Engineered Timber

8/4/2019 Engineered Timber

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ENGINEERED TIMBER

The words 'engineered timber' have become synonymous with the

sophisticated art of producing a wide range of structural forms using

computer technology to its limits in conjunction with 'state of the art'

manufacturing technology. Super-efficient jointing techniques have

evolved, the most versatile being effected using a variety of modern

adhesive formulations.

Engineered Timber is enjoying a new lease of life.

Glulam is perhaps the first form to be widely adopted and remains

the most versatile.

1

Contents Page No.

Preliminary Design Guidance 2

Structural Timber 7

Joint Design 8

Fire Resistance 9

Prevervation 10

Care and Maintenance 12

Site Care 13

Site Advice 14


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Preliminary Design Guidance

Glulam

Glulam has been developed over many years butimprovements in manufacturing techniques have

reduced costs and increased its viable range. It is

the original engineered form of wood and is made

by bonding together readily available section sizes

maximum 45 mm thick.

Glued laminated timber is available in straight or

curved profiles Size is limited only by transport

considerations and custom made shapes offer

unparalleled versatility. Appearance is an impor-

tant attribute. Continental manufacturers dominate

the market but a few specialist British companies

have limited capacity mainly used for unusualspecies (or shapes). Whitewood (spruce) is the

preferred material but Redwood is available at a

premium. Most beams have 45mm laminae but

some 33.3mm multiples are supplied.

For curved profiles lamination thicknesses are

reduced to facilitate bending say approx R/180

Permissible stresses

Comparative values for medium term duration (i.e.

roofs) at 300mm deep for conditions below.

18% moisture content: -

2


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Straight Beams

Simply supported straight beams are widely used

for purlins, lintels, flat roof joists and similar

applications. Relatively light weight, combined

with ease of fixing, make the use of timberpopular with contractors and visually attractive.

Simply supported beams are usually deflection

governed in design. It follows that beams which

are continuous over multiple supports are more

efficient with consequent cost savings. Care will,however, be needed to allow for the changed dis-

tribution of loading on the support structure.

Purlins & Lintels

Some alternative locations for local trimming or

longer span support.

3

Shaped Beams

For roofs that are nominally flat a generous fall is

strongly recommend. Timber beams can be

tapered from one end or both ways from centre.

This can be done with or without a camber which

can considerably enhance the appearance of

beams which might otherwise look quite deep mid

span.

Cantilever Beams

It is easy to taper Glulam. Balconies, canopies

and larger roofs will look better trimmed to a

structurally efficient profile.

Depth ratios are approximate to assist preliminary assessment

of proportions.

For breadth (thickness) assume approx H/5.


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4

Post & Beam

Simply supported or multiple span beams obvi-

ously need support and laminated columns are

frequently suitable. Some stiffness can be devel-

oped through the joints or knee braces can beadded to accommodate greater forces. Laminated

beams can also act in combination with other

materials and moment connections are quite

viable with steel or concrete.

A Frames & Trusses

Trusses can considerably reduce section size

compared with simple spans. Rafters can be held

together by timber or metal members, whilst in A

frame applications a concrete raft might be used

to accommodate thrusts. Tie rods will span sub-stantial distances without intermediate support but

sag-bars of small diameter can be introduced if

necessary with normal cost penalty.

Lifting the tie above eaves reaction points will

introduce secondary bending to the rafter exten-

sion. This will increase rafter sizes but can have

appearance advantages in certain applications.

Where some internal members are in compres-

sion these will usually be in timber but can still be

used in combination with steel tension members.

Complete timber trusses will often be chosen for

appearance and practical considerations. We

would suggest that the number of internal websbe kept to a minimum to reduce assembly costs.

Pyramids

Trusses are usually thought of as two-dimensional

but the same principles can be employed to form

four or more sided pyramids. Where the tension

from the reactions at eaves can be accommodat-

ed in steel, timber or concrete, appropriate care

should be taken to ensure that the centre lines of

force are correctly appraised. A laminated struc-

ture exerting outward thrust on top of a support

frame can result in eccentricity unless due care is

paid. Ties need not be limited to acting around the

eaves or directly across the void. Some very inter-

esting patterns can be evolved with an internal

network of steel or timber.


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5

Arches

Where the foundations or structural

frame are suitably designed two or

three pinned arches are readily

formed in Glulam and are verystructurally efficient. A circular layout

will produce a clear, efficient dome

a form which has been used for

very large spans in other parts of

the world.

Tied Arches

Tie rods can also be used torestrain thrust from arches where it

is difficult to accommodate in other

elements of the structure. In these,

as with other applications, the

warmth of timber creates a unique

environment which would be difficult

to reflect with other materials.

