structeng tech note timber frame.pdf

26|The Structural Engineer – 6 September 2005

technical note: timber frame

Timber frame is increasingly beingused for four to seven storey residen-tial construction but there is little

education for structural engineers in thedesign and construction of timber framebuildings.

This technical note addresses some ofthe structural issues associated with thedesign and detailing of multi-storey timberframe buildings and introduces the ‘RimBeam Method’ for achieving robustness forClass 2B buildings. It also identifies goodpractice to ensure a quality finishedtimber frame building, including detailingfor durability and differential movement,fire and acoustic design, constructionissues and the work of ‘follow-on-trades’.

The author, in his role as SeniorStructural Engineer at consultants CCBEvolution Limited, has had considerableexperience of the structural design, detail-ing and inspection of multi-storey timberframed buildings, constructed in the UKusing ‘Platform Frame’ techniques.

Platform timber frame v. framedstructures Conventional ‘Platform Frame’ relies on acellular plan form with all timber framewall and floor components fixed to eachother. Unlike other structural arrange-ments such as post and beam frames withinfrequent shear walls, ‘portalised’ framesor braced bays, timber platform framerelies on the diaphragm action of the floorsto transfer horizontal forces to a distrib-uted layout of load bearing walls.The loadbearing walls provide both verticalsupport and horizontal racking and shearresistance.

The platform timber frame method ofbuilding has many advantages includingspeed of construction due to the lack ofany ‘wet’ trades’, relatively low foundationloads and the predominance of ‘off-site’manufacture of floor ‘cassettes’ and wallpanels, which is not weather sensitive.Theuse of factory-made components, fullyassembled into cassettes, is potentiallysafer during construction and arguablyhas reduced dependence on skilled siteoperatives.

From the sustainability viewpoint, plat-form timber frame can lead to economicuse of materials, reduced site wastageduring construction and reduced energyrequirements during service1.The use of

timber from ‘managed’ sources is alsoinherently sustainable.

Performance proven by testHistory of use, and full-size testing carriedout by TRADA and BRE on the six-storeytimber-frame 2000 research project(known as TF2000) at BRE Cardington infrom 1995 to 2000 have demonstrated thattimber frame buildings have inherentresistance to accidental damage. ‘For stan-dard platform timber frame construction,the assessments and tests have verifiedreasonable robustness requirements, whenthe panels are keyed into each intersectionof the building and appropriately nailedtogether’2

Until a few years ago, the number ofstoreys in timber-frame buildings in theUK was limited by fire regulations.However, this restriction was lifted in19913 allowing up to eight storeys for thefirst time (in England and Wales) withoutany additional fire resistance require-ments other than those existing for manythree-storey buildings. Furthermore, fullscale fire tests on TF2000 also showed thatcompartmentation and building integrity

was maintained throughout the tests.TF2000 therefore opened up the possibili-ties for timber frame buildings in excess ofthe perceived technical limit of fourstoreys (Fig 2).

Permanent & temporary stabilityStability of load bearing wallsTimber wall studs rely on the sheathingand lining materials fixed to them toprovide lateral restraint against studbuckling. Plasterboard linings, however,are fixed later in the build process, prior towhich the timber studs will be carryingbuilding loads, which, in some instances,may be higher than the in-service loads asthe plasterboard packs can be located onfloors prior to being fixed. Unsheathedwall studs may not be adequately laterallyrestrained before plasterboard is fixed andthe designer should check the temporarycondition for lateral buckling of the studswith a construction imposed load to see ifmid-height noggins are required to reducethe temporary stud slenderness.

External wall studs also carry windloads, transmitted to them by the claddingvia wall ties or battens. Unless it can bedemonstrated that the stiffening effect ofthe cladding adequately prevents exces-sive stud deflection, stud deflection calcu-lations due to wind loads can be thegoverning load case for the design of 89deep wall studs, which are commonly usedfor external walls.This failure modeshould be checked by designers, but isoften overlooked.

