Englekirk (United States)

The 2009 NEHRP Provisions modified the definition of horizontal ground motion from the geometric mean of spectral accelerations for two components to the peak response of a single lumped mass oscillator regardless of direction. These maximum‐direction (MD) ground motions operate under the assumption that the dynamic properties of the structure (e.g., stiffness, strength) are identical in all directions. This assumption may be true for some in‐plan symmetric structures, however, the response of most structures is dominated by modes of vibration along specific axes (e.g., longitudinal and transverse axes in a building), and often the dynamic properties (especially stiffness) along those axes are distinct. In order to achieve structural designs consistent with the collapse risk level given in the NEHRP documents, we argue that design spectra should be compatible with expected levels of ground motion along those principal response axes. The use of MD ground motions effectively assumes that the azimuth of maximum ground motion coincides with the directions of principal structural response. Because this is unlikely, design ground motions have lower probability of occurrence than intended, with significant societal costs. We recommend adjustments to make design ground motions compatible with target risk levels.

This paper evaluates and compares between the seismic behavior of hybrid coupled wall systems that are 12- and 18-stories high. System response is studied using validated, transient finite element simulations that account for the effects of concrete cracking, crushing, and steel yielding. Suites of analyses are conducted to investigate pertinent parameters including interstory drift ratios, wall rotations, roof displacements, dynamic base shear magnification, strain demands, and coupling beam plastic rotations. The results for both systems confirm that hybrid coupled walls are particularly well suited for use in regions of high seismic risk. Although the responses of both structures were essentially dominated by the first vibration mode, higher mode effects manifested themselves by causing substantial base shear magnification. The effect was somewhat higher in the taller structure.

Carbon fiber composite connections for hollow-core precast concrete wall panels have been investigated with respect to bond behavior. A panel assembly made up of three precast concrete walls was loaded in the in-plane direction using a cyclic quasi-static load, which was transferred to the wall interface as a shear force. The application of the carbon-fiber-reinforced polymer (CFRP) composite was varied in the covered area, number and orientation of plies, and the concrete surface preparation. An expression, which compares well with the experimental findings, is presented for the effective bond length of carbon fiber composite sheets. The effective bond length of the connection depends on the peel-off shear strength of the concrete, the maximum tensile strain in the composite, the modulus of elasticity of the composite, and thickness of the composite plate.

Two mathematical models are presented to represent the idealized stress-strain behavior of both confined and unconfined grouted concrete masonry in compression. The models are based on experimental data from recent prism tests, and account for varying types of confinement reinforcing. The “Acceptable Fit” model is a simple model suggested for use in design. The “Best Fit” model is more complex but more accurate, and is appropriate for use in computer analyses. The models are compared to existing mathematical models for masonry behavior available in the literature.