SCIENTISTS at Napier University and Forestry Civil Engineering have spent three years exploring the properties of stress laminated timber arch (SLTA) structures, used for bridges. During this time they have built over 20 permanent bridges and load tested eight.

Professors Abdy Kermani, Professor of Timber Engineering at Napier University, and Dr Geoff Freedman, head of design at Forestry Civil Engineering, have produced a report on the performance of a 20m span stress laminated timber arch bridge.

The overall aim of this extensive research programme has been to develop structural uses for low grade, UK-grown, timber and it has been shown that arches, using timber in compression, is an extremely effective technique for bridges.

Timber is a very sustainable material, the report’s authors say, while also being a low-cost product. They say bridges on public roads can help increase public confidence in timber as a viable structural material. As part of a series of field and laboratory tests on SLTA bridges, they designed and constructed a 20m span arch bridge was designed and constructed at the Glentress Forestry Commission site near Peebles.

The bridge has since been subjected to a series of extensive static and dynamic loads evaluating its response to crowd and vandal loadings.

The results have confirmed predictions that the strength and stiffness of this type of construction was well beyond the strength normally expected from a slender timber structure.

The report’s authors say their extensive testing programme aims to develop reliable design guidelines for arch structures using UK softwood.

Spanning the gap

Having successfully built and tested a 15m span with 250mm timbers and established that elastic linear analyses provided comparative deflections it was assumed that this would hold true for larger spans. Commercial orders were received for three 20m span arch bridges so the decision was taken to build a test bridge to full scale (20m span) and use it as an exhibit at the June 2004 Royal Highland Show in Edinburgh.

Analyses and design calculations were carried out using a countryside crowd loading of 3.2kN/m2. This permitted a 200mm deep deck (an optimum solution in regard to material availability) with a natural frequency of just over 4.0 Hz.

One important aim for this research was to find uses for home-grown plantation timber so the bridge was built from Sitka Spruce. The permissible compression strength of this material is within the requirements of the design.

Because of the difficulties of measuring the lateral thrust for a full scale bridge with in situ foundations, and because the results from previous laboratory tests has shown thrusts to be as predicted by linear static analysis, they were not measured.

Vibration tests were carried out using three independent, but similar, techniques all employing accelerometers and transducers to measure the response to a hammer blow, crowd walking over or jumping on the bridge or sandbags being dropped onto the deck. These results were compared to the results from a finite element analysis.

Bridging the River Forth

Some 20 commercial bridges have been built in the field and eight scaled and full size test bridges in the laboratory. As construction techniques are tried and tested costs are reducing. There will be large variations in costs depending on familiarity with the technique but the system holds the promise of very low cost structures because the materials are not expensive, they are easily available and the skill level required is low.

The most recent 20m span bridge over the river Forth cost about £500 per m2.

Properly detailed SLTA bridges in the UK can have a design life of 50 years. They permit long spans that have only been achieved elsewhere by creating more expensive composite constructions such as glulam or box beams, which are either expensive or would harbour moisture in the UK thus promoting rot.

The results of this research have confirmed predictions that the strength and stiffness of this type of construction is well beyond the strength normally expected from a slender timber structure; as the bridge sustained loads well above its design load without sign of any distress.

The fundamental natural frequency of the bridge was approximately 3.5 Hz. This compared well with a finite element analysis value of just above 4.0 Hz.

This study has aimed to create the foundation for a detailed understanding of the actions of stress laminated arches. Current laboratory investigations at Napier University aims to explore the influence of several factors such as creep, moisture fluctuations, arch profile and stressing conditions on the stiffness, strength and dynamic response of the SLTA bridges.