Compartment tiesEach section of the building betweencompartment walls may need to be consid-ered separately for stability during theconstruction stage. During the permanentcondition, adjacent sections of buildingbetween compartment walls can beassumed to act as a whole and metalcompartment ties must be designed totransmit horizontal loads through thestructure and across compartment wallsin compression or tension only.

Metal compartment ties are generallyprovided across compartment cavities atnot exceeding 1200mm c/c to tie adjacentsections of the structure together.The tiesprovide load transfer if designed appropri-ately, but there is a limit to the amount ofload that can be carried by the tie becausethe size and spacing of the ties is limitedby acoustic regulations.

Racking resistanceIf plasterboard is to be used for racking inaddition to Category 1 sheathing boards, itshould be correctly fixed and fullysupported on all edges as specified by BS5268: Part 6.1:19964.

Typically, it is necessary to ensure thatthe lower half of the framing has adequateracking resistance without the contribu-tions of plasterboard, as reliance on tempo-rary bracing on large multi-storeyconstruction has proven to be morecomplex than low-rise projects5.

The design guidance given in BS 5268

Multi-storey timberframe constructionGuy Lewis (M) Associate Director, CCB Evolution Ltd.(formerly Chiltern Clarke Bond Ltd.) discusses the structuraldesign of timber frame buildings

Fig1. Five-storeytimber-framedstructuredesigned by CCBEvolution atFinboroughRoad, London forGilmac Design &Build Ltd

Fig 2. Six-storey TF2000timber framedstructure built byTRADA/BRE atCardington 1995-2000



Part 6.1:19964 currently limits the applica-tion of these codes to four storeys.A newcommittee is understood to be updatingthese codes for use up to seven storeys, butuntil this is issued, designers have to usejudgment in applying the rules given inthe current codes.

One aspect that should be considered isthe stiffness of multi-storey frames.Theexisting codes are based on small-scaletests and experience of low-rise construc-tion.The specific horizontal deflectionlimit given in the codes is height/300which, if realised, would lead to significantsway of multi-storey timber frames.Thedesigner must therefore use judgment inlimiting the expected sway of multi-storeybuildings during the design process.

Holding-down straps and soleplatefixings In addition to the racking and stud stabil-ity checks, sliding, overturning and roofuplift resistance also require checkingbefore claddings and linings are fixed andthe roof finishes are in place, at whichpoint the dead weight of the building maybe at its least.As well as proprietaryfixings at wall plate and soleplate levels,the nailed fixings at the intermediatelevels should also be checked.

The fixing schedule A fixing schedule should be provided bythe building designer to the fabricator andtimber frame erector to ensure that all thedesigned connections, and the fixingsrequired are identified for both factory andsite connections.The industry has stan-dards that have been established for low-rise construction but the higher loads frommulti-storey frames require increased nailfixings at member interfaces.The ‘experi-ence’ of factory operatives or site erectorsshould not be relied upon to ensure thatall interfaces are adequately nailed asexperience of the author has shown thatthis is not always the case.

Wall panel detailing and continuity ofvertical load paths Head binders laid on top of wall panelscan act as continuous structural beam

elements carrying joist point loads to thestuds below and should be designedaccordingly for non-coincident joists andstuds unless the manufacturing and erec-tion process is carefully controlled toensure that all joists will be aligned with asupporting wall stud.

Stud clusters must be provided beneathall beam and girder bearings to ensurethat bearing stresses are not exceeded.This load path must be continuousthrough floor zones, where solid membersmay need to be provided, and wall panelsat lower storeys all the way to a suitablefoundation.

RobustnessClass 2A or 2B building for dispropor-tionate collapse?Requirement A3 will now apply to allbuildings, not just those over four storeys.It applies to all buildings in England andWales, which had not started on site by 1December 2004. Most residential buildingswill fall into Class 2A or 2B of Table 11 ofBuilding Regulations Approved DocumentA6.

