Cathedral Hill 2 Challenges in the design of a tall all-timber building

In a very tall relatively light wood building, windloads dominate the design process and the control ofdeflections can be challenging. The use of verticalpost-tensioned rods for prestressing the shear walls isvery advantageous in this regard.Post-tensioning also greatly simplifies the connectionsbetween vertical segments, reducing their function toVOL 24· ISSUE 4 » NEW ZEALAND TIMBER DESIGN JOURNALshear resistance only.Without direct positive connections between verticalsegments of the shear walls, gap openings are possibleunder lateral loads, producing rocking behaviour,which stretches the rods and increases the restoringforce to recenter the building.In this project, seismic effects remained in the elasticrange, with a small amount of rocking on upperfloors, and so dissipaters to increase seismic-energyabsorption were not necessary. In fact, gap openingsand deflections under ultimate limit-state wind loadsare greater than under the design seismic loads, so itis preferable not to have dissipaters in any case, toavoid their replacement after an extreme wind event,more frequent than an extreme seismic event.Under the service-level wind loads, the designprecludes gap openings, to avoid possible occupantdiscomfort, although the gap openings, even underextreme wind loads, are very small, less than 4 mm.Non-linear Time-History Analysis results showedthat although the wind-induced deflection wasthe governing load case scenario, the dynamicamplification of shear and moments must not beoverlooked, especially for detailing purposes.