Coed Cymru - X329 Innovation Fund - Project Final Report April 2013

Building Structures with Round Timber Summary

Building with round poles is an ancient practice but producing efficient mechanical joints

between round poles is very difficult. In timber frame systems generally, the limiting factor will

usually be the connector. A wide range of non-domestic buildings and other structures could

utilise round timber if an efficient and cost effective method of connection were available.

Coed Cymru has previously worked with Clifford Jones Ltd of Ruthin to improve the process of

producing machined round timber by laminating two halves together. This was initially done to

reduce splitting, but access to the middle of the timber also offers the potential to embed

threaded rods and other materials into the timber before gluing it back together.

This has the potential to significantly improve connection strength and widens the scope for

roundwood timber in construction. This project has focused on the integration of the

connecting method into the timber to improve the integrity of the overall structure.

The results of the testing carried out by the project indicate that it is possible to induce failure

of the metal rods before the failure of the timber. This allows engineering calculations to be

produced based on existing peramiters and in principle allows the connectors to have wide

applicability in canopies, one off structures, and bridges etc.

The project investigated embedding steel connectors into round poles during the

manufacturing process in such a way that they bear on the end grain of the timber.

The resulting joints were compared in trials with conventional connections bearing on the long

grain of the timber and were found to be much stronger. Two different types of end grain

connector were then compared and a “T bolt” was found to be much stronger than a flat plate

and easier to install. Tests to establish the limits of this connector invariably resulted in the

shear of the threaded bar before the timber failed. This is a very satisfactory outcome.

Having successfully connected wood to metal, the next step was to suggest a range of simple

connectors or nodes which would enable the enhanced embedded connectors to join the poles

in 2 and 3 dimensional structures. Standard grades and profiles of steel were drilled, cut and

welded to produce a variety of nodes which are illustrated in the report. An enhanced wood to

wood connector is illustrated which could be used in timber trusses for bridges and wider spans

in buildings. This can also incorporate steel or other high tensile

elements in the laminate during manufacture.

Introduction

Welsh forests produce around one million cubic metres of timber per annum. The fencing

market is an important outlet for this material as treated round and half round posts are the

mainstay of agricultural fencing. A number of mills including Clifford Jones Timber of

Rhuthin,(CJT) use rounding machines to regularise the diameter of the poles and selected

material is also used to make playground equipment. The selected timber is straight with fine

branching and slower growth rates than sawlog material. It represents the best of the crop and

is available in lengths up to 7.2m and 225mm diameter.

Drying timber with the pith intact causes it to split along its radius which can be a hazard on

play equipment. CJT produce laminated poles from dry timber to reduce the tendency to crack.

During a visit to discuss this process it was suggested that steel connectors could be embedded

into the poles during the laminating process with the aim of making them less obtrusive and

more efficient.

Method

A series of practical tests was devised to compare embedded connectors bearing on the end

grain and conventional bolts bearing on the long grain. The tests used Sitka spruce, the most

widely grown species. This is a very low density softwood (circa 300kgs/m3 odw).

Test 1

This compared the indentation into long and end grain timber by drawing 30mm dia washers

into back to back samples of long and end grain using M12 threaded bar and nuts.

Test 2

Having established that the end grain was much more resilient, the flat washer was compared

with a bolt connector using the same procedure but with both bearing on end grain.

Test3

Further tests were devised to establish the performance limit of the T bar and a minimum

depth of embedding it into the end grain.

Test 4

The performance of the connector was tested under a shear load with and without a shear

connector on the end grain.

Results

Test 1 (End grain vs long grain in a simple pull test with back to back samples.)

The long grain broke up before the end grain was measurably dented.

Test 2 (Flat washer vs T bar into end grain)

The washers were drawn into the end grain to a considerable depth while there was minimal

denting with the T bar.

Test 3 (T bar vs large washer with hardwood bearing)

The M12 bar failed in shear in all tests. Reducing the depth to 50mm did not affect the result.

We did not manage to induce failure of the timber.

Test 4 (Shear test with and without shear plate)

No reaction was observed on test pieces up to 1100kgs load. One sample was retested with the

shear plate removed. Some cracking was heard at 800kgs and the sample was deemed to have

failed.

Having established that the T bar provided an efficient connection between wood and metal a

range of steel nodes was manufactured. These are illustrated below.

Discussion

Machine rounded green oak at Scolton Manor, Pembrokeshire

Evidence of the use of round timber poles in building in the British Isles dates back to the

Mesolithic period. The remains of circular huts excavated in Northumberland show that they

were built of birch 7,800 years ago. The size of the timbers used suggests these were

substantial buildings.

