Journal of Performance of Constructed Facilities

Concrete-filled steel tubular columns have been studied extensively worldwide. Such columns provide stable energy dissipation, which is ideal for earthquake engineering application. However, most applications use rectangular steel tubular sections. In the location of wall intersections, these sections often create sharp corners, which are architecturally displeasing. In some facilities such as schools and correctional facilities, sharp corners are even prohibited due to their potential hazard to occupants. To resolve the issue, concrete-filled crisscross steel tubular columns (CCSTCs) have been proposed. Three half-scaled specimens were constructed and tested under cyclic displacement loading history. The experimental results were used to calibrate numerical models of the components and further used to examine the seismic behavior of a three-story prototype office building located in Los Angeles. The results showed the proposed CCSTC has excellent energy dissipation and postearthquake performance, and hence it can be used as an alternative lateral force resisting system for seismic applications.

Earthquakes are rare natural catastrophes, causing severe impacts on society due to loss of lives, damage to built facilities, and business interruptions. After a large earthquake (mainshock), many induced events (aftershocks) may be triggered and cause additional damage or even collapse to mainshock-damaged buildings. To reduce seismic hazard and risk of aftershocks and extend the current performance-based earthquake engineering framework, the effects of aftershocks on the seismic performance of structures should be incorporated. Considering an extensive set of real and artificial mainshock-aftershocks sequences, the vulnerability of residential wood-frame houses in southwestern British Columbia, Canada, is examined analytically using incremental dynamic analysis. Subsequently, seismic loss estimation is conducted for four wood-frame house types with different seismic capacities. The analysis results indicate: (1) moderate effects of aftershocks (5–20%) on maximum structural response and damage extent, and (2) significant impact of structural seismic capacity on estimated seismic loss.

Excessive floor vibrations can occur in wood light-frame buildings and cause inconvenience to occupants in terms of unsatisfactory serviceability. There has been much unresolved debate about how to perform vibration serviceability design calculations for such floors. The research reported in this paper is built around field observations of the vibration behaviors of floors in building superstructure constructed using alternative contemporary wood materials using advanced vibration test and data analysis techniques, and synthesis of findings with existing knowledge. Field investigations were performed on residential units of two multioccupancy multistory buildings having distinctly different architectural and construction features. Those buildings are similar to many in North America. Detailed modal testing was performed with the objective of understanding effects associated with construction method choices made by engineers. Analysis of the data showed that architectural and construction detailing decisions play a major role in determining whether or not a particular building is likely to exhibit floor vibration serviceability problems. Deductions that flow from the new findings and synthesis of those with existing knowledge support the conclusion that empirical and semiempirical methods that engineers can use to make design decisions about floors tend to be unreliable. No attempt is made to specify appropriate engineering design methods but it is believed that what is presented will facilitate development of such methods.

This paper presents a research investigation of the period formulation in the national building code of Canada through testing and modeling of a light-frame wood structure. The experimental aspect of the study was focused on ambient vibration measurements, and a simple analytical shear-building model was developed to study the effect of multistory sway deflections. The results of the study showed that the calculated periods based on the code equation could be conservative if used to determine seismic base shear. Estimates for building stiffness are evaluated using the deflection equation for wood shearwalls from the current Canadian timber design standard. The calculated fundamental periods using the multiple-degree-of-freedom shear-building model are compared with the experimental results.

