method for generation of floor spectra directly from prescribed ground spectra. Further complementary developments in this method are made. These are: (1)The development of the method for generation of floor spectra at structural frequencies (an important resonance case); and (2)evaluation of nonstationarity of earthquake motions on floor spectra curves. Also, some approximations made in earlier work are further evaluated. As a result of these developments and evaluations, a comprehensive procedure based on stochastic principles is presented in the paper for definition of seismic design inputs for secondary systems. This procedure avoids the use of often used and recently criticized spectrum-consistent time history as seismic input for generation of floor spectra curves.

Tensile Strength of Concrete: Double-Punch Test

Abstract: The tensile strength of concrete is most commonly measured by the indirect split-cylinder test. Recently, an alternative new test, known as double-punch test, has been proposed. It is the purpose of this paper to summarize the present stand on the double-punch test method for the determination of tensile strength of concrete. The summary deals with two equally important aspects of progress — new solutions emerging from the analytical study and experimental results reported from laboratory tests. This paper attempts to evaluate critically this new test method and to determine the range of their applicability, relative merits, and limitations as compared to the well-established split-cylinder tests. Throughout this development and evaluation, it has been recognized that a balance must be struck between the requirement of realistic representation of tensile strength of concrete and the requirement for simplicity in practical use. It is considered that in both these respects, the double-punch method is satisfactory.

Nonlinear Analysis of Prestressed Concrete Frames

Abstract: A numerical procedure based on the finite element method for the material and geometric nonlinear analysis of plane prestressed concrete frames including the time dependent effects due to load history, temperature history, creep, shrinkage and aging of concrete and relaxation of prestress is developed The procedure is capable of predicting the response of these structures throughout their service load history as well as throughout elastic, inelastic and ultimate load ranges in one complete analysis Time dependent variation of concrete properties is recognized Concrete creep is evaluated by an efficient numerical procedure based on the age and temperature dependent integral formulation Varied material properties within a frame element is accounted for by a composite concrete and reinforcing steel layer system The contribution of prestressing steel is incorporated directly A series of numerical examples is presented

Cyclic Response Prediction for Braced Steel Frames

Abstract: An experimental and analytical investigation of the behavior of structural steel frames with K-braces subjected to severe cyclic loadings simulating seismic effects is described. The hysteretic behavior of a one-half scale three-story test frame is modeled analytically. Good agreement between the experimental and calculated results is shown to be possible provided the cyclic behavior of individual braces is accurately formulated. A development of a refined empirical brace model for this purpose is outlined.

Column Base Plates with Axial Loads and Moments

Abstract: The results are presented for an experimental and analytical study of steel column base plates subject to axial loads and moments. A total of 16 tests were made in which the variables were the anchor bolt size, the base plate thickness, and the ratio of moment to axial load. The results of these tests were used to review the two methods of design presently used, one based on the behavior at working load, and one based on the failure load. Both generally produced reasonable predictions of the capacity for the range of variables in this study. The investigation has demonstrated the need for limits of applicability for the two design methods.

Strength Variation of Laterally Unsupported Beams

Abstract: Nominally identical 25 rolled beams for each group of three different lengths are tested carefully under a concentrated load applied vertically at the midspan of the compressive flange of span center. The variation of the geometrical and material imperfections are measured and the influences of the various parameters on the scattered test points of the lateral buckling are reviewed. The main parameter which highly influences the variation of the ultimate strength is the variable value of actual plastic moment. The obtained test results are also compared with the existing test data and design formulas. The coefficients of variation of the strength increase with beam length and these values are considerably low as compared with the past scattered data. Finally, a design formula is proposed to ensure empirically the lower bound for the scattered test values.

Dynamic reduction of structural models

Abstract: This paper considers the errors associated with the reduction of the structural static degrees of freedom before performing a dynamic analysis. It is shown that the standard methods of performing the reduction can lead to significant errors especially for the member loads computed in a forced vibration problem. A modified reduction is developed which is shown to significantly reduce the errors. The standard and modified methods are applied to a cantilever beam, a gabled frame, and a piping system.

