Journal of Structural Engineering

The inelastic combined bending and torsion of I‐section beams is investigated in this paper using the finite‐element method. The elastic‐plastic load‐deformation relationships of beams under combined bending and torsion are determined by taking into account the effects of large deformations, material inelasticity, and initial conditions of residual stresses and geometric imperfections. The interactions between in‐plane bending, flexural‐torsional buckling, and torsion are studied. The effects of secondary bending actions about the minor axis caused by torsional rotations are important for the interaction between bending (with flexural‐torsional buckling) and destabilizing torsion. Interaction equations for the design of I‐section beams against combined bending and free torsion, and against combined flexural‐torsional buckling and destabilizing torsion, are proposed. It is found that a commonly quoted circular interaction equation for combined bending and torsion does not always provide a true lower bound for I‐section beams.

The primary objective of code torsional provisions is to limit to acceptable levels the element ductility demands and deformations in plan‐eccentric structural systems. Such levels are determined from well‐established seismic design criteria applied to the appropriately defined reference systems. Information is available from comprehensive recent studies focusing on the effectiveness of code torsional provisions in meeting that objective. The major controlling factors have been identified as the total design strength incorporated in plan‐eccentric structures and its code‐dependent distribution among the lateral load‐resisting elements. The results and conclusions of these related studies have been based on a comparison of the response of elements in plan‐eccentric systems with that of the corresponding element in the selected reference system. This paper demonstrates that when a reference model is adopted that adheres rigorously to the design regulations of codes, including provisions for so‐called accidental torsional effects, significant period dependency of inelastic seismic torsional response behavior is revealed. To demonstrate this, a reevaluation of the equivalent static torsional design procedures of codes is presented.

A semiempirical, mathematical model for assessing the load behavior of bridge guardrail systems is described. A full‐scale laboratory specimen was represented by a beam supported by coupled, elastic springs. Empirical flexibility coefficients were used to establish the spring constants for the post and deck subassembly of the specimen. This model was applied to a panelized, longitudinal‐deck bridge specimen. Laboratory load tests were conducted both without and with posttensioning features incorporated into the specimen. Longitudinal, transverse and angular loads were separately applied to the specimen. The model was successful at predicting transverse load‐displacement behavior. A stiffening due to posttensioning was evident. Due to erratic load‐slip behavior of the connection details, the modeling of longitudinal load‐displacement behavior was unsuccessful. The danger of a possible failure of the guardrail post at a low load level was observed under longitudinal loading. Loosening of the post to deck connection during the posttensioning was a factor in the occurrence of load‐slip behavior.

Extensive testing of control specimens and large‐scale models of high‐strength concrete composite beams, 3,048‐mm (10‐ft) length, reinforced with pre‐stressed prisms, have resulted in deformation readings that are consistent with those obtained from classical measuring methods such as electric strain gauges and linear voltage differential transformers. The present paper provides the research findings of fiber‐optic Bragg gratings as strain/stress sensors for monitoring the critical sections of a series of this type of composite beams. The optical fibers used in this study had a numerical aperture of approximately 0.22 and a V‐value (reduced frequency) of 2.2 at 1,300‐nm operating wavelength. The theoretical strain coefficient of the optical fiber had a typical value of 0.762, which agreed quite well with the calibration test performed. A successful packaging of the fiber‐optic Bragg grating was used and modeled. Good bonding of sensor to the structure was shown to be imperative for maximum transfer of load‐induced strain. The average strains in the main tension reinforcement embedded in the concrete beams, the plane section assumption of the strain distributions across the critical section depth, the flexural stiffness of the beams, and the crack widths at service loads are evaluated.

Free‐standing, unanchored liquid‐storage tanks have the tendency to lift off their base under the overturning influence of the hydrodynamic wall pressures generated during earthquakes. Fundamental to the analysis of these systems is a detailed investigation of the behavior of the partially uplifted base plate. As a step to such an investigation, a comprehensive analysis is made herein of the uplifting resistance of a uniformly loaded semiinfinite prismatic beam that rests on a rigid base. The analysis used represents an improvement over the previously proposed methods in that it provides for the effects of both material and geometric nonlinearities in a realistic and cost‐effective manner. The results presented demonstrate the importance to the load‐carrying mechanism of the membrane forces induced by the constraining action of the tank wall. An in‐depth parametric study elucidates the effects of the various system parameters on the uplifting resistance of the beam.

A method is presented for the analysis of the base plates in cylindrical liquid‐storage tanks that are uplifted under the overturning influence of the hydrodynamic wall pressures induced by earthquakes. Due consideration is given to the nonlinear effects associated with both membrane action and material yielding. The method is highly efficient and suitable for dynamic response analyses of unanchored and partially anchored tanks for which the solution of the plate problem must be obtained at numerous time steps. Both axisymmetric and asymmetric conditions of uplifting are examined. A comprehensive parametric study is carried out to elucidate the effects and relative importance of the various system parameters on the uplifting resistance. Cyclic loading patterns are included to highlight the nature of the hysteretic energy dissipation due to yielding.

