Journal of Structural Engineering

The inelastic rotation capacity of links in eccentrically braced frames (EBFs) is dependent on testing loading history or sequence. Current design rotations for links are based on experiments from the 1980s performed with somewhat arbitrary loading protocols. An AISC link loading protocol, derived from the standard moment frame loading protocol, has been used for link testing since 2002. This protocol results in lower link rotation capacities. Since no previous or current link protocols are based on EBF analyses, nonlinear time-history analyses were conducted to investigate cumulative rotation demands for links in EBFs. Results indicate the AISC link loading protocol is overly conservative, having 1.5 times the cumulative rotation demand and a much higher percentage of large cycles than is justifiable. A rational loading protocol for future link testing was developed based on cumulative and maximum link rotation demands observed from the time-history analyses. Experimental tests performed in a parallel study with this new protocol indicate that link design rotation capacities in the AISC Seismic Provisions are satisfactory.

Tests conducted in the 1980s showed that well-detailed short shear links can exhibit stable and ductile cyclic behavior. Recent tests of prevailing A992 rolled shapes revealed that shear links designed according to current seismic specifications can fail by ductile fracture in the link web, a mode of failure that was not observed in earlier tests. This paper investigates the observed ductile fractures through computational structural simulation. An existing criterion for judging the propensity for ductile fracture initiation in steel is modified based on published tests results for notched bars to better pinpoint the location of ductile fracture initiation. Validated finite- element analyses of previously tested shear links are conducted and the results postprocessed to evaluate the potential for ductile fracture of specimens with several different types of details. Reasons for the occurrence of web fractures in new A992 steel beams as opposed to older links are discussed. An alternative stiffener configuration that mitigates ductile fracture and is at the same time practical to construct is proposed.

A total of 12 large-scale link-column specimens were tested to study the cyclic loading performance of link-to-column connections in eccentrically braced frames. Four different link-to-column connection types with varying configuration and welding details were tested. Each of the four connection types was tested with three different link lengths to consider a wide range of force and deformation environments at the link-to-column connection. The specimens representing the pre-Northridge practice failed after developing only half of the inelastic link rotation required in U.S. building codes. Additional connection types were tested that implemented improved welding practices, as well as specimens that implement design and detailing modifications. This included connections that used the free flange concept and connections that eliminated the weld access holes. The experimental program and key test results are presented in this paper.

Trapezoidal steel box girders (also known as tub girders) may be at their critical stage during construction because the noncomposite steel section must support both the wet concrete and the entire construction load. A lateral bracing system is usually installed at the top flange level to form a quasi-closed box, thereby increasing the torsional stiffness during construction. Typical lateral bracing includes a single-diagonal type (SD type) and a crossed-diagonal type (XD type). The brace forces in box girders with XD type lateral bracing systems can now be estimated with good accuracy. In some horizontally curved box girders with SD-type systems, substantial axial forces, possibly up to 35% of the total force, are developed in struts due to induced torsion and distortion, which are neglected in previous studies. In this study, analytical equations were formulated to compute the brace forces developed in bracing members by taking into account bending and torsional actions of tub girders with SD-type lateral bracing systems. Brace forces in both diagonals and struts can be estimated by these new equations more accurately than with the previous procedures. Member forces due to bending and torsion computed using the new equations were compared with those evaluated by three-dimensional finite element analyses, and excellent correlation was found.

This paper describes the results of a series of experiments on semi-rigid and inelastic joints, and lateral-torsional buckling of the eaves region of portal frames comprising cold-formed single channels bolted back to back at the eaves and apex joints. The tests demonstrate the considerable ductility that is achieved in the back to back bolted connections, which should be sufficient in many cases to accommodate plastic analysis of the portal frames. Variables in the tests include the number of bolts in the connection, the lengths between lateral restraints, the width of the channel flanges, and the strength of the channels. Adjacent column and rafter channels are oriented successively in opposite transverse directions to take advantage of the counterbalancing moments and forces in the back to back connections. This system is important in enhancing the lateral buckling strength of the channels. An innovative method is proposed for connecting each purlin to the web of the column or rafter through a cold-formed angle. Experiments have shown this system to be capable of restraining the frames from failing due to lateral-torsional buckling. This eliminates the idea of having fly bracings, which is normally done in practice to restrain torsional instability.

