Timber construction for high-density housing

Status

finished

Abstract

Task

The newly revised Viennese building code, in effect since April 2001, now finally allows timber mixed-construction buildings to be erected up to five-storeys (four timber floors on a mineral ground floor with high structural demands for fire protection). This opens new applications for timber construction in the building categories of higher density housing. To date there exist but very few such examples in Austria and the German-speaking realm in general; therefore, new knowledge and experience in designing and constructing multistorey timber structures must be developed. Furthermore, these construction methods can only gain ground in the rental housing sector if their cost effectiveness proves comparable to the market leaders brick and reinforced concrete.

To this end, alternative technological solutions for load-bearing wall and floor structures were examined and economically compared in a one-year research programme. The project was financially supported through the Austrian Federal Ministry of Transport, Innovation and Technology in conjunction with a research initiative programme titled "Building of Tomorrow", as well as through a grant from the Austrian Federal Ministry of Economy and Labour.

Point of departure for the investigation and development of structural design options was a five-storey housing project with 150 apartment units that is slated for realisation in Vienna by Sozialbau, Austria's largest non-profit building organisation.

The research work focussed on load-bearing partition walls and floor systems between apartments, i.e. the critical components of multistorey timber housing both in structural and economic terms (relatively high static loads, potentially high dynamic loads in case of earthquake, strict requirements with respect to sound isolation and fire protection, and frequently high moisture levels due to occupancy).

Work Programme and Results

The timber constuction industry also contributed to the participation of research centers, building designers, and regulatory authorities. All project participants were involved in the various work phases, as well as in project meetings to monitor group and individual progress.

The first project stage involved analysing the structural repertoire of multistorey timber construction and evaluating a number of examples in terms of case-specific demands. As a result, the configuration of wall-floor junctions was identified as particularly critical (settling, force transfer). Subsequent design development, therefore, gave preference to structural solutions without "clamped" floor joists.

In the concurrent second stage, experiments were performed to obtain fundamental information about sound transmission and moisture migration. This entailed making detailed measurements of sound propagation through existing housing buildings as well as through prototypes built specifically for the research project. Special attention was given to measuring the propagation via flanking paths in adjacent components, thus establishing the first available data base for calculating noise reduction properties according to the European Standard 12354 for this type of construction.

Analysis of the moisture migration characteristics for various exterior wall-floor junctions showed that indiscriminately transferring construction principles for single family dwellings to multistorey housing can, under circumstances, lead to problematic conditions in the timber building components. With the aid of simulations modelling thermal and vapor behaviour within the wall-floor joint, alternative solutions were found that demonstrate a stable vapor balance within the allowed moisture range for timber, despite comparatively high interior humidity.

In the course of the research project, general requirements as stated in the Viennese building code, in particular their application to timber structures, was thoroughly discussed and documented.

With respect to construction cost consideration, an instrument for directly and meaningfully comparing timber structures with conventionally built housing was developed.

Development and comparative evaluation of various timber frame and solid wood panel structures comprised the third project stage. Here in particular, solid wood components were implemented to develop new alternatives to existing construction systems with laminated sheet components. The wood materials used in fabricating the prototype can be processed with standard carpentry skills and simple joining machinery, without any specialised lamination echnology.

The developed construction types, two solutions proposed by a manufacturer, and several reinforced concrete designs were evaluated in economic terms according to a uniform set of criteria.

In the fourth and final project stage, a two-storey prototype was erected and thoroughly examined with respect to noise control as well as structural behaviour under static and dynamic conditions. The carpentry-based, wood panel construction type, as developed in the previous phase, was selected for the experimental evaluation. Measurements showed that this structural type fulfills the given acoustic and static-dynamic requirements. Furthermore, the structure demonstrated a pronounced plastic behaviour.

Conclusions

• Given the same thermal performance requirements, both optimised timber frame construction and the developed solid wood wall system can compete economically with concrete construction.
  
• Craft-based, solid-wood solutions can compete economically with timber frame construction, despite significantly higher material usage.
  
• Single-layer wall assemblies provide a decisive economic advantage over two-layer assemblies in timber. Generally in timber construction, the use of large components reduces costs.
  
• Prefabricated wall components spanning the building height are economically at least comparable with single-storey wall elements under appropriate circumstances. Individually assembled, pretreated floor beams are comparable in cost with prefabricated floor system components.
  
• Partially bending resistant joints between continuous wall and floor components can assume a bracing function in timber construction, thus signigficantly improving the critical load case under earthquake conditions.
  
• By employing mineral prefixed sheathing, the high noise reduction requirements that are applicable, especially for adjacent building components, can be achieved with single-layer, continuous wall elements in timber construction.
  

Contact

Vienna University of Technology
Institut für Tragwerkslehre und Ingenieurholzbau (Prof. Winter)
Karlsplatz 13/254
A 1040 Wien,
Tel.: +43 1 588 01-25401
Fax.: +43 1 58801-25499
E-Mail: sekretariat@iti.tuwien.ac.at

Vienna University of Technology
Institut für Baustofflehre, Bauphysik und Brandschutz
Abteilung Bauphysik (Prof. Dreyer)
Karlsplatz 13/206
A 1040 Wien,
Tel.: +43 1 588 01-20602
Fax: +43 1 588 01-20698
E-Mail: thomas.bednar@tuwien.ac.at

Schoeberl & Poell OEG
Ybbsstraße 6/30
A 1020 Wien
Fax: +43 1 726 45 66
Fax: +43 1 726 45 66-18
E-Mail: office@schoeberlpoell.at