Feasibility study of a wood-concrete hybrid super tall building and optimization of its wind-induced behaviour

Abstract

The demand for sustainable high-rise buildings is growing. Such sustainable high-rise could be realized by the use of mass timber for the structural design instead of more conventional building materials such as steel and concrete. Timber is a renewable resource which can be CO₂ neutral if reforestation takes place to close its carbon cycle. In addition, the light-weight of timber reduces the loads on the foundation, and the timber could be used as an architectural feature as well. The height boundaries for tall timber buildings are currently extending, as illustrated by the ongoing realization of a 70 metres tall timber building in Amsterdam. However, the light-weight of timber make tall timber buildings prone to dynamic wind loading. In addition, the current trend to design slender high-rise further increases the wind-induced dynamic response of the building.

In this thesis, the technical feasibility of a super tall hybrid wood-concrete building was evaluated and its wind-induced dynamic behaviour was optimized. To this end a 300m tall building of timber and concrete was designed for construction in the city-centre of Rotterdam, The Netherlands. Due to the absence of seismic activity in the area, wind loading was identified as the governing parameter for lateral stability design. The structural design was therefore optimized to satisfy serviceability criteria for lateral drift and occupant comfort. Based on these requirements, the structure was designed as a reinforced concrete core surrounded by a glued-laminated timber (GLT) frame and floor slabs consisting of a cross-laminated timber (CLT) panel with a thin concrete top layer. Lateral stability was ensured by an outrigger/belt-truss system at three levels, resulting in a significant increase of the global stiffness in the structure, and in a reduction of the maximum lateral inter-storey drift by a factor two.

In order to fully design a 300m tall wood-concrete hybrid building, design aspects such as the floor plan, core layout, lateral stability system and timber frame were designed first. The floor plan layout and storey height were based on a typical office building. The length from the perimeter to the core of the building was set to 9 metres and a storey height 3.75 metres was applied. These dimensions ensured that enough sunlight would fall into the office spaces. The entrance level with double storey height was performed in reinforced concrete to create a more open layout and to achieve a higher safety in the case of accidental blast loading.

Stiffness optimization of the structure was carried out in order to satisfy the serviceability criteria for lateral inter-storey drift and displacement. Consequently, a parametric study of the cross-sectional dimension of the columns, of the thickness of the reinforced concrete core wall, and of the outrigger/belt-truss layout was performed. The cross-sectional dimension of the columns and the thickness of the core wall were tapered down over the height of the structure. A diagonally- and orthogonally-oriented layout of the outrigger trusses were compared. For the final design the orthogonally-oriented layout was applied, because it resulted in more lateral stiffness of the structure. Each outrigger level consists of 8 outrigger trusses which are orthogonally oriented with respect to the reinforced concrete core, and a belt-truss surrounds the perimeter of the structure. The outrigger levels were also designed to accommodate the mechanical, electrical and public health system (MEP) facilities.

Due to the large tension forces caused by lateral wind loading the connection design of the GLT frame is paramount for the feasibility of a hybrid wood-concrete tall building. The outrigger trusses transfer large tension and compression forces to the perimeter columns, making the outrigger connections critical for the overall stiffness of the structure. Therefore, the connections in the outrigger truss, as well as the belt truss, were designed with two slotted-in steel plates and dowels. Beam-to-columns and beam-to-wall connections were designed with only one slotted-in steel plate and dowels. Column splices were carried out with glued-in rods and bolts. In-between the column splices adjustment devices were installed to compensate for the vertical differential shortening of the reinforced concrete core and mass timber frame. Consequently, an additional steel plate with the required adjustment thickness could be placed between the bolted steel plates of the connection. Continuous columns were applied over a height of 4-storey levels in order to reduce the number of expensive steel-timber connections. Due to transportation constraints, the length of the columns was limited to 15 metres. 
Vortex shedding and end-effects of the wind due to lateral wind loading cause peak response accelerations mainly in the across-wind direction. For a return period of 50 years of wind loading to satisfy the serviceability criterion for the occupant comfort, the peak response acceleration should be lower than 0.390 m/s². This criterion is dependent on the natural frequency of the structure which is equal to 0.130 Hz. To improve the structure's dynamic behaviour, a passive tuned mass damper (TMD) and a chamfered corner modification were applied, resulting in a decrease of the peak acceleration to a level satisfying the occupant comfort criterion.

The tall building was considered as consequence class 3 (CC3), meaning that a fire-resistance time of at least 120 minutes should be guaranteed. To this end, the mass timber structural elements were covered with protective cladding and a sprinkler system was applied in the building. The reinforcement bars in the concrete core wall were placed at a 35 mm cover to satisfy the required fire resistance time. The fasteners in the dowel-type connection were sealed with wooden plugs and the slotted-in steel plates are hidden and covered by the timber beam element. Dowels, bolts and steel plates in the column splice connection were protected by a double layer of gypsum plasterboard. During a fire, the protective cladding would eventually fall off and charring of the mass timber element would start. The developing char-layer would protect the rest of the cross-section against the fire load. In the GLT beams, a charring depth of 34 mm developed after 120 minutes of fire loading, and as a consequence all steel plates, connectors, and dowels were placed at a minimum distance of about 40 mm from the outer surface.

The designed tall and slender hybrid wood-concrete structure satisfied the serviceability criteria for lateral displacements and accelerations, and the critical connections in the timber frame are able to resist the large forces in the structure. However, a hybrid wood-concrete super tall building required shape optimization and a tuned mass damper to avoid peak acceleration levels exceeding the occupant comfort criterion. Furthermore, the design process established in this case study could serve as a roadmap for the design of future hybrid wood-concrete super tall buildings.