Three Pinned Portals

Curved laminated three pinned

frames will probably be familiar and

have considerable versatility from

the smallest swimming pool costing

a few thousand pounds to larger

pools, churches, community centres

and industrial applications, both

large and small.

Filling in the corners is fairly expen-

sive and the use of a separate col-

umn and rafter, both saves moneyand may lighten the appearance

where the member depth could

otherwise be quite large.


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6

Framed corners can be economical andhave the advantage of fabrication from

straight components.

Curved corners may be intrusive theuse of finger joints can improve head-

room.

Large finger joints are widely used on

the continent. The relevant Standard is

BS EN 387: 2001


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7

Structural Timber

References

The principal standards governing the design and production of structural timber.

BS EN 1995: Design of timber structures.

2005 Part 1.1 General rules and rules

for buildings

A limit state code which has now been published

and can be used for design on a voluntary basis.

Will eventually supersede national codes.

BS 5268-2: Structural use of timber.2002 1996 Code of practice for

permissible stress design,

materials and workmanship.

Introduces much of BS EN 1995-1-1 in a

permissible stress form as a transitional

document. Some new material. Formalises the

EN production standards.

BS 5268: Recommendation for

Part 4 calculating fire resistance of rsSection 4.1 timber members.

1978

BS 4978: Specification for visual strength

1996 grading of softwood.

+ Amd No. 1:

BS EN 301: Adhesives, phenolic and

1992 aminoplastic, for load bearing

tmber structures: classificationand performance requirements.

Type 1 adhesives are suitable for exposure to

weather and/or high temperature. Phenolic

Resins (PRF) are dark coloured. Aminoplastic

Resins (MUF) are light.

BS EN 336: Structural timber, coniferous

2003 and poplar.

Sizes, permissible deviations.

Solid timber tolerances

BS EN 338: Structural timber strength Structural

2003 timber strength classes

Defines the properties of C and Dclassifications

BS EN 385: Finger jointed structural timber.

2003 Performance requirements and

minimum production requirements.

BS EN 386: Glued laminated timber -

2001 performance requirements and

minimum production requirements

Glulam manufacture. Supersedes BS 4169

BS EN 390: Glued laminated timber - sizes.

1998 Permissible deviations

Glulam Tolerances

BS EN 519: Structural timber Grading

1995 requirements for machine strength

graded timber and grading

machines.


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8

Joint Design

High machining and jointing costs can often be

the sum of many apparently small contributions.

These pointers to better detailing also cover some

performance criteria.

Note the pattern of forces

The choice of jointing systems should take into

account the extremes of loading conditions not

just worst cases.

Be aware of visual constraints

Appearance is often very important and may

preclude some types of fastener.

Keep machining simple

Mixing hole dimensions in the same componentrisks error and slow drilling timber.

Single crosscuts are preferred to double or

compound cuts.

Notches, grooves & slots add to setting out & pro-

cessing times.

Avoid special fabrications

Even the simplest plate or angle can cost far

more than its proprietary counterpart and has to

drawn, priced etc.

Separate the artwork

If special fabrications are unavoidable make it

easy for the details to be copied for tendering and

subsequent processing controls.

Standardize the fastenings

Using the same bolt with or without connectors

can maintain drilling & appearance but take vary-

ing loads. Repeat grouping/spacing helps jigging.

Keep the steelwork slim

Try comparing the bolt shear/hole bearing/timber

embedment capacities. Unless tension or buckling

govern, thick plates are seldom justified.

Limit plate sizes

Space the fastenings to the timber rules. Then

keep minimum steel edge distances beyond them

(normally 2d). Any more could be asteful and cre-

ate unnecessary tolerance restrictions.

Weight is not free

For ease of processing complex or fast track

inquiries many fabricators price on weight lone

based on experience of the average labour con-

tent. Even carefully costed items have a material

content. Dont read too much into that disappoint-

ing allowance for a weight reduction.

Fastener density

A greater number of small fastenings tend to be

more efficient than fewer large ones per unit area.

(E.g. Trussed rafter plates v bolts) Spacing rulesare more easily satisfied with smaller units but

labour content may rise.

Shrinkage

Timber moves with variations in moisture content

across (but not along) the grain. Varies with

species but around 1% size for 5% m.c. Plate

sizes & orientation should recognize this and

avoid stress concentrations in large

sections. Joist hangers, for example, should not

be bolted top and bottom.

Avoid eccentricity

Timber takes load far better to the grain. Make

sure centrelines of action intersect.