Round houses built from round timber was the characteristic building type until the arrival of

the Romans. The Iron Age site at Castell Henllys in Pembrokeshire is the best known example

of a reconstructed site in Wales. The most recent reconstruction is at Melyn Llynnon in

Anglesey. A circular frame of oak posts and ash rafters interwoven with hazel supports the

wattle and daub walls and a thatched roof of Norfolk reed

http://www.coedcymru.org.uk/archaeology.html. While excavation has revealed much about

the scale and occupation of the buildings, it has been for experimental archaeology to suggest

how the buildings were constructed, particularly the detail of the connections and how the

frame was braced.

Roundhouse reconstruction, Melyn Llynnon,

Ynys Mon

The simplest method available to our ancestors

was to select timber with the correct shape and

use the crucks and forks in the tree to form the structure and provide connections. Final

connection between pieces was then provided with bindings of rope, pliant rods or hide or by

cutting through one of the timbers and shaping the other to fit (this is the mortice and tenon

joint we still use today). These fixing methods are relatively inefficient when compared to the

strength of the timber itself and it is

the connections which limit the size of

structures which can be built in this

way.

Structures made from round timber

have continued to be used in

construction, particularly where the

timber bears the load in compression,

ie as a post or strut. Round timber

works less efficiently as a beam where

the lower surface is under tension and

more likely to fail. With sawn timber

this can be overcome by increasing the depth of the beam relative to its width but with round

timber the width and depth increase together, and therefore the structure is unable to bear its

own weight. But in most cases it is still the joints which are the weakest point.

Mortice and tenon joint

Round timber works well as a strut Not so well as a beam

If the efficiency of the joint can be enhanced this reduces the effect of the key limiting factor.

The simplest method is to insert a

steel plate into the end of the pole

and secure it with bolts.

This method works well with square

section timber but round poles limit

the spacing of the bolts which can

lead to the end of the pole splitting

under load and the failure of the

joint. Binding the end of the joint in

steel wire is a crude but effective

method which has been used to

reduce the risk of splitting.

Steel Plate and Wire connector

Picture of timber drying shed at Cilrhedyn

In any system, no matter how strong the timber is, the key limiting factor is the connector and

the project has focussed on improving the integration of the connecting method and the timber

in the context of improving the integrity of the overall structure. Access to the middle of the

roundwood during laminating offers the opportunity to embed different options of steel

connectors into the timber. This significantly improves connection strength and widens the

applications of roundwood timber in construction.

A very efficient connection is provided by bonding steel (or other tensile material) into the end

of the pole using epoxy adhesives (this was the subject of research projects in the 1990’s which

lead to Cowley Timberwork and Rotafix developing patented systems). These have the

advantage over steel plates with cross bolts in that the load is applied to the end grain of the

timber which is very resistant.

An alternative which also bears on the end grain of the timber is the

‘T’ anchor (referred to by furniture makers as a ‘barrel nut’).

This provides a very efficient method of transferring load to and from

a round pole enabling the structure to make best use of the timber.

The system does have two disadvantages:

(i) The perimeter of the pole is compromised by a large hole

when the barrel nut is inserted which reduces its strength and

detracts from its appearance

(ii) The alignment of the nut and bolt is critical if they are to meet correctly in the

timber. Achieving accurate end grain boring of timber, especially softwood, requires

special equipment and a degree of luck.

Producing round poles of uniform diameter by a process known as machine rounding is well

established technology and is widely used by manufacturers of fencing and play equipment.

Selected straight poles are fed into the rounding machine which removes the bark and the

outer layers of timber, increasing consistency. This process works for green (unseasoned)

hardwood and softwood and is capable of producing a good finish in one or two passes through

the machine. The timber selected tends to be slower grown from suppressed trees with finer

branching. Faster grown coarser material is preferred by the sawmills. As a result the machine

rounded poles are of superior quality in density, knot size and finish compared to most

carcassing timber.

The one disadvantage of using round poles which retain the centre of the tree is their tendency

to split along the radii when drying. This is a particular problem for play equipment if drying

produces deep wide fissures and sharp splinters. Splitting can be reduced by sawing the round

poles along their length and drying the ‘D’ sections. These can then be moulded to a precise

half round and glued back together to form a perfect round. Re-profiling the dry timber

enables much better finishing of the surface. However this also provides the opportunity to

embed connectors and tensile elements on the inside of the pole and is the focus of this

project.

Embedded “T” bar connector

This is a significant advantage as the core material is less important than the periphery in

determining the strength of the pole. Half round grooves moulded into the faces to be glued,

can accommodate bolts or longer rods or cables which can extend the whole length of the pole

or be anchored at the ends to provide connections.

