FEMA P695 Research Articles

Seismic performance assessment of steel frames with slack cable bracing systems

Slack cable collapse prevention system is a cost-effective and simple method for seismic upgrading of existing structures. In this study, the effectiveness of adding slack cables to 2, 4, 8, and 12-story steel frames are assessed. The pushover analyses were performed to evaluate the general behavior of the original and braced frames. Considering 22 pairs of far-field ground motion records per FEMA P695, the nonlinear time-history analyses, as well as Incremental Dynamic Analyses (IDAs), were conducted. The maximum compression force of the first story columns, base shear demand, and the Collapse Margin Ratio (CMR) were determined and compared for original and braced frames. The fragility curves were developed and compared for the structures. Obtained results show that the slack cables postpone the collapse of the frames without increasing the initial stiffness of the structure. The average of maximum values of column axial force for the frames with slack cables are close to original frames, while it has a considerable increase in typical cable brace system cases. All original frames exceed the 10% probability when subjected to Maximum Considered Earthquake (MCE). However, the probability of collapse for the braced structures in MCE decreased significantly to less than 10%. A significant improvement in the collapse safety could be observed in the braced frames. The slack cable technique has demonstrated almost similar performance to the typical cable braces and even superior protection in terms of structural column axial forces and base shear.

Investigation of Seismic Coefficient Factor for Divergent Bracing Frame with TADAS Damper Utilizing the FEMA P695

Investigation of Seismic Coefficient Factor for Divergent Bracing Frame with TADAS Damper Utilizing the FEMA P695

Consistent one-bay frame simplified model for efficient seismic evaluation of steel moment frame buildings with equal and unequal bay lengths

A Consistent One-Bay Frame (COBF) simplified model for the efficient estimation of Engineering Demand Parameters (EDPs) in Steel Moment Resisting Frames (SMRFs) with equal and unequal bay lengths is proposed. The geometry of the Substitute One-Bay Frame (SOBF) simplified model has been used, and the elastic and inelastic properties of the beams and columns in this model are determined using an energy consistent approach in a way that the energy balance is established between the members of the original SMRF and the SOBF simplified model. This way, the SOBF simplified model, which originally could only be used for MRFs with equal bay lengths, can now be used for MRFs with any geometric arrangement for bay lengths. Two 9- and 20-story SMRFs with equal bay lengths and two others with unequal bay lengths have been employed as structural models. The concentrated plastic hinges with modified Ibarra-Medina-Krawinkler (IMK) deterioration model have been used to apply strength and stiffness deterioration in numerical models. The accuracy of the proposed simplified model in estimating the EDPs along the height of the structure has been evaluated using Nonlinear Time History Analysis (NLTHA) under 22 pairs of FEMA P-695 earthquake records at the three seismic intensity levels of SLE, DBE, and MCE. The proposed simplified model has been compared with the Consistent Generic (CG) simplified model previously developed for SMRFs. This comparison suggests that the proposed simplified model is more efficient and reliable. Evaluating the seismic performance of the proposed simplified model using the conventional Incremental Dynamic Analysis (IDA) method and the Endurance Time Analysis (ETA) method at all of the seismic intensity levels reveals that using the COBF simplified model along with the ETA simplified method not only reduces the Degrees of Freedom (DOFs) of the original SMRF and the number of the NLTHAs but also leads to a proper estimation of the EDPs of the original SMRF from low seismic intensity level to collapse intensity level. Therefore, it is very helpful to use it for reducing the complexities of the new generation of Performance-Based Earthquake Evaluation.