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9

Fire Resistance

Timber is Predictable

The charring rate of timber exposed to fire is

predictable. BS 5268 : part 4 sets out the criteria

by which the strength of the residual section may

be assessed following specified periods of

endurance. This enables designers to ensure that

the required fire resistance can be achieved.

Design Criteria

The load bearing capacity of timber beams 70mm

thick or greater should be calculated based on the

residual section and stresses of *2.25 x long term

dry stress (* x 2.00 for members under 70mm).

Permissible deflection is relaxed to span/30 and is

not normally relevant unless gross distortion might

de-stabilise other elements of the structure.

Charring Rates

For periods of 15 to 90 minutes a charring rate of

20mm in 30 minutes is assumed for beams and

columns with one concealed face. This rate

relates to most coniferous species (except

Western Red Cedar 25mm in 30 mins). The

nominal rate for a specified range of Hardwoods

is 25% slower at 15mm in 30 mins. When all

faces are exposed the charring rate is increased

by 25%.

Permissible Loads

The load to be supported is either the maximum

permissible design load or the load which the

member is required to support in normal service.

It has become custom to interpret this apparent

ambiguity to suggest that full snow load would be

unlikely under these circumstances and to reduce

the imposed load by two thirds accordingly.

Effect on Size

In practice beams 90mm thick and over will

usually endure 30 min fire without modification.

Longer periods will probably lead to an increase in

thickness although this may be partially offset by

a reduction in depth.

Bond Strength

The adhesives used in the manufacture of Glulam

and LVL comply with Type 1 of BS EN 301, are

not flammable and do not lose their integrity when

exposed to prolonged elevated temperatures.

90 x 315 mm

Glulam

After 30 Mins

Fire

After 60 Mins

Fire


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10

Preservation

The European View

BS EN 1995-1-1 will increasingly govern the

structural use of timber, requires designs to reflect

the envi ronmental climatic conditions from a

choice of three classes:-

Service Class 1. Conditions in which most timber

will attain an average moisture content not

exceeding 12% - corresponding to a temperature

of 20oc with relative humidity of the surrounding

air only exceeding 65% of a few weeks per year.

Service Class 2. Conditions in which most timber

will attain an average moisture content not

exceeding 20% - corresponding to a temperature

of 20oc with relative humidity of the surroundingair only exceeding 85% of a few weeks per year.

Service Class 3. Climatic conditions leading to

higher moisture contents than in service class 2

but excluding continuous hot and wet conditions

such as in cooling towers.

The average moisture content of timber (A) likely

to be attained in service and (B) which should not

be exceeded at the time of erection are defined:-

Service Environment (A) (B)Class

3 Covered, generally unheated 18% 24%

2 Covered, generally heated 15% 20%

1 Internal, continuously heated 12% 20%

BS EN 1995-1-1 with support standard EN 335-1

does not require treatment of timber in C1 and

most of C2. Although the per capita use of Glulam

is far greater in other European countries, the

non-essential use of toxic chemicals tends to be

avoided.

Choice of Preservative

If it is still felt that preservative should be applied

for reassurance, then for low hazard conditions

(say up to cat.2B or C) the industry standard

method of applying preservative to Glulam is by

flood coating, deluging, spraying etc to the point

of refusal.

Organic solvent based formulations are preferred

for Glulam and for joinery since they do not

discolour the timber or its finishes.

Organic solvent based formulations are preferred

for Glulam and for joinery since they do not

discolour the timber or its finishes.

They avoid swelling, shrinkage and similar prob-

lems associated with water borne varieties. A

protective envelope or shell is provided rather

than deep penetration of the component.

Treatment of individual laminations prior to bond-

ing is not viable since most would be removed by

the planning, which must immediately precede

lamination.

Glulam is usually made from European

Whitewood, which is Restraint to treatment.Where slightly greater risks are envisaged than

European Redwood which is Moderately

Resistant can be specified but with an increase in

cost and extended delivery period. Knots will be

more prominent with Redwood.

Where a high risk is unavoidable, such as

bridges, then pressure impregnation with water

borne CC treatment can be applied to laminations

(max.33mm) prior to bonding. The salt retention

can be specified to suit a desired life in excess of

60 years. The process normally requires use of

Redwood for satisfactory penetration and when

extra drying and other processing costs are taken

into account, involves significant cost increases.

CC treated timber is characterised by green

colouring which fades to some extent.

Some manufactures are able to supply Glulam

made from very durable or durable hard wood

species and not necessarily from tropical sources.

Significant cost increases will, however, be

incurred with possible extended delivery.