Distributed mass damper (DMD) systems are discussed as a method of suppressing lateral motions of superstructures during wind storms and earthquakes. Potentially, DMD systems are a technology that is economical enough for widespread application to buildings or other structures. Focus is placed on lightweight superstructures as a reflection of the trend toward the use of ultra-lightweight floor slabs in high-rise buildings. Results of model-scale experiments are presented that show that tuned mass damper (TMD) systems that add between 1.3 and 2% to the total superstructure gravitational mass are effective methods of increasing damping in superstructures and reducing peak lateral accelerations during forced vibration events. In those experiments, tuned sloshing dampers (TSDs) were employed in conjunction with floor and roof plates that simulated ultra-lightweight slabs constructed from cross-laminated timber (CLT), which is a new material option in North America. The use of TSDs was a surrogate for TMDs in general, but such devices are expected to be economic and can easily be tuned to match free vibration frequencies of superstructures. A concept for incorporating DMD arrays as parts of multimaterial ultra-lightweight floor slabs is presented in the context of high-rise building superstructures having moment-resisting frameworks made of steel, reinforced concrete, or other materials that work in conjunction with the slabs and shear walls to form superstructures. The main conclusion is that DMDs are a practical and potentially economic approach for suppressing undesirable motions of lightweight high-rise superstructures. Continuing work is focused on developing and optimizing low-cost TMDs and design of DMD systems for specific buildings or other structures.

Composite, timber-concrete slabs as structural floor systems for office and residential buildings, offer several technical advantages over traditional, exclusively timber floors. The connection between timber and concrete in current systems made of spruce relies on mechanical steel fasteners to transfer shear forces between the two materials. The mechanical properties of beech-laminated veneer lumber (LVL), which are higher and more reliable than spruce, allow the development of an innovative timber-concrete composite slab made of a thin beech-laminated veneer lumber plate and a concrete layer. The connection between the two materials is accomplished with notches in the timber plate instead of mechanical steel fasteners. The structural performance of this system was evaluated with a series of bending tests. The results demonstrated that this type of timber-concrete composite system was able to reach high load-carrying capacity and that the structural behavior and failure modes of the composite elements were strongly influenced by the geometry of the notched connection.

A buckling restrained knee braced truss moment frame (BRKBTMF) is a novel steel structural system that combines steel trusses with buckling-restrained braces (BRBs) to form an alternative seismic force–resisting system. The combination of the steel trusses and the BRBs allows the BRKBTMF to have a large interior opening without the need to provide ductile detailing for the steel trusses. This makes the design very efficient and economical. More importantly, through the use of BRBs as the designated energy-dissipation devices, the damage is controlled within the BRBs, which can be replaced easily after the earthquake. This will drastically reduce the structural damage, repair cost, and repair time, which makes this structure more efficient and resilient toward future earthquakes. In this study, a modification of the performance-based plastic design (PBPD) procedure was derived to design the member sizes of the BRKBTMF. A parameter study was conducted by varying the inclination of BRBs to optimize the initial construction cost and expected financial loss during the maximum credible earthquake shaking. A prototype four-story office building located in Berkeley, California, was selected for this study. State-of-the-art site-specific hazard analysis, finite element models, and damaged fragility models were used to evaluate the seismic performance of the BRKBTMF with different BRB inclinations. The result shows excellent seismic performance in the proposed BRKBTMF, where the structure is able to satisfy both the drift and force limits. The parameter study shows that the design forces in the steel trusses are not affected by the inclination of the BRB. Instead, the design forces in the BRBs and columns increase as the inclination of the BRB becomes more horizontal. This results in larger column sizes and thus higher initial construction costs. The cost of the BRB is not significantly affected by the inclination of the BRB. This is because as the BRB inclination becomes more horizontal, the cross-sectional area of the BRB increases, but the length of the BRB decreases. The results of the nonlinear dynamic analysis show that the peak dynamic responses (peak interstory drift and floor acceleration) of the BRKBTMF are very similar among the different BRB inclinations. This shows that the proposed PBPD design methodology is very effective in designing the BRKBTMF. The detailed parameter study also shows that the BRB strain is the lowest when the BRB inclination is around 30° or 90°. As the BRB inclination gets closer to 60° (using the geometry presented in this study), at the same interstory drift demand, the axial strain demand in the BRB is the highest. This results in a high degree of damage to the BRBs, which translates to the highest degree of structural damages and financial losses during the maximum credible earthquake shaking. Hence, it is recommended that optimal choices of BRB inclination are either horizontal or connecting directly to the end of the columns.