Class 2A buildingsThe Building Regulations state that forClass 2A buildings, robustness will beachieved by providing effective horizontal

ties, or effective anchorage of suspendedfloors to walls.

For Class 2A buildings, the approach isto adopt good building practice of provid-ing lateral restraint to walls and commonanchorage details of floors to walls.Thedesign process should involve checking thecapacity of the component interfaces (e.g.panel rail to soleplate, soleplate to floordeck, floor joists to head binder and headbinder to panel rail) against the variablehorizontal wind forces.The timber framedesigner should therefore be providing arobust connection at each and every junc-tion as part of the normal design process7.

For conventional timber frame build-ings of cellular plan form UK TimberFrame Association (UKFTA) has recom-mended that the effective anchorage offloors to walls will be achieved with aminimum density of nails equivalent of3.1mm dia at 3.3/m/run-of-wall junction7

(Fig 3).

Class 2B buildingsThe Building Regulations state that forClass 2B buildings, robustness will beachieved by providing effective horizontalties together with effective vertical ties orby checking that upon notional removal ofa load bearing wall (one at a time in eachstorey of the building) the buildingremains stable and that the area of floor atany storey at risk of collapse does notexceed 15% of the floor area of that storeyor 70m2, whichever is the smaller, anddoes not exceed further than the immedi-ate adjacent storeys.

Where the notional removal of lengthsof walls would result in an extent ofdamage in excess of the above limit, thenthe use of a ‘key element’ design approachfor an accidental design loading of 34kN/m2 applied in the horizontal and verti-cal directions to the ‘key element’ shall beadequate.Trada’s 2003 publication5

provides design guidance for Class 2Bbuildings where notional removal of load-bearing walls is part of the design check tocomply with Regulation A3.

For conventional timber-frame build-ings of cellular plan form UKTFA hasrecommended that the effective anchorage

Nail density and type to be determined by design but minimum nailing to be provided as follows:

FOR CLASS 1&2A BUILDINGS:3.1mm dia @ 300mm centres (3.3 per m run of interface)

FOR CLASS 2B BUILDINGS:3.1mm dia @ 200mm centres (5 per run of interface)

An increase in nail diameter isallowable, but a reduction incentres is not

Examples of nailing locationsand specifications at each interface are shown opposite

bottom rail

sole plate

decking

rim beam

waist bandnailed toperimeter

headbinder

top rail

16

pane

lpa

nel

panel to sole plate – 3.1Øx75 @ 400c/ cin pairs x 50mm stagger

sole plate to deck – 3.1Ø x 75@ 300c/ c in pairs x 50mm stagger

deck to rim beam 3.1Ø x 50@ 300 to each perimeter member(equivelent to 150c/ c)

bottom flange of t joist tohead binder – skew nail with 3.1Ø x 75 @ 300 max.

deck edge to header – skew nail with3.1Ø x 75 @ 300 max.

head binder to panel – 3.1Ø x 75 @ 300c/ c in pairs x 50mm stagger

Finishes (omitted for clarity) are to provide fire protection to floor and conform with relevant acoustic criteria

Fig 3. Diagram ofexploded floordetail showingminimum nailingdensities

Fig 4.Seven-storeytimber-framedstructuredesigned byChiltern ClarkeBond Ltd atNewcastle-under-Lyme for TaylorLane TimberFrame Ltd swhich used top-hung open webjoists supportedon a rim beam

6 September 2005 – The Structural Engineer|29


of floors to walls will be achieved with aminimum density of nails equivalent of3.1mm dia at 5/m run-of-wall junction7

(Fig 3).In checking the robustness of timber-

frame buildings, engineers are to applyjudgment-based thinking to the likely 3-dimensional structural behaviour of abuilding backed, where appropriate, with a2-dimensional structural assessment ofdiscrete elements.The TF2000 full-sizetesting has shown that this approach isconservative but appropriate to determin-ing the robustness of platform-frameconstruction in buildings such as themedium-rise TF2000 building5.

