Experimental investigation and numerical modeling of rocking cross laminated timber walls on a flexible foundation

Abstract

With recent developments of engineered wood products such as cross laminated timber, mass timber buildings in areas of high seismicity are becoming feasible and offer benefits such as faster construction and unique architectural features. It has also opened the door to creating seismically resilient lateral systems that sustain minor and reparable damage during large earthquakes. This paper presents a numerical modeling procedure for mass timber rocking wall lateral system with distributed spring elements to model base rocking. This modeling technique was validated by comparing the behavior with results from the full‐scale two‐story shake table test on a mass timber building with coupled CLT rocking walls completed as part of the NHERI TallWood Project. Although the lateral system performed better than expected during testing, it was determined that the flexibility of the foundation actually limited damage to the rocking wall panels. By adjusting the numerical model to represent a rigid foundation boundary condition, it was determined that the wall system would have experienced more damage, but would have still met design performance objectives had it been tested on a rigid foundation.

Development of a Numerical Model to Consider the Foundation Flexibility Effects in CLT Rocking Walls

“…Equating Equation ( 1) with the expression gives an effective length of the wall equal to the thickness of wall. It's important to mention here that several past studies have proposed an effective length of two times the wall thickness for CLT walls [7,11,16]. Results with one and two layers exhibited almost identical results in this study and are not shown here for brevity.…”

“…The CLT panels in each wall were joined by five equally spaced Ushaped flexural plates along the height of the wall to provide a minimum energy dissipation ratio of 0.3. Modulus of elasticity and yielding stress of each U-shaped plate were 2x10 5 MPa and 414 MPa. Post-tensioning was provided by using four 19 mm bars in each wall with an average initial post-tensioning force equal to 40% of yield strength (53 KN for each bar).…”

“…Details of the test structure, rocking wall and foundation are shown in Figures 2 and 3. Further information about the structure, testing setup and dynamic testing can be found in [8].…”

“…[9,10] proposed empirical relationships to find out the plastic hinge length for rocking timber walls. A plastic hinge length of two times the wall thickness has also been proposed and used in the past [5,11] while for concrete rocking connections, a plastic hinge length equal to the half the length of the walls has been proposed [12,13]. All these empirical relationships try to a provide a measure for the axial stiffness of the rocking wall assuming the foundation to act as rigid, however, both the case study structures selected in the current work showed a permanent deformation due to yielding in the foundation, rendering the available empirical formulas unsuitable.…”

“…During the testing of the 2-story structure, it was observed that the base beam, used as a foundation for the rocking wall, yielded with significant permanent deformation. This unintended inelastic behavior of foundation increased the fundamental period of the building [5], resulting in lower-than-expected damage in the structural members especially at the wall toes; however, the displacement demand on members 1 Visiting Fulbright Scholar, Washington State University, m.qureshi2@wsu.edu 2 PhD Scholar, Washington State University, Pullman WA, USA g.acunaalegria@wsu.edu increased. Based on these results, it is important to explore the effect of foundation flexibility on the overall lateral behavior of rocking wall structures and develop guidelines to achieve optimum solutions with minimal damage and manageable drift demands.…”