Cross-laminated timber (CLT) products are gaining popularity in the North American market and are being used in midrise wood buildings, in particular, in shearwall applications. Shearwalls provide resistance to lateral loads such as wind and earthquake loads, and therefore it is important to gain a better understanding of the behavior of CLT shearwall systems during earthquake events. This paper is focused on the seismic performance of connections between CLT shearwall panels and the foundation. CLT panels are very stiff and energy dissipation is accomplished by the connections. A literature review on previous research work related to damage prediction and assessment for wood frame structures was performed. Furthermore, a test program was conducted to investigate the performance of CLT connections subjected to simulated earthquake loads. Two different brackets in combination with five types of fasteners were tested under monotonic and cyclic loading protocols. In total, 98 connection tests were conducted and the monotonic load-displacement curves and hysteretic loops were obtained. In this paper, an energy-based cumulative damage assessment model was calibrated with the CLT connection test data. Finally, a correlation between the damage index and physical damage is provided.

Timber-concrete-composite (TCC) systems, commonly with timber at the bottom and concrete at the top, are efficient solutions for floors because of their advantages compared to pure timber or pure concrete slabs, one of the advantages being better fire resistance. This paper presents experimental and numerical investigations on the fire resistance of a novel TCC setup with timber beams at the top and a concrete layer at the bottom. This setup has several advantages in facilitating the connection to the surrounding structure and mechanical installations. The results from fire tests on a subsection of the system are reported. The concrete side of one test specimen was subjected to a fire according to the standard temperature-time curve and proofed to achieve a 60-min fire rating. Subsequently the temperature behavior was numerically modeled by performing a transient thermal analysis with temperature dependent material properties. The temperature profiles for the TCC system was accurately modeled allowing for the validated model to be used for future optimizations and predicting the fire resistance ratings of optimized systems.

The development of hybrid structures is constantly being pushed forward because of the need for more efficient components that incorporate multiple functions. Structural requirements lead to state-of-the-art timber-concrete-composite (TCC) floor systems, which consist of a concrete layer on top of timber beams or decks. In bending, the TCC concrete layer is in compression and the timber is in tension. When multifunctionality requirements such as building physics or fire protection are added, other solutions of the structural form of hybrid systems can emerge that challenge common engineering conceptions. This paper presents research on the connection system of a novel TCC setup that integrates thermal and fire protection functionality, in which timber beams are placed at the top and a concrete layer at the bottom. The results from full-scale bending tests, and shear and tension tests on a subsection of the system, show that the grooves cut into the timber beams transmit the shear forces and sufficiently connect the concrete deck vertically to the timber. Subsequently, the structural behavior was numerically modeled and the agreement between experimental and numerical results allow for the validated model to be used for geometric optimizations, and predicting the performance of optimized systems.

It is well known that the stiffness properties of diaphragms and lateral load resisting elements (LLREs) influence the load distribution between LLREs under lateral load induced by earthquake or wind. Where a more sophisticated method of calculating the load distribution in a lateral load resisting system is used, often it is based on the concept of a beam on an elastic foundation. This approach could be tedious to apply in design when there are more than a few LLREs. A multiple-spring model (MSM), whereby the translational springs are used to model the diaphragm stiffnesses and the stiffnesses of the LLREs, is proposed. The model was validated with test and finite-element results of a particular benchmark building. The lateral load distribution between LLREs with various stiffness ratios of diaphragm to LLRE was also investigated. The results show that, contrary to common belief, the forces transferred by a semirigid diaphragm to supporting LLREs may be higher than those predicted by flexible and rigid-diaphragm assumptions. Therefore, using the envelope force approach may lead to underestimation of the design forces in the shear walls.