One method adopted to satisfy thenotional removal of wall panels and usedby the author in the design of multi-storeystructures, can be best described as ‘TheRim Beam Method’.

The Rim Beam MethodThis method allows joisted floor structuresto be assembled in the factory as ‘cassettes’with a ‘rim board’ used to connect the endsof the joists together for transportationand which remains as a vertical loadtransfer element in the completed struc-ture.A separate ‘rim beam’, which isusually installed loose on site, spansbetween intersecting return walls or ‘keyelements’ and acts as a bridging member.

Calculation checks are carried out onthe principal of notional removal of wallpanels, one at a time, between intersectingreturn walls or defined key elements. Forexternal panels the minimum length ofwall to be considered is 2.4m, with nomaximum length. For internal walls themaximum length of wall to be consideredis 2.25H where H is the clear height of thepanel between lateral supports.

The rim beam is incorporated loose inthe floor zone at the end of all simply-supported joists to prop the wall panel andfloor structure at each level, followingnotional removal of a wall panel betweenintersecting return walls or defined keyelements beneath the rim beam. Unlessthe joists are ‘top-hung’ over the rim beam,the floors at the same level as the rimbeam are only nominally connected to thisbeam by the nailing together of the twomembers, and are therefore assumed tocollapse (or cantilever if continuous joistshave been used, albeit with significantdeformation).A check should be carriedout to ensure that the resulting floorcollapse will not be disproportionate to theevent and will constitute less than 15% ofthe floor area of that storey or 70m2,whichever is the smaller.

The rim beams are supported at walljunctions by solid stud groups. It is impor-tant that the rim beam supporting thegreater load (the one supporting the floorjoists) has a full bearing on at least twostuds at the panel junction and to achievethis, the wall panels are often lapped inthe opposite manner to the rim beams. Ifno stud clusters are present below the rimbeam bearing, proprietary or fabricatedhangers are provided off adjacent rim

beams to ensure that a support is avail-able.

The rim beams and their connectionsare designed to support the full dead loadof a single level of floor structure plus 33%of the imposed loads on the floor.A dura-tion of load factor of k3 = 2.00 and deflec-tion limit of L/30 are applicable for thisload case.

Continuous joist spans (I-joists andopen-web joists are easily available andtransportable in lengths of up to 11m) areused wherever possible to avoid the needfor rim beams on internal supports.Whereinternal load-bearing walls are notionallyremoved, the joists are assumed to act indouble span at each level and support thefloor loads described above plus the weightof a (now non-load bearing) wall panelsupported off the double-spanning joists.

In this way, a continuous ‘perimeter’ ofloose rim beams is provided at externaland compartment walls, with ‘cassetted’floor decks being located inside of the rimbeam perimeter.The need to provide a

horizontal tie force, that is part of the ‘postand beam’ design approach in concreteand steel, is avoided by the notionalremoval of load-bearing elements.The rimbeams are tied back to the floordiaphragm with the minimum nailingdensities described earlier.

Other methods of achieving the supportfor walls and floors following notionalremoval of wall panels are possible such as:• the use of ‘room-size’ floor cassettes with

cassette rim boards acting as rimbeams, bridging over removed wallpanels;

• ‘loose’ floor construction with joistshangered from loose rim beams, whichbridge over removed wall panels;

• where joist span-lengths are repetitiveusing alternate double-spanning joistswith selected walls designed as deepbeams.

However, the rim beam method has theadvantage of enabling floors to be fabri-cated as cassettes and enables fairlycomplex building plans and large roomsizes to be accommodated.

The design chosen will probably rely onengineering judgment to some extent.Assuch it is important that it is peer-reviewed by experienced senior engineersand is fully documented, quoting sourceswhere appropriate8.