Seismic performance factors for timber buildings with woodframe shear walls

Seismic performance factors are an engineering tool to estimate force and displacement demands on structures designed through linear methods of analysis. In Chile, the NCh433 standard provides the regulations, requirements, and factors for seismic design of several structural typologies and systems. However, when it comes to wood frame structures, previous research has found that the NCh433 provisions are highly restrictive and result in over-conservative designs. Therefore, this paper presents an experimental and numerical investigation aimed at proposing new, less restrictive seismic performance factors for wood frame buildings. Following the FEMA P-695 guidelines and a novel ground motion set for subduction zones, this research embraced: (1) testing of several full-scale specimens, (2) developing of detailed and simplified numerical models, and (3) analyzing the seismic performance of a comprehensive set of structural archetypes. 201 buildings were analyzed and results showed that changing the current NCh433 performance factors from R = 5.5 & Δmax = 0.002 to R = 6.5 & Δmax = 0.004 decreases the average collapse ratio of wood frame structures by 13.3% but keeps the collapse probability below 20% for all the archetypes under study. Besides, it improves the cost-effectiveness of the buildings and enhances their competitiveness when compared to other materials, since savings of 40.4% in nailing, 15.9% in OSB panels, and 7.3% in timber studs were found for a 5-story building case study. Further analyses showed that the buildings designed with the new factors reached the “enhanced performance objective” as defined by the ASCE 41–17 standard, guaranteeing neglectable structural and non-structural damage under highly recurring seismic events. Finally, dynamic analyses revealed that the minimum base shear requirement Cmin of the NCh433 standard is somewhat restrictive for soil classes A, B, and C, leading to conservative results compared to archetypes where the Cmin requirement did not control the structural design.

Study of the seismic behavior of rigid and semi-rigid steel moment-resisting frames

Steel moment resisting frames are prevalent seismic force resisting systems that are used extensively in high seismic regions worldwide. New design procedures suggested by codes have resulted in extensive use of semi-rigid connections in moment resisting frames. These connections have some degree of flexibility. This study focuses on seismic performance of steel moment-resisting frames with different degrees of moment connection rigidities. Three moment frames (5, 10 and 15 stories), each with different levels of connection rigidities (full, 80%, 70% and 60% rigidities) are analyzed under the 22 far-field ground motions presented in FEMA P695. Detailed modal analysis, non-linear static (pushover) analysis and incremental dynamic analysis (IDA) were conducted. Results reveal that semi-rigid frames have lower base shears and higher structural deformations, but higher energy absorption capability. Lastly, detailed performance analysis was conducted. The results show that probability of exceeding the fully operational performance level is higher in semi-rigid frames as compared to rigid connections. The inclusion for the rigidity in the beam-column can have an impact on the performance of steel moment frames which cannot be ignored during the design and retrofit of steel moment-resisting frames.

The Response Modification Factor for Seismic Design of Integral Abutment Bridges

The response modification factor (R factor) is a crucial parameter for calculating the design seismic forces applied to a bridge structure. This factor considers the nonlinear performance of bridges during strong ground motions. Conventional bridge structures rely on the substructure components to resist earthquake forces. Accordingly, there are R factors available in the design codes based on the type of bridge substructure system. Lateral load resisting system of Integral Abutment Bridges (IABs) in the longitudinal direction is more complex than ordinary bridges. It involves the contributions from soils behind the abutments and soil/structure interaction (SSI) in addition to existing rigid connection between the superstructure and abutments. There is no R factor available in any design code throughout the world for IABs in the longitudinal direction that considers all these parameters. In this research, the Federal Emergency Management Agency publication FEMA P695 methodology has been applied to estimate the R factor for IABs. It is found that 3.5 could be a safe and valid R factor in the longitudinal direction for seismic design of such bridges.

Evaluation of seismic performance of rotational-friction slip dampers in near-field and far-filed earthquakes

In this study, the performance of rotational-friction slip dampers in steel structures with different heights is investigated by the use of fragility curves. The use of dampers is one of the methods for vibration control of structures by simultaneously increasing both the structural stiffness and damping. Rotational-friction slip dampers are among the passive control devices that dampen the earthquake energy through their stable cyclic behavior. To study the performance of these devices in steel structures, 3, 6 and 9-story steel moment frame structures are designed, and the mentioned dampers are attached to the structure by Chevron braces. To account for the earthquake uncertainty, with the aid of incremental dynamic analysis (IDA), the damper-equipped structure is subjected to both near-field and far-field ground motion records. The acceleration and drift engineering demand parameters are selected as the functions to quantify the damage states, and the design, modeling and material properties uncertainties are considered in accordance with FEMA P-695. Evaluation of statistical results and comparison of the fragility curves, shows that the probability of failure at different damage states decreases when the dampers are added to the structure. This decrease is more remarkable in low-rise structures and near-fault ground motions.