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11

Points to Watch

Subjects which merit particular car in building

design include: -

Vapour Barriers High Humidity Areas

DPCS Splash ZonesInsulation Jointing Steelwork

Flat Roofs Roof Overhangs

Enclosed Valleys Condensation

Gutters Ventilation

Down Pipes Decoration

Timber below FFL Maintenance

In more hazardous locations, Glulam made from

European redwood is better than whitewood. It is

more receptive to preservatives, but should still be

finished with applied coatings, as recommended

for joinery inn similar locations. Most methods of

preservation provide a protective envelope of

varying depth. Detailing should provide shelter

from moisture and allow good ventilation and

drainage. Columns should not continue below

DPC level and in damp zones, shoes should not

enclose the timber, but have open sides or be

centrally slotted in the member. End grain is

relatively absorbent and extra care should be

taken in end sealing or capping with a similar

species.


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12

Care & Maintenance

General

Routine inspection is recommended.

Routine maintenance will normally consist of a

systematic inspection at say six or twelve monthintervals. Only when exceptional conditions

prevail will further action be required.

Decoration

Structural timberwork needs no special attention.

The decoration of structural timberwork can be

undertaken to the same programme as other

internal refreshing coats. At low levels a wipe

clean (stain or gloss) surface may be advisable to

extend redecoration intervals.

Fire ProtectionBe aware of special coatings.

Special coatings (intumescent) are sometimes

specified. It is critical that these are neither

removed nor degraded by the application of non-

compatible materials. A record should be kept of

the exact specification employed and redecoration

should comply with the manufactures recommen-

dations.

Excess Moisture

Look out for roof or plumbing leaks.

Care should be taken to ensure that ntreated soft-

wood is not maintained at over 18% moisture con-

tent. If a source of moisture such as plumbing or

weathering fault had been undetected for some

time, the source should be corrected promptly,

and the timber allowed to dry out. Checks should

then be made for possible start of rot or similar

degrade. If in doubt, consult the supplier for

appropriate remedial action (repair and/or the

application of preservative.

Dry Environment

Vigorous air conditioning can have side effects

For most internally heated conditions timber

should stabilize at about 12% moisture content.

(Normal Glulam supply level). Timber shrinks

when dried. Rapid drying below 12% can lead to

some surface splits, and in exceptionally warm dry

conditions splits can be expected to occur. They

can sometimes look severe but are hardly ever of

structural significance. (Grading rules can permit

fissures half the width of the member or more).

Remedial

Do not rush

Any cosmetic action should not be undertaken

until say 9- 12 months after commissioning centralheating. If stopping is judged necessary hard set-

ting varieties should be avoided to allow future

movement to occur with generation of secondary

stresses.

Bolted Joints

May need tightening

Nuts should be checked for tightness about twelve

months after construction. This check should

receive particular attention where signs of excep-

tionally dry conditions have been noted.


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13

Site Care

Storage

Timber structures and components for visually

exposed applications should be stored:

On level bearers well clear of the ground

Use clean dry timber free from girt and of grease,

supported high enough to clear rain and mud

splashes.

With Supports evenly spread

Locate bearers at ends and near joints. Ensure

self weight is uniformly supported to avoid distor-

tion.

Using Strips or blocks between componentsSpacers help to prevent the trapping of dirt or

water between timber faces, but must be in line

vertically.

Covered with dark sheets

Secured against wind. Rain normally does not

harm in the short term, but prolonged exposure

can lead to swelling and staining. Sun darkens

exposed wood quickly. Partial exposure or partial

masking can be very difficult to blend in later.

With attention to wrappingTransit wrapping products during delivery, han-

dling and erection only. Ideally, if should be

removed promptly to avoid moisture traps and

ensure even weathering. However, site manage-

ment will often prefer to retain for protection

against wet trades. In this case small holes should

be cut in the underside of retained wrapping to

drain any water pockets. It is also particularly

important to make an keep good any disturbance

around joints.

Handling

When handling and erecting structural timber

components for visually exposed use, care shouldbe taken to ensure that they are:

Evenly supported

Use webbing slings to avoid local bruising. Locate

with care to ensure balance support. Control with

guy lines.

Evenly exposed

Mud, plaster, banding, temporary bracing, partial

wrapping etc, can all leave light patches when

removed or made good. Other materials should

be removed promptly.

Fixed with rustproof fittings

Particularly nails in temporary bracings or nearby

timber. Rain can create dark stains from any

unprotected ordinary steel.

Securely braced

Preferably with permanent bracing. Wire guy lines

with turnbuckles or timber members may be need-

ed to resist sudden high winds etc. (When delays

in permanent framing are unavoidable)

Covered if necessary

When prolonged exposure is expected close

wrapping may be desirable but difficult to secure

without water traps. Black polythene can be

draped over with a continuous top

batten and open soffit.


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