PDF of Wind Pressures on Low-Rise Buildings

Abstract: Probability distributions of wind pressures acting on points or areas of low-rise building models have been measured in turbulent boundary layer flow simulating wind over open country and suburban terrains. Results show extensive positive and negative tails for high negative and positive mean pressure coefficients, respectively. Probability density functions of wind speed have also been found positively shewed at lower heights. It is shown that the Gaussian distribution underestimates the high-pressure peak factors, whereas a Weibull model fits the data adequately. Analytical expressions for the number of upcrossings of any pressure level and the extreme value distribution are presented. Comparison with experimental data shows a good agreement.

Perforation of Reinforced Concrete Slabs by Rigid Missiles

Abstract: This paper presents an extension of the penetration theory proposed by the National Defense Research Committee (NDRC). The extension is based on experimental studies, performed over the last few years and covering projectiles with lower impact velocities and larger diameters and weights than originally considered by the NDRC. The missiles are considered to be rigid, that means that no deformation of the missile is taken into account. The paper gives a new expression for the calculation of the perforation thickness for the above mentioned range of missiles and a new formulation of the velocity function is proposed, allowing the estimation of residual velocity after perforation. The new formulation preserves the advantages of the NDRC formula and leaves the basic expression unchanged. The impact time and the reaction force are calculated based on the new velocity function. The results are compared with the experiments.

Wind Loading on Cooling Towers

Abstract: A wind-tunnel technique for the assessment of wind induced stresses in cooling tower shells has been developed. It includes simulation of the atmospheric wind and aeroelastic models of the towers under test as part of a complete site model. It is shown that the simulation of the steady and fluctuating wind loadings and the dynamic properties of the models are in good agreement with results from full-scale towers. The predictions of wind speeds for failure of the tower shell given by the method are shown to be in good agreement with full-scale evidence. Stresses in cooling tower shells are sensitive to changes in wind loading distribution arising from aerodynamic interactions in complex groups of towers. If the resonant frequencies of the shell are low enough, vibration arising from turbulence produced by the wakes of buildings and other cooling towers can produce significant contributions to the stresses.

Revised Procedure for Estimating Along-Wind Response

Abstract: A revised version is presented of a procedure for calculating the along-wind response of tall buildings previously developed by the writer. This version: (1)Incorporates recent improvements in the modeling of mean wind profiles and of turbulence intensity; (2)includes a correction in the Monte Carlo integration algorithm employed to obtain the rms values of the fluctuating response and thus results in more accurate values of the calculated along-wind response; and (3)is simpler to use. A numerical example is given illustrating the use of the procedure.

Wind Effects on Cooling-Tower Shells

Abstract: The design of natural-draft cooling towers is dominated by wind action. With respect to the response of the structure the wind load may be divided into a static, a quasistatic, and a resonant part. The effect of surface roughness of the shell and of wind profile on the static load is discussed. The quasistatic load may be described by the variance of the pressure fluctuations and their circumferential and meridional correlations. The high-frequency end of the pressure spectra and of the coherence functions are used for the analysis of the resonant response. In general, the resonant response is small even for very high towers. It increases overlinearly with wind velocity. Equivalent static loads may be defined using appropriate gust-response factors. These loads produce an approximation of the behavior of the structure and in general are sufficiently accurate.

Automatic Active Control of Structures

Abstract: Some of the benefits gained from using closed-loop control in controlling flexible structures are illustrated. It is shown that a closed-loop active control provides the active damping and stiffness needed by any kind of current response of the structure. That leads to the idea of designing a closed-loop control system for a simplified structural model, neglecting those factors which complicate the design. The efficiency of the designed control system is then checked afterwards by considering those factors which have been neglected previously. The design may be revised if the controlled response, considering the neglected factors, is not acceptable. A revision proves to be rarely needed. The procedure outlined in the paper enables one to find easily a closed-loop control for nonlinear or high-order structural systems. An automatic controlled bridge is presented as an example.

Strength Decay in R.C. Beams under Load Reversals

Abstract: An experimental investigation to determine the effectiveness of intermediate longitudinal reinforcement in limiting shear strength decay of reinforced concrete members is presented. Test specimens consisted of fourteen exterior beam-column subassemblies. Variables included shear stress, percentage of beam transverse reinforcement, percentage of longitudinal reinforcement and shear span to depth ratio in addition to the presence of two layers of intermediate longitudinal reinforcement in half the specimens. Test results indicated that intermediate longitudinal reinforcement provided significant and useable improvement in shear strength retention for members with maximum shear stresses between three and six times the square root of the specified compression strength of concrete.