Seismic design coefficients and factors for coupled composite plate shear walls/concrete filled (CC-PSW/CF)

ASCE7–16 (2016) [4] defines three seismic performance factors to approximately predict the inelastic response of a seismic resisting system. These factors are the response modification factor, R; deflection amplification factor, Cd; and the system over-strength factor, Ωo. The research presented here was conducted, using FEMA P695 methodology, to determine the value of the above factors for a special seismic-force resisting system defined as Coupled Composite Plate Shear Walls-Concrete Filled (CC-PSW/CF). The ASCE 7–16 (2016) [4] and AISC 341–16 (2016) seismic provisions provide specific requirements for the use of planar composite steel plate shear walls in seismic regions. However, the ASCE-7–16 standard does not differentiate between coupled and non-coupled walls. Coupled walls can benefit from the added energy dissipation provided by their coupling beams and are accordingly expected to exhibit better seismic hysteretic behavior than uncoupled walls. Therefore, coupled walls systems should arguably have a higher response modification factor value. In this paper, the FEMA P695 approach taken for determining the seismic parameters of CC-PSW/CF is presented. To enhance confidence in the results, the Incremental Dynamic Analyses needed as part of the P695 procedure were conducted in parallel using two different non-linear hysteretic models. Complementary studies were also conducted to investigate the sensitivity of results to assumptions related to damping and yielding models. Results show that values R = 8, Cd = 5.5, and Ωo = 2.5 would be appropriate for CC-PSW/CF.

Collapse probability of code-based design of a seismically isolated reinforced concrete building

In this study, the collapse probability of a Reinforced Concrete (RC) seismically isolated building designed according to ASCE 7-16 is given using the Incremental Dynamic Analysis (IDA) and Adaptive Incremental Dynamic Analysis (AIDA) methods. In a region where spectral demands of different intensity levels are known, thirty ground motions are selected and gradually changed from first to last seismic intensity level to match hazard-consistent properties in the extreme events, as given in the AIDA procedure. The collapse probabilities were obtained using FEMA P695 based IDA and AIDA methods by unidirectional analysis results, separately. The fragility curve obtained using bidirectional analysis results of the AIDA method was adjusted to the fragility curve acquired utilizing unidirectional analysis results by a correction factor to consider the accidental torsion effects. It is determined that the collapse probability of isolators is over the ASCE 7-16 acceptable limit, which is 10% for building in Risk Category of I and II at the Risk-Targeted Maximum Considered Earthquake (MCER) level. Also, it is shown that reducing the displacement capacity of isolators according to ASCE 7-16 causes the probability of collapse to exceed the 10% limit considerably. When the displacement capacity was increased to 1.25 times of total maximum displacement, the collapse probability decreases below the acceptable limit at the MCER level. The analysis results reveal that the collapse probability of isolators is sensitive to ground motion suites. Despite the isolator displacement capacity, the fragility curves of the superstructure drift demands, beam plastic rotations, and column tensile strains provide sufficient exceedance probabilities at the MCER level.

Evaluation of rocking motion on the seismic performance of a linked column frame system

A linked column frame (LCF) is a seismic-load-resisting system with ductile behaviour. In this system, the link beam, operating as a shear fuse, reduces or eliminates damage to other components of the structure under various hazard levels. In this study, by combining rocking motion and changes in the linked column pattern, an innovative seismic-resisting system was developed. The aim was to improve the performance of the LCF system by reducing damage to the flexural beams and columns and to concentrate damage in the link beams. For this purpose, a three-storey model based on the SAC building was designed based on storey drift demands. The model was evaluated by incremental dynamic analysis (IDA) according to Fema P-695 instructions using OpenSees. Results of changes in the linked column pattern from the IDA indicated a 20% increase in the structural capacity of the LCF model. Compared with the model without rocking motion, the model with rocking motion resulted in reduced structural capacity, shear and horizontal acceleration of the storey, and maximum drift. This novel LCF seismic system provides self-centring and damage control by using features such as rocking motion.