Designing for durabilityThe vast majority of timber frame build-

Fig 5. DiagrammaticRim BeamMethod framedrawing (see‘Panel 1: Rimbeam’)

Fig. 6. Ground floorsoleplate detail(Courtesy NHBCStandards Section6.2, April 2002)

Panel 1: Rim beam

D.C Event 1: Notional removal of internal wallpanel IO of maximum length 2.25H where H isthe clear height of the panel between lateralsupportsContinuous joist spans J1-J5 avoid the need for rimbeams on internal supports. On removal of thesupporting wall IO the joists act in double span ateach subsequent level and support the floor loadsplus a single storey height of (now non-loadbearing) wall panel I1-I5 supported off the double-spanning joists.

D.C. Event 2: Notional removal of external wallpanel E3 (party walls similar) betweenintersecting return walls or defined keyelementsRim beams are incorporated loose in the floor zoneat the end of cassetted floor joists. Followingremoval of wall panel E3, unless the joists are ‘top-hung’ over the rim beam, joists J4 are assumed tocollapse or cantilever and a check should carriedout to ensure that the resulting floor collapse willconstitute less than 15% of the floor area of thatstorey or 70 sqm, whichever is the smaller. Rimbeam R4 is designed to support panel E4 and floorjoists J5 by ‘bridging’ over the notionally removedwall panel. Subsequent rim beams R5 support wallpanels E5 and joists J6.

Rim beams are designed to support a single storeyof wall panel plus the full dead load, plus 33% ofthe imposed loads of a single level of floorstructure. Continuous joists are designed tosupport a single storey of wall panel plus the fulldead load, plus 33% of the imposed loads on thatfloor. A duration of load factor of k3=2.00 anddeflection limit of L/30 are applicable foraccidental load case.



ings in the UK use softwoods rated as non-durable for the structural components.However, properly designed andconstructed timber buildings do not relyupon preservative treatments fordurability9.

A correctly detailed wall framework,with adequate ventilation, is designed sothat it will maintain an equilibrium mois-ture content considerably less than the22% threshold that could allow fungalgrowth to occur. It is, however, commonpractice for the structural components inthe external wall to be preservativetreated as an insurance against any futurefailure of the weather-resistant cladding.

Wall framing above DPC level is gener-ally classified to Risk Category C2 andtreatment would generally be by doublevacuum organic solvents or water-basedmicro-emulsions. Soleplates carry a higherrisk category of C3, where organic solventpreservatives are required.

Furthermore,Trada and insuringbodies such as NHBC and Zurich recom-mend that all structural timber, irrespec-tive of its preservative treatment ordetailing, should be located a minimum of150mm above finished ground level.Thiscan have a number of effects on the detail-ing of the lower storey of timber-framewall panels, often requiring ‘durable’plinths of concrete or masonry to beprovided around the full external perime-ter of the building to lift the timber-frameelements 150mm above finished groundlevel (Fig 6).

Detailing for differential movementWhy does differential movementhappen?The coefficient of thermal expansion fortimber along the grain is small. Thethermal conductivity of wood is muchlower than steel. For these two reasons itis normal in timber buildings not to useany movement joints to cater for temper-ature movement, but care must be takento avoid cumulative cross-grain move-ment10.

Kiln-dried softwood timber is typicallyinstalled in buildings at 20% moisturecontent. Over time this will reduce downto around 10% in the internal walls of aheated building.As it dries out, cross-sectional timber in the structure shrinksand the whole structure settles.

Differential movement can thereforeoccur between the timber-frame inner walland masonry outer cladding, resultingfrom shrinkage of the structural framedue to drying out of the cross-sectionaltimber elements (rails, binders, floor androof joists) and the expansion of clay bricksor shrinkage of concrete blocks due tothermal changes and swelling or drying-out.Additional movements occur due toslight elastic shortening, creep and‘bedding-in’ of structural elements.

Movement may be reduced by usingengineered wood joists or super-driedtimber at 12% moisture content but it isalso important to ensure that detailing iscorrect to allow for settlement as it will not

usually be possible to omit all kiln-driedsoftwood from the building make-up asthis would not be cost effective.

