Implementation of low damage construction: What are the challenges?

Christchurch earthquake events have raised questions on the adequacy of performance-based provisions in the current national building code. At present, in the building code the performance objectives are expressed in terms of safety and health criteria that could affect building occupants. In general, under the high intensity Christchurch events, buildings performed well in terms of life-safety (with a few exceptions) and it proved that the design practices adopted for those buildings could meet the performance objectives set by the building code. However, the damage incurred in those buildings resulted in unacceptably high economic loss. It is timely and necessary to revisit the objectives towards building performance in the building code and to include provisions for reducing economic implications in addition to the current requirements. Based on the observed performance of some buildings, a few specific issues in the current design practices that could have contributed to extensive damage have been identified and recommended for further research leading towards improved performance of structures. In particular, efforts towards innovative design/construction solutions with low-damage concepts are encouraged. New Zealand has been one of the leading countries in developing many innovative technologies. However, such technically advanced research findings usually face challenges towards implementation. Some of the reasons include: (i) lack of policy requirements; (iii) absence of demonstrated performance of new innovations to convince stakeholders; and (iv) non-existence of design guidelines. Such barriers significantly affect implementation of low damage construction and possible strategies to overcome those issues are discussed in this paper

Prediction of spatially distributed seismic demands in specific structures: Ground motion and structural response

The efficacy of various ground motion intensity measures (IM’s) in the prediction of spatially distributed seismic demands (Engineering Demand Parameters, EDP’s) within a structure is investigated. This has direct implications to building-specific seismic loss estimation, where the seismic demand on different components is dependent on the location of the component in the structure. Several common intensity measures are investigated in terms of their ability to predict the spatially distributed demands in a 10-storey office building, which is measured in terms of maximum interstorey drift ratios and maximum floor accelerations. It is found that the ability of an IM to efficiently predict a specific EDP depends on the similarity between the frequency range of the ground motion which controls the IM and that of the EDP. An IM’s predictability has a direct effect on the median response demands for ground motions scaled to a specified probability of exceedance from a ground motion hazard curve. All of the IM’s investigated were found to be insufficient with respect to at least one of magnitude, source-to-site distance, or epsilon when predicting all peak interstorey drifts and peak floor accelerations in a 10-storey RC frame structure. Careful ground motion selection and/or seismic demand modification is therefore required to predict such spatially distributed demands without significant bias

Prediction of spatially distributed seismic demands in specific structures: Structural response to loss estimation

A companion paper has investigated the effects of intensity measure (IM) selection in the prediction of spatially distributed response in a multi-degree-of-freedom structure. This paper extends from structural response prediction to performance assessment metrics such as: probability of structural collapse; probability of exceeding a specified level of demand or direct repair cost; and the distribution of direct repair loss for a given level of ground motion. In addition, a method is proposed to account for the effect of varying seismological properties of ground motions on seismic demand that does not require different ground motion records to be used for each intensity level. Results illustrate that the conventional IM, spectral displacement at the first mode, Sde(T1), produces higher risk estimates than alternative velocity-based IM’s, namely spectrum intensity, SI, and peak ground velocity, PGV, because of its high uncertainty in ground motion prediction and poor efficiency in predicting peak acceleration demands

Probabilistic seismic indoor injury estimation

Most injury models in existence either estimate injuries at a regional level and/or focus only on fatalities. In regions with good engineering practice, the likelihood of building collapse is rare and hence fatality risk is also correspondingly low. Research has shown that in such situations non-fatal injuries are likely to result in larger economic loss than fatalities due to their higher incidence, despite non-fatalities having lower consequence. A new building-specific method of indoor injury estimation is proposed in this paper. Injuries are considered due to: (i) occupants being struck by toppling contents; and (ii) occupants losing balance and falling. This model considers the spatial distribution of occupants in the building, time-occupancy relationships, and the severity of injury to occupants. A simple room layout is used to demonstrate the application of the model

Cost Effective Computer Vision Based Structural Health Monitoring using Adaptive LMS Filters

Structural health monitoring (SHM) algorithms based on Adaptive Least Mean Squares (LMS) filtering theory can directly identify time-varying changes in structural stiffness in real time in a computationally efficient fashion. However, the best metrics of seismic structural damage are related to permanent and plastic deformations. The recent work done by the authors uses LMS-based SHM methods with a baseline non-linear Bouc-Wen structural model to directly identify changes in stiffness (modelling or construction error), as well as plastic or permanent deflections, in real-time. The algorithm validated, in silico, on a non-linear sheartype concrete structure using noise-free simulation-derived structural responses. In this paper, efficiency of the proposed SHM algorithm in identifying stiffness changes and plastic/permanent deflections under different ground motions is assessed using a suite of 20 different ground acceleration records. The results show that even with a fixed filter tuning parameters, the proposed LMS SHM algorithm identifies stiffness changes to within 10% of true value in 2.0 seconds. Permanent deflection is identified to within 14% of the actual as-modelled value using noisefree simulation-derived structural responses. Accuracy of the proposed SHM algorithm mainly relies on providing high-speed structural responses. However, due to a variety of practical constraints, direct high frequency measurement of displacement and velocity is not typically possible. This study explores the idea that emerging high speed line scan cameras can offer a robust and high speed displacement measure required for the modified LMS-based SHM algorithm proposed for non-linear yielding structures undergoing seismic excitation, and can be used for more precise estimation of the velocity using measured acceleration and displacement data. The displacement measurement method is tested to capture displacements of a computer-controlled cart under 20 different displacement records. The method is capable of capturing displacements of the cart with less than 2.2% error