Seismic performance of mid-rise thin concrete wall buildings lightly reinforced with deformed bars or welded wire mesh

The increasing demand of housing in urban areas in Latin America has driven the construction of a significant number of buildings using reinforced concrete (RC) walls with thickness equal or lower than 100 mm, with a single layer of reinforcement provided by a welded-wire mesh (WWM) or by deformed bars (DB). Several concerns related to the lack of ductility, scarce evidence on its behavior during earthquakes, and the lack of clarity of design guidelines in earthquake-resistant codes for this structural system have risen in recent years. This research aims at providing evidence on the seismic risk of thin concrete wall buildings reinforced with two types of reinforcement. A six-story building, constructed in Bogotá, Colombia with walls having 100 mm thickness and detailed with WWM is used as case study. After gathering relevant information from the building structural drawings, a nonlinear model was created in OpenSees using the shear-flexure interaction multiple vertical line element (SFI-MVLEM). To evaluate the effect of steel ductility, a second benchmark model of the studied building was created using deformed bars (DB) as reinforcement. Incremental dynamic analyses were conducted on the models using the far field ground motion suite provided by the FEMA P-695 and a set ground motions for subduction zones, which served as input for the development of fragility functions for the building. The results show that the fracture of reinforcing steel is a frequent failure mode of the building reinforced with WWM, whereas the failure of the building reinforced with DB was controlled by the drift limit of the walls. The findings also show that probabilities of failure for the ground motions scaled to the maximum credible earthquake are 41% and 25% for the building reinforced with WWM and DB, respectively. These large probabilities suggest that the use of thin RC walls for mid-rise buildings should be limited in seismic prone areas, especially those detailed with WWM.

Quantifying the Ductility-Related Force Modification Factor for 10-Story Timber–RC Hybrid Building Using FEMA P695 Procedure and Considering the 2015 NBC Seismic Hazard

In this work, a 10-story uncoupled (10S-U) hybrid seismic force resisting system, consisting of cross-laminated timber (CLT) walls and reinforced concrete (RC) beams, is considered. Required design ductility factor Rd, in congruence with the National Building Code of Canada, was developed using FEMA P695 collapse risk procedure. Two trial Rd factors, Rd=2 and Rd=3, were first used to design the hybrid building for seismicity of Vancouver, BC, and 3D numerical models were developed in Open System for Earthquake Engineering Simulation (OpenSees) finite element framework. The energy dissipation of the structural system was enhanced using buckling restraining brace hold-downs and energy dissipator connection between the panels. The rocking response mechanism governed and, as a result, the cyclic pushover results show recentering capability. A suitable set of 30 ground motion records that reflect the seismic hazard of Vancouver, British Columbia, was selected in congruence with the 2015 National Building Code of Canada (NBC). Using incremental dynamic analysis, the collapse risk and collapse margin ratios were obtained to check the suitability of the two proposed Rd factors. The Rd=2 factor was shown to be acceptable for the 10S-U structural system.

