Journal

Are Radiata Pole Columns a Better Solution for Multistory Timber Buildings?

John Chapman Lecturer, School of Architecture, University of Auckland. Vance Bentley 5th year Barch Student, School of Architecture, University of Auckland

Aim of study

To further our understanding in the efficient use of timber as the main structural material in multistory buildings.

Introduction

This article is intended as a preliminary study of the use of radiata poles as the column elements in timber multistory buildings. If radiata poles prove to be practical then a series of allied issues will need to be considered. It is intended that the other structural elements such as beams and floor systems would be timber or timber based.

New Zealand has an increasing number of plantation trees that will need felling over the next 20 years. If timber multistory can be made a viable option, an additional market will become available to the timber industry.

We have specified radiata timber poles as structural elements extensively in NZ -especially over the last 35 years. They are mainly used in house piling, pole platforms, power poles, retaining walls. Many poles of retaining walls exist in a continual state of relatively high bending stress. In these contexts they have gained the reputation of having reliable strength.

To date most of the research into multistory timber buildings has concentrated on gluelam elements and also fixed joints involving gluing. This study will avoid gluelam and onsite gluing of elements, as much as possible and consider methods of mechanical fixing. Fixed joints using gluelam are more expensive than those of steel or reinforced concrete. Glued joints require a high degree of skill and attention to construct and involve more risk than the jointing of steel or reinforced concrete.

Another perpetual problem with timber structures are joints involving members with large tension forces. The ends of tension members generally require a large number of bolts and/or nailing which can be awkward and costly. To overcome this issue, it is proposed that the resistance to wind and earthquake forces be provided by steel cross bracing.

A building with timber pole columns presents its own set of design problems. Gluelam elements have the advantage of being virtually completely straight and with defined section dimensions. Poles, while generally straight and regular, all have variations of section and to some extent straightness.

For a building system to be practical the structural elements must be manufactured in a factory with consistent dimensions and be easily erected. Can such a system be achieved when associated with poles that are all slightly irregular?

It is hoped that the final outcome of this study is a building system that is attractive economically, ecologically and will assist the multistory buildings in timber to be a viable if not preferred option when compared to steel or reinforced concrete.

Issue 4 Volume 9

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