00350 Cecilia Alonso - Timber Design Society

PERFORMANCE OF A MECHANICALLY LAMINATED
TIMBER ARCHED SUSPENSION BRIDGE
Cecilia Alonso 1 , Abdy Kermani 2
ABSTRACT
The first constructed bridges were probably made of
wood. When sympathetically designed, timber bridges
are often easy to construct, economical, aesthetically
pleasing and have a long life expectancy. In order to
enhance its environmental advantages, specified timber
must be locally sourced from sustainably managed
forests. In the UK, with low-grade home-grown
softwood and no established glue-lamination industry,
mechanical lamination can offer a cost competitive and
structurally efficient solution to overcome the long-span
and design requirements of pedestrian and vehicle bridge
construction. 12
Stress-Laminated Timber (SLT) is a relatively new
technique mainly used in bridge decks in which
rectangular sawn timber laminates are post-tensioned
transversely with high-strength steel rods. The
compressive pressure generates friction forces between
the laminates, creating a solid wood plate with
orthotropic characteristics. Therefore, the planks do not
have to be continuous over the whole bridge span. Butt
joints can be introduced, allowing the deck to be
constructed using short lengths of timber. In addition,
SLT decks with butt joints may be cambered to offset the
dead-load deflection and the additional vertical creep.
However, it has been found that the application of flat
slab-type stress-laminated decks is limited to bridges of
6 to 9m spans using normal timber sizes up to 250mm.
Composite pre-stressed T-beams and box section decks
have been developed in order to extend the possibilities
of SLT, but these systems are not suitable for the UK
due to the wet weather or the requirements for
construction of bespoke glulam members.
1 Cecilia Alonso, Napier University, 10 Colinton Road,
Edinburgh EH10 5DT, UK. Email: cecilia.alonso@yahoo.es
2 Abdy Kermani, Professor and Director of Centre for Timber
Engineering (CTE), Napier University, 10 Colinton Road,
Edinburgh EH10 5DT, UK. Email: a.kermani@napier.ac.uk
In order to overcome the limitations of the flat deck
stress-laminated systems in the UK, the performance
characteristics of SLT bridges, in an arched form
developed at Edinburgh Napier University, were recently
studied. The extensive research revealed that the
utilisation of timber in an arching action where timber
predominantly acts in compression and end-bearing
rather than in bending, drastically improves the strength
and stiffness of the bridge. But as the benefits of the
arching action increase with the rise to span ratio, the
ease of access over the deck due to increase in slope
becomes a limiting design criterion.
This research has focused on the development and
examination of the structural performance of a flat SLT
bridge deck suspended from two screw-laminated timber
arches. A mechanically laminated composite timber
arched bridge with suspended flat deck was designed as
a half-scale of a 12m span bridge for pedestrian use, and
was subjected to a series of analytical and experimental
investigations under static loading conditions.
20mm thick off-cuts of low grade, UK grown Sitka
spruce timber, was used for this purpose. Screwed
lamination of short planks of timber was selected to form
the supporting arches. The laminates were cut at a
predetermined angle so that compression is mainly
transferred through end-bearing at the butt joints whilst
the fasteners transmit shear forces between the
laminates.
The half-scale of a 12m span pedestrian bridge
comprised a 6m span, 540mm wide 100mm deep flat
SLT deck suspended from two 110mm wide x150mm
deep screwed laminated arches all using 20mm thick offcuts
of Sitka spruce timber. The horizontal thrust was
carried by steel tie bars, fixed to steel bearing plates at
either ends of each arch. Their elasticity was designed to
simulate the horizontal settlement of the supports. The
bridge was subjected to a series of four-point and
eccentric line loads up to approximately 2.75 times the
design load (5kN/m 2 ) and the load-deformation
characteristics were monitored and recorded.

edistribute loads around a localised failure, laminated
timber showed an outstanding post-critical load capacity,
even after strength failure had occurred.
A finite element model was developed using a
commercially available analysis programme. The close
agreement between the analytical and experimental
results demonstrated that the structural behaviour of the
laminated timber structures can be accurately predicted
using linear elastic analysis.
Figure 1: Maximum measured vertical deflection of the
deck under a line load at various positions.
The results showed that the timber bridge behaved
elastically well beyond the design load. The maximum
deflection of the deck under the design load
corresponded to only a 1/1000 of the span for the
symmetric loading and 1/850 for the asymmetric
loading.
The bridge was then loaded to failure under a quarterpoint
loading. The bridge supported an eccentric load of
50kN until the collapse of a secondary member. After
replacement of the damaged component, the model was
again loaded under a four-point loading up to 50kN. This
last test confirmed the superior recovery capacity of the
laminated timber bridge after being subjected to loads
close to failure. Additionally, it was found that the
failure of the bridge under a symmetric loading would be
governed by out-of-plane buckling of the laminated
arches.
The study, carried out at the UK’s Centre for Timber
Engineering at Edinburgh Napier University successfully
demonstrated that large span stiff and strong structures
can be constructed by means of short lengths of low
quality timber. In comparison with glued-lamination
methods, typical for these structures, mechanical
lamination does not require expensive and nonenvironmentally
friendly gluing techniques, and hence
provides low-cost construction using locally available
timber and semi-skilled labour. Maintenance and
repairing of these structural systems could simply be
reduced to the replacement of the damaged laminate(s),
as end of life disposal and recycling processes are
enhanced. Individual laminates can be easily transported
to the site for the construction of large span bridges in
areas of difficult access.
Once the design and production techniques become well
established, the construction of medium to long span
bridges using mechanically laminated timber in public
paths and roads would further enhance confidence in the
use of locally produced timber as an economical and
sustainable material.
Due to the ductility of the structural system, the
dispersion of wood natural defects and the ability to
Figure 2: Deformed shape of the physical and computational model subjected to a quarter-point loading.