Effect on claddings, linings, windowsand openings and vertical servicesFor cladding supported off the timberframe (e.g. timber boarding) the differen-tial movement of the timber frame mustbe accommodated in the cladding systemby providing movement joints between thecladding panels at suitable locations.Asthe frame moves the supported elementsof cladding will move with it.Any abut-ment at the base of the cladding (e.g. atthe junction with a terrace, low-level roofor ground-supported cladding) will alsorequire sufficient movement capacity totake up this frame movement.

For cladding supported independentlyof the timber frame (e.g. masonry), thedifferential movement of the timber framemust be accommodated by providingmovement joints between the claddingand the timber frame at suitable locations,usually at window cills, at eaves level andat the bottom of any openings. Claddingwall ties must also be designed to accom-modate the timber frame movement.

As a result, any material or componentwhich is attached to the timber-framestructure and overhangs or projectsthrough the masonry cladding must havea gap beneath it to allow differential move-ment to take place without damage to thestructure or the cladding.

Trada Timber Frame Construction Ltd.9

recommends that gaps of at least 3mm,11mm and 19mm are allowed for at thebottom of openings in the ground, first andsecond storeys respectively of a threestorey building, and 21mm at the eaveslevel.These are typical allowances. Eachframe should be assessed individually bythe summation of the actual quantities ofcross-grained timber occurring in thestructure for shrinkage and allowancesmade for elastic shortening, creep and‘bedding-in’ of structural elements.

Continuous linings in stairwells andshafts are also prone to buckling as thetimber frame shrinks. Linings shouldtherefore allow for horizontal joints(backed by solid members) at floor levels toaccommodate the frame movement.

Furthermore, site supervision must beadequate to guarantee that proper move-ment joints, as specified by the designteam are incorporated.

Vertical services such as SVPs andRWPs, which are connected to the timberframe, should also be able to accommodatemovement of the structural frame, whichmay be in the region of 8mm per storey.Engineers responsible for the specificationof these items need to be aware whenspecifying pipe collars and continuouselements in the building of this tendencyto ‘crush’ any vertical services.

Fire designThe achievement of fire resistance Correct plasterboard fixing and correctlocation and installation of cavity barriers

and fire-stops is essential to achieve thedesign fire performance. If the plaster-board fixings are inadequate, the boardswill fail early during fire. Plasterboardmay also be required for the rackingresistance, in which case it is essentialthat the Fixing Schedule provided by thetimber-frame engineer is complied with.Close site supervision is essential toensure that those fixing plasterboard areaware of the increased fixing require-ments.

Acoustic designTesting has shown that the standard ofworkmanship is crucial in ensuring thatthe expected acoustic performance is met.For acoustics, maintaining structuralseparation and correct installation of insu-lation is essential.Air sealing is also essen-tial. If air can pass through gaps in theconstruction then so can noise.

ConstructionProtecting the works from rainTimber frame is typically installed atmoisture contents of about 18 % althoughsome elements may be at 20% during thewinter months. Soleplates can becomesaturated if standing water is allowed andtimber should be protected from gettingwet as much as is reasonably practical byremoving standing water.

The use of taped joints on chipboard orOSB sub-decking will not guarantee thatwater will not seep into joints but maysignificantly reduce the chances.All open-ings and gaps are sources of water ingressand the use of membranes such asVisqueenTM should be considered toprotect openings before windows areinstalled.

Storage of timber-frame panels, trussrafters, loose timber and engineeredtimber should ensure that members arenot in contact with the ground and arecovered against driving rain using venti-lated covers.

Dry lining and closing-in of timberframe should only be carried out when themoisture content of the timber is below16%. Closing-in of damp timber may leadto mould growth and in some instancesdecay. Saturated timber deck boards, Ijoists, glued-laminated timber and engi-neered timber materials should all bereplaced. Forced drying of these productsis likely to create problems with perform-ance and may cause warping or excessiveshrinkage.