A methodology is presented for conducting forensic investigations of earthquake‐damaged structures. Causes of unexpected damage, indicated by differences between predicted (expected) and observed damage, are significant to the engineering and construction professions. Due to the existence of uncertainties associated with randomness and vagueness, methods for conducting probabilistic and fuzzy‐set damage‐assessments are discussed. Measures are presented to compare expected‐ and observed‐damage assessments that are probabilistic or fuzzy. Methods for identifying, studying, and ranking causes of unexpected damage are presented. The use of fuzzy expert systems is proposed as a structured method for using expert knowledge to assess the effect of observed distress on structural integrity. Desiderata are proposed to ensure reliable conclusions. A fundamental desideratum is proposed to maintain uniform levels of precision in the required analyses. Results are presented in layers to determine the professional significance of the unexpected damage. This methodology is applied to the forensic investigation of a three‐story reinforced‐concrete office building damaged in the 1987 Whittier Narrows earthquake.

A study of the principal effects of base uplifting on the seismic response of laterally excited, unanchored, cylindrical liquid‐storage tanks is made. The analysis is implemented by a procedure that is highly efficient and believed to adequately represent essential aspects of the problem. The method takes due account of the continuously varying area of base contact and the nonlinearities associated with membrane action and plastic yielding in the base plate. Critical responses of representative tanks are evaluated for increasing intensities of an earthquake ground motion, and the effects and relative importance of the problem parameters are highlighted. The response quantities examined include the pseudoacceleration of the system, which controls the magnitude of hydrodynamic pressures, the uplift of the plate at points along its junction with the shell, the plastic rotation at the plate boundary, and the maximum compressive stress at critical points in the tank wall. It is shown that the base uplifting may significantly reduce the hydrodynamic pressures, but this reduction may still be associated with increased axial stresses in the tank wall and large plastic rotations at the junction of the shell and base plate. These actions are particularly prominent for tall, slender tanks.

Limitations of present procedures for the design of reinforced concrete structures to withstand seismic loads are discussed. An integrated seismic design procedure (ISDP) is proposed, the main features of which are (1) The automatic selection and scaling of design ground motions given the earthquake magnitude, epicentral distance, and the type of overlying soil strata; (2) prediction of the inelastic behavior of reinforced concrete elements when subjected to random lateral loads up to and including failure; and (3) calculation of cumulative damage that is directly correlated to observed states of damage from laboratory tests of reinforced concrete elements. The analytical tools required to implement these capabilities are discussed. New seismic design criteria are proposed, which are based on acceptable damage indices as functions of earthquake energy content and structural importance. The proposed seismic design approach is demonstrated by means of a practical example involving the seismic retrofit of a spiral reinforced bridge column. This example incorporates new and existing analytical tools, and demonstrates the feasibility of the proposed approach.

Ground snow loads have traditionally been estimated from data, in the form of accumulated water‐equivalent snow depth (density), collected by the National Weather Service (NWS) and the Soil Conservation Service (SCS), at so‐called first‐order weather stations. Extensive data related to snow depth, with the exception of water equivalents, are available from other weather stations (other NWS stations, cooperative state and local agencies' stations, etc.). In this paper, we present a method, using first‐order station data, to relate water‐equivalent depth to snow depth and daily temperature. A locality with a similar weather pattern as the first‐order station that maintains an appropriate weather database (i.e., snow depth and daily temperature) was identified. Using the developed relationship between water equivalents, snow depth, and daily temperature, we convert snow depth to density (i.e., water equivalents) for the local data source. This procedure allows us to significantly expand our ground snow load database, better determine the proper statistical distribution for annual maximum events, and more accurately estimate design ground snow loads. We illustrate the method using climatological data from a NWS first‐order station (including water equivalents), and a nearby cooperative state weather station.

Current methods used in the design of nailed connections do not directly relate to the types of joints found in manufactured housing. These methods do not account for the construction practices used today, such as power nailing equipment, the use of nails with coated shanks, or the fabrication of joints with filler materials separating main wood members. A total of 640 joints were tested to investigate the effects of these five variables on lateral and withdrawal strength of nails: (1) Two wood species groups; (2) low‐ and high‐humidity environments; (3) hand‐ and power‐driven nails; (4) uncoated and coated nail shanks; and (5) presence or absence of filler material in a joint. Tests verified that lateral strength increases with increasing wood density and that as environments become less humid and the moisture content of wood decreases, lateral strength and stiffness of nailed connections increase. In the dry environment, the withdrawal strength of coated nails was almost 90% higher than that of uncoated nails in joints with an oriented strandboard (OSB) filler and almost 40% higher when no filler was used. In high‐humidity conditions, no differences were observed between joints with coated nails and those with uncoated nails. In most other cases, the OSB filler reduced withdrawal strength by an overall average of 25%. This reduction is proportional to the amount of penetration into the solid wood member; therefore, nail length should be increased when an OSB filler is present.