The compression behavior of cylindrical steel members is affected by geometric imperfections resulting from manufacturing, handling, and damage during installation and service. This paper summarizes the full-scale laboratory tests and the finite element simulations to quantify the effects of both small-scale global and large-scale local geometric imperfections on the compression behavior of nonslender cylindrical steel members. Systematic measurements of the small- and large-scale imperfections on 300- $\mathrm{mm}$ internal diameter pipe specimens with 6.17- $\mathrm{mm}$ wall thickness were used to characterize the imperfections for nonlinear finite element simulations of axial compression. Excellent agreement is shown between the analytical results and the detailed force, deformation, and strain measurements on the test specimens loaded well beyond their peak loads. The analytical and experimental results are summarized for the load reduction effects of the global imperfections as a function of the amplitude, expressed as a percentage of the wall thickness, and the various patterns of the local surface imperfections. Recommendations are made with respect to the imperfection effects on the compression load capacity, methods of modeling the imperfections, and the implications with respect to the design, inspection, and the serviceability decisions in the field.

The Civil Engineering community has long needed methods of accurate global displacement measurement for use in construction and more recently in the areas of structural health monitoring. Global Positioning Systems (GPS) provide one answer to this challenge, with rapid advancements in the available sampling rates and tracking resolution. However, as a relatively new dynamic sensing technology, GPS performance must be thoroughly validated before its application in full scale. This paper details an experimental program with this aim, documenting the performance of a high-precision GPS in terms of its background noise and dynamic tracking ability, while providing position quality thresholds suitable for qualifying the accuracy of tracking in full scale. Given the performance observed in these studies, considerations for full-scale deployment are offered as well as an assessment of its efficacy in monitoring the dynamic motions of a tall building through the Chicago full-scale monitoring project.

A new method for damage identification in large, massive civil structures is presented, which is based on the idea that dead load is redistributed when damage occurs in the structure. The method uses static strain measurements due to dead load only as input to the identification procedure. An analytical model of a fixed-fixed beam is developed in which the damage is represented by a section of reduced flexural rigidity. The damage state is determined by the location, length, and severity of the stiffness reduction. A forward analysis of the beam response is first presented to illustrate how the dead load is redistributed for different damage scenarios. The inverse problem is defined by a constrained optimization problem and is solved using a genetic algorithm. The proposed method correctly identified damage in the beam for a wide range of locations and damage severities. The identification procedure, in general, has a greater degree of success with increasing damage severity. Results show that damage is difficult to identify when it is close to the inflection point of the undamaged beam, where the dead load strain is zero. The effect of measurement noise on the ability to identify damage is investigated in the companion paper.

The effect of measurement error on the results of a new method for damage identification in large, massive civil structures is presented. The damage identification procedure is based on the redistribution of dead load in the structure that takes place when damage occurs. Damage is modeled in the flexural member by a section of reduced flexural rigidity. Static strain measurements are used as input to the procedure. The damage identification parameters are determined using a genetic algorithm. The effect of measurement error is investigated using Monte Carlo simulation. The measurement error is modeled as a Guassian, zero mean random variable. When the model element length is equal to the damage zone length, the results are in good agreement. The correct location and severity is determined even for significant levels of measurement error. The error-to-strain ratio is a convenient parameter for establishing when the error is too significant: results show that for error-to-strain ratios below about 40%, the procedure is able to determine the approximate location and severity of damage accurately. Damage is correctly identified, in an average sense, for the realistic case of when the model element length is not equal to the actual damage zone length. False-positive tests are conducted: the procedure is not prone to incorrectly identify damage when it does not exist, even in the presence of significant measurement error.

Fourway is a simple graphical tool for estimating the conditional dependence of decision variables involved in the seismic design process on the earthquake intensity. The tool was formulated for use with the Pacific Earthquake Engineering Research Center’s performance-based earthquake engineering framework. The tool simplifies the development of decision fragilities by exact determination of first moments and approximate determination of second central moments (variance) of the corresponding probability distributions, without the need for numerical integration of intermediate probabilistic models. For commonly assumed probability distributions of the interim variables, the Fourway method was compared to closed-form and numerical solutions, including measures for evaluating the variance approximations. It is shown that Fourway is a useful first-pass assessment tool for approximating the parameters of the decision variable distributions. Furthermore, Fourway is a tool for evaluating the effects of varying the interim model parameters on the final decision variable fragilities. Examples are provided for the case of a reinforced concrete highway overpass bridge in California.