Improved seismic hazard model with application to probabilistic seismic demand analysis

An improved seismic hazard model for use in performance-based earthquake engineering is presented. The model is an improved approximation from the so-called 'power law' model, which is linear in log-log space. The mathematics of the model and uncertainty incorporation is briefly discussed. Various means of fitting the approximation to hazard data derived from probabilistic seismic hazard analysis are discussed, including the limitations of the model. Based on these 'exact' hazard data for major centres in New Zealand, the parameters for the proposed model are calibrated. To illustrate the significance of the proposed model, a performance-based assessment is conducted on a typical bridge, via probabilistic seismic demand analysis. The new hazard model is compared to the current power law relationship to illustrate its effects on the risk assessment. The propagation of epistemic uncertainty in the seismic hazard is also considered. To allow further use of the model in conceptual calculations, a semi-analytical method is proposed to calculate the demand hazard in closed form. For the case study shown, the resulting semi-analytical closed form solution is shown to be significantly more accurate than the analytical closed-form solution using the power law hazard model, capturing the 'exact' numerical integration solution to within 7% accuracy over the entire range of exceedance rat

Influence Of HF2V Damping Devices On The Performance Of The SAC3 Building Subjected To The SAC Ground Motion Suites

Recent advances in energy dissipation for structural systems can create structural connections that undergo zero sacrificial energy absorbing damage, even at extreme story drifts. However, questions exist around the ability of such structures to re-center after a major event. In this paper, the seismic performance of the as-designed SAC LA3 seismic frame with rigid moment connections at the beam ends is compared with the same frame using semi-rigid connections with high force-to-volume (HF2V) lead dissipators. Non-linear dynamic analysis is preformed using Abaqus™. With respect to re-centering, the presence of the gravity frames in the model is also considered. It was found that the placement of dissipators, ignoring the effect of gravity frames, caused a 12% increase in period due to the decreased stiffness of the connections. During design level ground shaking the semi-rigid connections with HF2V dissipators have slightly lower accelerations, up to an 80% increase in peak drift, and a 200% increase in the permanent displacement compared to the as-designed case, but no structural damage is expected. When gravity frames are considered, the floor accelerations decrease further, the peak displacements do not significantly change, but the residual storey drift ratios reduce to approximately 0.17%. This result is less than one half that of the as-designed frame, where typically gravity frame effects are not considered. The addition of braces with a stiffness 20% of the pushover stiffness ensures that the structures can re-center after any given event to within construction error. The realistic non-linear dynamic analyses combining HF2V lead dissipators with gravity frames and well-designed non-structural elements creates a system with almost no structural damage and low residual displacements

Location of Plastic Hinges in Axially Loaded Steel Members

New Zealand and Australian steel structure design standards contain equations to encourage yielding at the ends of steel members rather than along their lengths. This paper evaluates the accuracy of these equations using a commercially available computer program as well as an analytical procedure. The analytical procedure considers non-linear geometric effects and material effects of the member stiffness by considering stability functions in conjunction with residual stress effects. New equations to prevent yielding away from the member ends, which are less conservative than the current code equations, are developed. Simplifications of these equations being considered for adoption into the New Zealand steel structure design standard are described

Slab Effects on Beam-Column Subassemblies - Beam Strength and Elongation Issues

This paper describes the effect of composite slabs in increasing beam strength and its implications for design. It also discusses the “beam-growth” phenomena, which can detrimentally influence the performance of a frame with reinforced concrete or precast concrete beams, and its impact on steel beams with RC slabs. From the subassembly testing conducted the slab increased the beam strength by around 40%. However the slab could not maintain strength at large drifts without degradation with transverse or longitudinal decking placed around the columns. This indicates that while transverse or longitudinal slabs should not be considered in design to size the beam, they should be considered in the beam overstrength calculations for the design of other members. Also, both rational considerations and experimental results 2 indicate that beam growth effects tend to be small for composite steel beams because the steel beams are able to yield in both tension and compression

Is the Asymmetrical Friction Connection (AFC) a low damage dissipater?

Asymmetrical Friction Connections (AFC) are used in structures in earthquake zones to dissipate energy without causing major damage to the structural members. This means that the structure itself does not require replacement after a major seismic event. Testing of these connections has been undertaken and degradation in strength has been observed. However, (i) reasons for this degradation have not been clear, (ii) a means of assessing the strength degradation has not been available, (iii) the importance of the strength degradation (which is related to the amount of strength degradation) has not been described, (iv) the ability to reinstate the joint using new bolts is not known, and (v) effective friction factors for the connection after connection reinstatement are not known. This paper describes the testing of AFC specimens with high hardness shims (i.e. Bisalloy 500) under increasing cyclic displacements to address the issues stated above. Tests were conducted twice with the same setup. In the second test, the change in performance as a result of the first test was able to be observed. Then the bolts were replaced and tests were conducted twice more