Existing structures, including offshore platforms, often do not meet the same standards as those mandated for new designs. It is also recognized by the engineering community that acceptance criteria for existing structures: (1) Should not necessarily be the same as those for new designs, and (2) should be dependent on the consequences of structural failure. Because of a large population of aging offshore platforms worldwide, reassessment of platforms to determine their fitness for purpose has gained considerable attention by the oil and gas industry and regulatory agencies in the United States and overseas. The reassessment process could be time‐consuming and costly because it requires many steps such as: gathering information on design and physical condition of the platform, modeling of all important damage found, structural evaluation of the platform, calculation of reliability indices, and determination of mitigation and repair schemes. Therefore, a reassessment process covers a large span of technical topics and could require more effort and expertise compared to a new design.

Existing structures, including offshore platforms, often do not meet the same standards as those mandated for new designs. It is also recognized by the engineering community that acceptance criteria for existing structures: (1) Should not necessarily be the same as those for new designs, and (2) should be dependent on the consequences of structural failure. Because of a large population of aging offshore platforms worldwide, reassessment of platforms to determine their fitness for purpose has gained considerable attention by the oil and gas industry and regulatory agencies in the United States and overseas. The reassessment process could be time‐consuming and costly because it requires many steps such as gathering information on design and physical condition of the platform, modeling of all important damage found, structural evaluation of the platform, calculation of reliability indices, and determination of mitigation and repair schemes. Therefore, a reassessment process covers a large span of technical topics and could require more effort and expertise compared to a new design.

Six pretensioned concrete columns, either with fiberglass or steel tendons, $200\mathrm{mm}\times 200\mathrm{mm}\times 2.4m(8\mathrm{in}\text{.}\times 8\mathrm{in}\text{.}\times 8\mathrm{ft})$ long, were tested to failure under axial and eccentric loading. Four were fiberglass pretensioned columns, two with steel square spirals and two with fiberglass square spirals. For comparison, two identical steel pretensioned columns were also tested. The fiberglass tendons were made of S‐2 glass/epoxy, and the fiberglass spirals of E‐glass/epoxy. The slenderness ratio $(l/r)$ for the pretensioned columns was 40, and the eccentricity ratio $(e/h)$ 0.25. The columns were instrumented to measure strains in the concrete, prestressing strands, and lateral deflections at midheight. The results of the tests revealed that the performance of the fiberglass and steel pretensioned columns was very similar, although the confinement provided by the fiberglass spirals was inferior to that of steel. The test results also showed reasonable agreement with nonlinear finite‐element analyses using the PCFRAME program.

A system of cables supported by linear elastic springs and subjected to a concentrated static load is investigated. A method to determine tensions in individual cables and the location in space of the system under load is presented. The cables are assumed to be massless and inextensible. Friction is ignored in the formulation. The method is based on satisfying compatibility and equilibrium conditions for the cables and intercable connections, respectively. A system of coupled, nonlinear algebraic equations is solved numerically using the Newton‐Raphson algorithm. A good initial guess for the unknown quantities, based on the physics of the problem, is provided. The method is applicable to intermediate support cable logging systems. In these systems, suspended cables supported by existing trees (intermediate supports) are used to transport freshly cut logs over sensitive terrain. A model of an example intermediate‐support cable logging system is analyzed by the proposed approach, and the resulting behavior is discussed.

A simplified procedure for computing the dynamic response of flexible tanks containing two liquids under a lateral base motion of arbitrary temporal variation is proposed. It requires the input of ground acceleration in the rigid tank solutions to be replaced by the pseudoacceleration function corresponding to the fundamental mode of vibration of the tank‐liquid system. The procedure yields results with good accuracy for tanks with height‐to‐radius ratio ranging from 0.3 to 1.2, a practical range of height‐to‐radius ratio of most tanks. This simple procedure is an extension of the method used for the evaluation of response quantities for flexible tanks with only one liquid. Response quantities that may be computed by the proposed approach include the hydrodynamic pressure, base shear, base moment, and foundation moment. The effect of the soil‐structure interaction on the response quantities can be taken into account. Only the impulsive components of response quantities are considered in the proposed approach, since the convective components can be computed by considering both the tank and its supporting medium to be rigid.

A constant coefficient difference equation plus boundary conditions is used to replace the usual matrix formulation, to find the shear flows for a torsional analysis of regular multicellular members. This alternative mathematical model facilitates new design insights, one of which is that thick end webs reduce the shear flows to zero in all interior webs. An exact formula (in the form of a constant plus a hyperbolic cosine function) is given for the case in which all webs are identical. The general, $m+1$ cell, case is confirmed by checking the degenerate case of two cells, i.e., $m=1$. Use of the formulas is illustrated with a numerical example. The example and results are expressed in terms of dimensionless parameters so that units, English or metric, are not relevant.