Collapse performance assessment of steel moment frames designed to Eurocode 8

This paper reports on a study performed on steel moment-resisting frame buildings with the aim of evaluating the safety margins against collapse of buildings designed according to Eurocode 8 (EC8) when subjected to maximum considered earthquake ground motions. To this end, the methodology proposed in FEMA P695 is employed. A set of 360 archetype buildings are designed according to EC8 using code-prescribed behaviour factors for medium and high ductility class structures. Additionally, behaviour factors determined according to an Improved Force-Based Design (IFBD) methodology are also adopted. IFBD allows for a more rational selection of the behaviour factor and is fully compliant with the EC8 prescriptions. The results from the seismic performance assessment of the structures allow for an improved understanding of the seismic behaviour of steel MRFs designed according to EC8 and highlight the efficiency that is achieved with the adoption of the IFBD methodology, both in terms of reductions of steel weight as well as in terms of the consistency between the design assumptions and the ductility demand that is explored from the buildings. Moreover, the results also indicate that moment-resisting frame buildings designed according to EC8 are characterized by acceptable safety margins against collapse.

Ductility and overstrength of shape-memory-alloy reinforced-concrete shear walls

The unique properties of superelastic shape-memory-alloy (SMA) bars have motivated researchers to investigate their use as reinforcing bars for concrete elements. They were found to decrease seismic residual deformations, while increasing seismic inelastic deformations. This characteristic deformation behaviour requires an assessment of the seismic design parameters of SMA reinforced concrete walls. This paper addresses this requirement by evaluating their ductility and overstrength factors. A total of 972 walls were analyzed under a quasi-static lateral load. Suitable values for the overstrength and ductility factors were estimated for two proposed locations of SMA bars. FEMA P695 was then utilized to evaluate the seismic safety margin for case study buildings, which were designed based on the estimated seismic design parameters.

Soil-structure interaction effects on the seismic collapse capacity of steel moment-resisting frame buildings

FEMA P695 proposes quantification of soil-structure effects as a field of research to adjust the collapse margin ratio. The Winkler foundation with elastic and inelastic stiffness is used to address interaction effects. Three-, nine- and 20-story steel moment-resisting frame buildings are selected, and the collapse margin ratio of the buildings for site classes C and D are evaluated. The nonlinear soil behavior leads mostly to an increase in seismic collapse probability compared to linear soil behavior and fixed-base conditions. Finally, simplified adjustment equations are proposed for estimating the collapse margin ratio of structures considering the soil-structure interaction effects.

Seismic collapse safety and response modification factor of concrete frame buildings reinforced with superelastic shape memory alloy (SMA) rebar

Seismic performance of concrete frame buildings reinforced with superelastic Shape Memory Alloy (SMA) rebar is evaluated in terms of Collapse Margin Ratio (CMR). CMR is the ratio of the ground motion intensity at the median collapse to the Maximum Considered Earthquake (MCE) ground motion intensity at the fundamental period of the building frame. The results are compared with the performance of concrete frames reinforced with regular steel rebar only. Collapse safety assessment is performed using both nonlinear static pushover and nonlinear Incremental Dynamic Analysis (IDA) with 20 ground motion records. For analysis, three different stories (3, 6 and 8) of Reinforced Concrete (RC) buildings, each with four different types of reinforcement detailing, are considered: (i) steel reinforcement (Steel) only and (ii) SMA rebar used in the plastic hinge region of ground floor columns and steel rebar in other regions (SMA-CM) (iii) SMA rebar used in the plastic hinge region of all the beams and steel rebar in other regions (SMA-BM) (iv) SMA rebar used in the plastic hinge region of both beams and ground floor columns (SMA-BM-CM). Results indicate an increase in collapse capacity for 3 and 8 story concrete frame structures while using SMA as reinforcement. The seismic response modification factor for concrete frame reinforced with shape memory alloy has also been evaluated based on an acceptable collapse margin ratio according to the guidelines provided in FEMA P695.