A timber-steel hybrid system, where cross-laminated timber (CLT) shear panels are used as infill in steel moment resisting frames, is analytically investigated. The proposed hybrid system combines ductile behavior of steel moment frame with lighter and stiffer CLT panels. Initial parametric study is carried out with a single-bay-single-story model using pushover analysis to investigate effect of CLT panel thickness, crushing strength, and confinement gap. Results showed that the parameters studied have a significant influence on system’s ultimate strength, ultimate drift capacity, and post-peak behavior. Subsequently, parametric studies are carried out for three-bay hybrid buildings with three-, six-, and nine-story hybrid buildings for varying panel configuration (a given gap, panel thickness, and crushing strength) values. The parameters varied and are design ductility level (limited ductility and ductile) and infill pattern. A monotonic pushover analysis was performed to develop a preliminary ductility and overstrength values based on both steel yield of frame and panel crushing force in the links. Overall, the results suggest that addition of infill bays is less beneficial in ductile moment frames; CLT infill panels are better suited to lower ductility systems. Ultimately, a ductility factor of 2.5 is recommended for the infilled frame system, and an overstrength factor of 1.25 appears to be warranted.

Story limit of residential light wood-frame buildings in the province of British Columbia was raised from four to six in 2009. The increase in height leads to more flexible buildings, potentially requiring the stiffer elevator shaft and stairwell core made from reinforced masonry to reduce building deflection under lateral loads. Guidelines for designing such hybrid buildings connected with ductile connections are currently lacking. As a first step towards achieving this goal a six-story light wood-frame building was designed assuming different scenarios of attachment to a stiffer core. Seismic responses to 10 ground motions of the designed six-story light wood-frame building with and without connection to a reinforced masonry core were obtained through the use of a finite-element modeling procedure through commercial software. A user-developed subroutine to model the hysteretic performance of wood shear walls, masonry core and connections was used in the analyses. Results show that the maximum lateral drift of wood shear-wall subsystem reduces to 79, 62 and 48% on average of that of pure wood structure in hybrid buildings with masonry core designed to 25, 50 and 100% shear resistance of the wood subsystem respectively. It is concluded that seismic design of the wood subsystem in hybrid building can be based on the use of the same seismic force modification factor for light wood frame system published in the the building code.

This article presents the seismic performance of a timber frame with three-dimensional (3D) rigid connections. The connections were made with self-tapping screws and hardwood blocks were used to support the beams. The frame was designed to resist high seismic excitations with the goal of controlling the drift. The moment-rotation characteristics of the connections were measured in the laboratory by applying static cyclic loads. The frame made of laminated wood beams and columns, and cross-laminated lumber deck, was subjected to seismic, white noise, snapback, and sinusoidal sweep excitations. The synthetic seismic excitation was designed to contain a considerable amount of energy close to the frame’s first natural frequency. The structure showed no significant damage up to a peak ground acceleration of $1.25g$. Failure of the frame occurred due to shearing of the columns with a peak ground acceleration of $1.5g$. The designed structure fulfilled with current serviceability limits up to $0.8g$.

Design analyses are presented to demonstrate the technical advantages of substituting ultralightweight slab materials like cross-laminated-timber (CLT) for reinforced concrete (RC) floors and roofs of low-rise and high-rise hybrid building superstructures. Such substitution reduces the gravitational masses of slab by at least two-thirds without reducing functionality as bending or diaphragm slabs. Specific illustrations of design impacts of using CLT are given for hybrid building superstructures where steel or RC frameworks are the primary means of resisting effects of gravity forces associated with occupied built spaces. Results shown apply to two-story buildings in which effects of lateral forces associated with wind or seismic loads are resisted entirely by a steel moment framework, and six- and twenty four–story buildings in which RC shear walls within building cores primarily resist effects of such lateral loads. Consideration is also given to how fire engineering design decisions can impact structural performances of high-rise buildings incorporating combustible slab materials like CLT. The possible fire design solutions considered being complete encapsulation of CLT slabs using noncombustible claddings, and inclusion of RC fire floors vertically separating the superstructure into fire compartments having dimensions typically permitted for combustible construction. In the broad sense, analyses presented here suggest that it is feasible to reduce total gravitational and modal masses of superstructure by at least one-third, with concomitant reductions in superstructure and foundation construction costs. In practice engineers can have many types of lightweight slab construction techniques to choose from, apart from using CLT. Approximate percentage reductions in sway and superstructure force flows quoted herein will scale in proportion to the square-root of the ratio of the masses of RC floor slabs to masses of any substitute floor construction.