Tolerances and erectionTimber frame is an accurate buildingmethod. However, accurate foundationsare essential to a successful timber frame.Any faults at this stage only become exag-gerated as each storey is erected.Foundations and slabs must be checkedfor level and square, and if foundations arenot within recommended tolerances, theymust be rectified before panel erectionstarts.

The correct levelling of timber sole-plates is essential to ensure that subse-

6 September 2005 – The Structural Engineer|31


quent wall panels are erected plumb andto the correct levels. Soleplates must becontinuously packed along their entirelength. If shims are used to achieve thecorrect level, these should be located underall load bearing studs and the soleplateshould still be continuously grouted.

Recommendations for constructiontolerances are given by Trada TechnologyLtd9 and UKTFA11.

‘Loading-out’ and ‘first-fix services’Plasterboard pallets are usually ‘loaded-out’ on the floors as the structureprogresses.This has the advantage ofplacing materials at the correct level in thebuilding for future fixing, but also helps to‘bed-in’ the frame as it is erected.

However, lightweight joisted floors areparticularly susceptible to irreversibledamage from large stacked loads duringconstruction. Pallets of plasterboard caneasily exceed the design imposed loadingallowance of a domestic floor of1.5kN/sq.m (equivalent to just 160mm ofstacked plasterboard of density 950 kg/cum).Where large stacks of material arerequired, temporary props should beinstalled to support material packsthrough all floors to a suitable foundationand adequate temporary bracing providedto the supporting wall panels.

Timber-frame structures are also proneto damage from those installing ‘first-fix’services, where adequate provision forthese services has not been allowed for inthe design. I-joists can generally accommo-date holes of certain sizes through theirwebs, but their flanges should not be cut.Head binders and the top and bottom railsof wall panels are also important struc-tural members, but these are often cutthrough during services installation withlittle regard for their structural function.

Early coordination between the maincontractor and timber-frame engineer, andclear guidance within the timber framespecification and health and safety plan isessential to ensure that an otherwiseadequately designed structure is notrendered inadequate by the work of follow-on trades.

‘Add-on items’Staircases and liftsStair flights must be designed for the rele-vant structural loading category (1.5, 3.0or 4.0kN/sq.m in accordance with BS6399-1:1996). Often a domestic style stair-case is provided where a stair flightdesigned for an increased imposed loadingshould have been provided.The fire resist-ance requirements of timber stair flightsalso needs to be agreed with the FireOfficer at an early stage in the design toensure that timber flights can satisfacto-rily be used.

Timber-framed lift shafts should beadopted to avoid the problems associatedwith differential movement between thetimber-framed structure and rigidmasonry or concrete lift shafts.

Timber framed structures are notgenerally designed to support the vertical

loads imposed by lift equipment (e.g. liftcars, guides and hydraulic rams etc). Liftsto be installed in timber-framed buildingsshould therefore be ‘self-supporting’ forvertical loads.This will generally requirethat the lift car is provided with its ownbraced steel supporting structure whichtakes all vertical loads to suitable founda-tions.

The detailing of this supporting struc-ture and its connection to the timber-frame structure must take account of thepotential for differential movement duringconstruction and in the first 36 months ofoccupation, due to drying-out, creep andbedding-in of the superstructure.

BalconiesBalconies are usually either ‘no access’Juliet balconies fitted back to the masonryor timber frame or self-supporting ‘walk-on’ balconies with structural posts used tosupport the balcony vertically, independ-ently of the timber frame.

Cantilevered balconies are not recom-mended as ‘back span’ beams and struc-tural timber posts will be required to bebuilt into the timber frame to support thecantilevered elements.This generallybreaks up the floors, prohibiting the use ofregular floor cassettes and generallyrequires engineered timber posts in theexternal walls to resist the large elasticreactions, which may lead to differentialshrinkage problems relative to the soft-wood framing. Uplift in the back-spansmust also be carefully checked, as the deadweight of a timber floor may be inade-quate to resist such uplift forces.