Hybrid braced frame with buckling-restrained and strong braces to mitigate soft story

The steel concentric braced frames (CBFs) are the common structural system for buildings. Unlike high strength and stiffness, brace buckling in CBFs results in high nonlinear behavior, which causes damage concentration in one or a few stories during a severe earthquake. This paper proposes a hybrid CBF bracing system to reduce soft-story mechanisms and improve the structural behavior of the CBF system. The proposed system is composed of a buckling restrained brace (BRB) or an energy dissipation device and a strong brace (SB) in one span. The strong braces are designed to have an elastic performance during an earthquake. The direction of BRB and SB are changed, inversely, story by story in a zigzag form. This configuration leads to paradoxical results: stiffness and ductility. This improves both the soft story mechanisms and the buckling capacity of CBFs. In this study, a set of 6-story structural models with different bracing patterns including inverted-V, multi-X, zipper, hexa (hexagonal bracing configuration), and strongback braced frames were evaluated through nonlinear static (pushover) and dynamic analyses (IDA). The collapse performance of structural models was assessed by comparing the seismic behavior of conventional CBFs with similar bracing configuration based on FEMA P695 criteria. From the evaluation of collapse fragility curves, the hybrid bracing system had more safety margin against collapse during severe earthquakes than conventional CBFs.

Seismic performance factors of a dual system with IMRF and cable-cylinder bracing

In recent years, numerous approaches have been adopted to improve the performance of structural systems during earthquakes. Cable-cylinder bracing is an innovative displacement-restraint bracing method using wire ropes or cables that are bundled together with a cylinder at their junction. As designing structures with cable-cylinder braces based on the current design guidelines requires design parameters for the system, in this study, Seismic Performance Factors (SPFs) consists of the response modification factor (R), the over-strength factor (Ω0), and the deflection amplification factor (Cd) are quantified according to the methodology proposed by FEMA P695. Following this procedure, a set of 2-, 4-, 6-, and 8-story archetypes were designed based upon presumed SPFs. The performance of each archetype was then examined through conducting nonlinear static analysis (pushover) and Incremental Dynamic Analysis (IDA). Collapse probability of archetypes by utilizing fragility curves and considering uncertainty sources have been assessed. Required parameters were evaluated for the designed archetypes and the values were compared with the accepted criteria to confirm the initial assumed SPFs for the cable-cylinder bracing system.

Performance of Optimum Tuned PID Controller with Different Feedback Strategies on Active-Controlled Structures

In this study, multi-story structures with different combinations (on each floor and only the first floor) of active tendon control systems driven by a proportional–integral–derivative (PID) controller were actively controlled. The PID parameters, Kp (proportional gain), Td (derivative gain), and Ti (integral gain) for each structure, were optimally tuned by using both the harmony search algorithm (HS) and flower pollination algorithm (FPA), which are metaheuristic algorithms. In two different active-controlled structures, which are formed according to the position of the PID, the structural responses under near-fault records defined in FEMA P-695 are examined to determine the appropriate feedback which was applied for displacement, velocity, acceleration, and total acceleration. The performance of the different feedback strategies on these two active-controlled structures is evaluated. As a result, the acceleration feedback is suitable for all combinations of the active control system with a PID controller. The HS algorithm outperforms the optimum results found according to the FPA.

Seismic force-modification factors for mid-rise wood-frame buildings with shearwalls using wood screws

Wood-screw shearwall (WSS) is generally considered to have limited ductility under seismic load. Conservatively, minimum ductility-related force-modification factor, Rd, of 1.0 and over-strength related force-modification factor, Ro, of 1.3 are used in the National Building Code of Canada for seismic design. In this study, the structural performance of WSS’s with different thicknesses of wood-based panels and sizes of wood screws was investigated by conducting monotonic and cyclic loading tests. The combined force-modification factor RdRo was estimated based on shearwall test results. To verify the estimated modification factors, mid-rise light wood-frame buildings (LWFBs) with different numbers of storeys, wall aspect ratios, magnitudes of gravity and diaphragm flexibilities were designed and analysed numerically. Nonlinear dynamic analyses were performed with 22 scaled Far-Field earthquake time-history records. The seismic response of the mid-rise LWFBs with WSS’s under design hazard level met the design criteria of strength and deformation, and the collapse margin ratios obtained by using a simplified FEMA P695 method complied with the requirements. The study indicates that it is appropriate to use an Rd factor of 1.5 and an Ro factor of 1.7 for seismic design of WSS’s.