A consistent approach is proposed for deriving inelastic design spectra and estimates of maximum displacement directly through statistical studies of inelastic response spectra, without the need to resort first to elastic spectra. The main finding is that the seismic response coefficient, or dimensionless yield strength, $C_y$ , that will maintain the mean ductility ratio, $\overline{\mu }$ , equal to a prescribed value can be estimated by an expression of the form $C_y(T,\overline{\mu })=C\left(T\right){\overline{\mu }}^{-n\left(T\right)}$ . $C$ is interpreted as a mean unreduced inelastic spectrum, and $n$ depends only on the elastic natural period, $T$ , of the structure. Explicit formulas for both $C$ and $n$ are obtained for a set of 87 accelerograms recorded in California. $C$ differs from the mean 5% damped elastic spectrum. Another significant result is that $C_y(T,\overline{\mu }=2)$ can be closely approximated by a highly damped mean elastic spectrum, i.e., $\overline{C_e}(T,\xi )$ , with $\xi$ , the critical damping ratio taken to be 30%. Based on these two results, $C_y$ can be conveniently written as $C_y(T,\overline{\mu })=\overline{C_e}(T,30){(\overline{\mu }∕2)}^{-n\left(T\right)}$ . This means that the seismic coefficient of an inelastic system can also be expressed approximately in terms of a highly damped mean elastic spectrum divided by a reduction factor that depends only on $\overline{\mu }$ and $T$ . With $C_y$ determined, it is straightforward to approximate consistently the normalized mean maximum relative displacement of the structure as the product of $C_y$ and $\overline{\mu }$ ; the approximate results differ by less than 10% from the corresponding exact values, for elastic natural periods between 0.1 and $3.0\mathrm{s}$ and mean ductility ratios up to 5.

To implement the concept of a smart structure, a smart active control system is presented. Both fiber Bragg grating (FBG) sensors and neural networks are utilized to construct a system similar to the human being. The smart system is comprised of three parts: a structural condition surveillance system, three converters, and a controller. By distributing as many sensors as possible in the key areas in buildings, FBG sensors can be used for assessing structure soundness while acting as dendrites in a neural network system. Comparison of structural responses under an uncontrolled system, traditional optimal linear quadratic gain, and the proposed NEURO-FBG control system verifies the advantages of using this novel technique approach. The system robustness is evaluated under both time delay and disconnection problems. Analytical results demonstrate that the NEURO-FBG system can effectively control the response of the structure and provide a more reliable system than ordinary active control.

Experimental results for a full-scale steel frame structure with a smart active control system under shaking-table testing are presented. To this end, a full-scale steel frame structure with the proposed control system is examined under earthquakes simulated by a 5 m $\times$ 5 m shaking table with several representative earthquakes arranged to assess the performance and robustness of the proposed control system. Signals detected from fiber Bragg grating sensors showed superior performance to conventional strain gauges in structure surveillance. Experimental data recorded during earthquake excitation, for structural strain, also demonstrated the improved performance of the proposed control system over the traditional active control. Experimental results demonstrated that the control system can be applied to buildings after the whole testing process.

The provision of ductile connections is a critical element that is often lacking in the design of a heavy timber structure to resist seismic loads. During the last decade, a novel connection was developed that combined reliable high strength and stiffness with excellent ductile properties, the so-called tube connection. The steel tubes fit into oversized holes that ease the assembly of the connection members. By expanding the diameter of the steel tube, a perfectly tight fit can be realized. To prevent timber splitting when the tube diameter is increased, densified veneer (ply)wood is glued on the timber members as reinforcement. This paper reports the results of an experimental study to determine the hysteresis behavior of the tube connection. Unprecedented energy dissipation capabilities were found in comparison with other timber connections. Applied in timber frame structures, its seismic performance will be enhanced considerably and a more competitive structural timber design is within reach.

A consistent basis for statistically reducing fiber-reinforced polymer (FRP) composite material property test data for load and resistance factor design (LRFD) of composite structures is developed in this paper. A two-parameter Weibull probability distribution is recommended for modeling both strength and stiffness properties. Consistent with practice for other materials used in structural engineering applications, the 5-percentile value of strength of coupons and components, as well as modulus values, appropriately adjusted for the small size of typical data samples, is recommended for the nominal value of strength for LRFD applications. Statistical uncertainty due to small sample size in the data reduction process is accounted for through the use of a data confidence factor, which is the tolerance limit defined as the 80th percent lower confidence level of the 5-percentile value of the population. With the nominal values so determined, appropriate resistance factors for LRFD-based design codes can be derived and limits for qualification and acceptance criteria for FRP composite structural products can be established.