‘Walk-on’ balcony frames may be tiedback to the structural timber frame ormasonry cladding for horizontal restraintonly but if connection is made directly tothe structural frame, the connectionsshould be detailed for differential verticalmovement. If the balcony fixes only to themasonry cladding, a check of the brick-work ties should be carried out and onlyType 1 ties in accordance with BritishStandard DD140-212 should be used.

The balcony design can be steel frameor a combination of timber and steel. It isrecommend by the NHBC that the verticalelements are not timber due to durabilityconsiderations13. Joists can be preservativetreated or durable timber sections.

CladdingLightweight cladding should be adopted atall inset wall locations and external wallsvisible above lower roofs to avoid support-ing ‘heavyweight’ masonry off the timberstructure.The timber frame at these loca-tions would be detailed to support theloads from a ‘lightweight’ cladding.

Products exist on the market, whichappear similar to face brickwork, but arein fact a ‘lightweight’ system, which maybe supported by a timber frame wall.Aproprietary system of brick slips supportedin a horizontal metal ‘carrier tray’ fixed to,and supported by, the timber frame wasrecommended by CCB Evolution onFinborough House, where London Stock

brick slips were used to match the colourof neighbouring buildings.

For masonry cladding, BS DD140-212

requires that Type 6 timber frame wallties are only suitable for buildings up tothree storeys in height (not exceeding 15min height) where the anticipated differen-tial movement between frame andcladding does not exceed 18mm.Abovethis limit,Type 1 ties, which are able toaccommodate greater differential move-ment due to the fact that they are fixedinto continuous vertical channels, shouldbe specified.

Drained and ventilated cavities for thetimber frame are essential for good timberframe performance. In no circumstancesshould the timber frame be erected inhorizontal contact with masonry orconcrete, even with DPM membranes

ConclusionsTimber frame is a highly efficient methodof building for manufacturers, buildersand their customers. However, tomaximise its potential, the whole teammust understand the importance of keydesign issues and details.The ‘Rim BeamMethod’ is presented as a way of satisfyingBuilding Regulations Approved DocumentA6 for the notional removal of wall panelsfor Class 2B buildings and has beensuccessfully used in a range of timberstructures designed by CCB Evolution upto seven storeys high.

Problems with multi-storey timberframe structures have occurred as a resultof poor detailing in relation to accommo-dating differential movement.The benefitsof timber frame can therefore be negatedby a lack of attention to detail duringdesign, lack of adequate specification andcontrol of erection and a lack of under-standing of key issues on site.

The work of ‘follow-on trades’ can alsohave a significant impact on the structuralintegrity of the completed timber framestructure and these trades need carefulsite supervision. se

1. ‘Fit for the future’ – The Structural Engineer,82/17, 7 September 20042. ‘Verification of the robustness of a six-storey timber-frame building’

– Milner et al, The Structural Engineer, 76/16, 18 August 19983. Approved Document A,B,C,E & L, Building Regulations 1991

HMSO, London 1992, England & Wales 4. BS 5268 Part 6.1:1996 Structural use of timber – Code of practice for

timber frame walls – Part 6.1 Dwellings not exceeding four storeys5. Multi-storey timber frame buildings – a design guide, Trada/BRE, 20036. Approved Document A, Building Regulations 2004, HMSO. London

2005 (England & Wales)7. Design guidance for disproportionate collapse, UKTFA Technical

Bulletin No.3 March 2005 8. ‘New approach to disproportionate collapse’, The Structural Engineer,

82/23-24, 7 December 20049. Timber frame construction, Trada Technology Ltd. 3rd Ed., 200110. ‘21st century timber engineering – the age of enlightenment for

timber design, Part 1: An introduction to timber’ The StructuralEngineer, 82/23-24, 7 December 2004

11. ‘A pocket guide to timber frame construction’, UKTFA,1st ed., 200412. BS DD140-2: 1987 Recommendations for design of wall ties13. Technical Newsletter, April 2004, Issue 29, NHBC

REFERENCES

structeng tech note timber frame.pdf

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