Paired “T” bar connectors

A connector acting against the end grain of the timber is much more efficient than a

conventional connector acting against the long grain, which is softer and likely to split. A rigid

‘T anchor’ is simple to make and to fix accurately, and is very strong. It transfers the load from

wood to steel connectors very efficiently and it is possible to install two or more in a single pole

without compromising the peripheral timber. ‘T’ anchors consisting of 20mm mild steel rod

and M12 threaded bar drilled and welded through it were tried in comparison with nuts and

washers and found to be much stronger.

Test procedure for “T” bar connectors

When the size of the washers were increased and hardwood (ash) bearers included, the M12

bar sheared (this failure occurred in a series of teats at 80-100 ft lbs measured with a torque

wrench.) On no occasion was the top of the ‘T’ pulled significantly into the end grain of the

timber and attempts to induce splitting using shallow anchors were not successful.

Examples of tests comparing washers and “T” connectors

Proprietary single sided shear connectors were included to reduce the risk of failure of the joint

under shear.

Single sided shear connector

Wood to Wood Connectors Using ‘T anchors’

To work most efficiently the ‘T anchor’ needs to draw the end grain on to a flat bearing surface.

If this surface is long grain timber it can be reinforced with a flat washer or shear plate. The

simplest way found to achieve this was to taper the pole carrying the anchor to a spigot and

locate it into a shallow flat drilled into the other plate. This functions as a mortice and tenon

joint but minimises the material cut away in making the mortice, while reinforcing the tenon

with a steel rod

Simple wood-to-wood connector

In the example shown the two poles are set at right angles but any geometry can be used

enabling braces to be included in compound structures. The application envisaged for this

technique is a truss which could form part of a building, bridge or gantry.

This can be further enhanced by including high tensile rods in the lower beam of the truss (the

upper beam is in compression and doesn’t need it)

Round beam with steel reinforcing rods

Wood to metal connections using ‘T anchors’

Two and Three D connector using standard steel components.

An efficient connection is made by drawing the end grain of the pole onto a rigid metal plate,

the more of the end grain that impinges on the plate, the better the joint. If more than one

connector is employed the strength increases particularly if they are placed away from the

centre of the pole. The ability of the joint to withstand shear will be improved by a single sided

shear plate. The provision of a simple and efficient wood to steel connector enables a variety

of structures to be developed using welded steel brackets.

These have the advantage of simplicity and familiarity. The performance of the steel and the

welds is established by convention and novel applications can usually be resolved by

calculation. In this way a structural engineer can predict the performance of a structure using

standard tables without the need to test every component. Fixing steel brackets to concrete

and other materials is also established practice.

Note: creating a close contact between any impervious material and the end grain of timber will

encourage a build up of moisture increasing the incidence of fungal decay. Sealing the end

grain with bitumen or a proprietary end grain sealant is essential.

Where complex geometry is envisaged it may not be practical or necessary to achieve broad

contact with the metal surface. Where many poles converge on a single connector it may be

possible to minimise the size of the connection and simplify its manufacture by tapering the

ends of the poles where they converge and protecting the ends from splitting with a ring or cap

3D Hollow sphere connector 2D ring connector

Conclusions

1 Embedding T bar or plate connectors into round poles during lamination is a simple process.

2 Anchoring the connector into the core of the timber avoids compromising the main structural

timber around the periphery .

3 Connecting to the end grain is much more effective than connecting to the long grain.

4 The T bar provides a much better connection than the flat plate (washer)

5 The threaded portion of the T bar fails before the anchor is compromised even in a shallow

conection. The threaded steel rod has now become the limiting factor, not the timber.

6 This is now a very strong and simple connector.

7 A variety of nodes, compatible with the T bar connector, can produce a variety of 2 and 3

dimensional structures using standard steel profiles and fabricating skills.

Next steps

These will be exhibited at TimberExpo in Sept 2013

As at 2013 the European convention for calculating the performance of round poles has fallen

behind sawn timber and timber compounds in the process of updating standards but should be

reviewed shortly. Tests conducted to establish the stiffness of round poles derived by

laminating two half rounds will be analysed and some basic structures will be built.

In order for the system to gain wider market access it is envisaged that a system of formal

quality assurance will be required. Previous experience with the Ty Unnos system used the

TRADA Q Mark in similar circumstances and the route to CE marking is now open to the

manufacturers in that case.

Acknowledgements:

Thanks to Alan Jones, Richard Jones and their colleagues at Clifford Jones Timber Ltd who have

developed the capability for producing laminates from homegrown timber.

NEO Fabrication for developing steel connectors

Ian Jones ‘Idesign’

Thomas Martin of Burroughs

This work was supported with grant aid from Welsh Government

Further reading

http://www.forestry.gov.uk/pdf/crwoodproducts27.pdf/$file/crwoodproducts27.pdf