CA2176450A1

CA2176450A1 - Structural elements

Info

Publication number: CA2176450A1
Application number: CA 2176450
Authority: CA
Inventors: Brian Thomas Ray
Original assignee: Lancefield Pty Ltd
Current assignee: Lancefield Pty Ltd
Priority date: 1996-05-13
Filing date: 1996-05-13
Publication date: 1997-11-14

Abstract

A structural beam (10,20) comprising a plurality of transversely extending discrete timber pieces (12) arranged in alignment, each timber piece (12) having opposed parallel faces which abut with equivalent faces of adjacent pieces, aligned apertures are formed in the pieces and a longitudinally extending prestressing cable (14) passes through the aligned apertures under tension so as to press the aligned pieces together.

Description

21~S~

blr~u~:L ~J~tAL ELEMENTS
Ihe present invcntion relates to s~ructural elctnents.
In accordance wlth onc aspect of the present invention tbere is provldcd a struaural beatn comprisin~ a p~urality of trans~etscly extending discrete timber pieces arranged in alignmcnt, e~ch tinnber piece having opposed tr2nsYerse~y eYtending parallel faces which 5 abut with e~uiYalent faces of adjacent pieces, a re9p~ctiYc bearing plate at each cnd of the bearn, an aperture formed in each piece 6uch that the tespectiYe apettures in the beam are aligncd and a ~ ~e ' ''~ exteGding pre9tresslng cable passes throu~h the a~ignedapettu~e and is anchored on tite bearin~ plate5 under tension so as to p-ess the ali~ned pieces to~ethet with the transversely extending parallel f3ces in abuttin~ relatiorl with 10 equivalent faces o~ adjacent pieccs.
The present invention will rtow be described, by vlay of cxample, with reference to the ~; b drawings, in whicJt:-Figure 1 is ~t side eleYation of a structutal bearn in accordance with t'ne present invention;Figure 2 is a plnn ~iew of the beam of Figure 1;
15 Figure 3 i9 an end ~lew of the bea~n of Figute 1;
Figure 4 is a side clevation of a struaural beant ia accordance with a further ~ o-of the present invention;
Fi~ure S is a plan vie v of the beanl of Figure 4;
Figure 6 1s ~tn end view of a first ~ t~ of the ~ ù~ t; ~ of Fieures 4 and S;
20 Figure 7 is an end Yiew of a second ~ - ~ . ." of the ' " of Pigures 4 and 5;
Pignre 8 is an end view oi' a t~tird ~ of tite ~ u~ of Pigures 4 and 5;Figure 9 is a plan view of a sttuctural plate ~n accordance wit~t the present invention;
Flgure 10 is a side elevation of the plate of Figure g fron~ t~ first side;
Figure 11 ~s a side elevation of the platc of Figtlre 9 from a second side;
25 Pigure 12 is a sectional view along the line A A of Pigure 14 o~ a struaural bo~ beam in accordance with t~te present invent;on;
Figure 13 is a plan view of the beam of Pigure 12;

~ 217~S~

Figure 14 is an end elevation of tho bealn of Figure IZ;
Pigure ~5 is a plan vicw of a beam and slab construction in accotdance with the prcscnt invention;
Figure 16 is r~n cnd elevation of the ~ shown in figure 15;
S Figute 17 is a plan ~ iew of a T-~eam and slab construction in accoT~ianoe with the present i~venffon;
Flgure 18 is an eEId ele~-ati~n of the construction of Figu}e 17;
Pigurc lg is an undemeath plan Yiew of a structurai br~x girder in accotdance with the present invention;
10 Figute 20 is an ond clevation of thc bo~r girder of Figur~ 19;
Fi~ure 21 is a section aioElg ti~e line B of Figure 23 of a poie in accordance with the present invention;
E;~ure 22 is a side devation of thc pole of Fi~urc 21 vieweû from A of Figure 23; and Figure Z3 is a plan vicw of the pol~ of Figure 21 in ~ccoEdance with the present15 invention.
In Fi~ures 1 to 3 of the r J;~ dtlwings there is shown a prestEessed timber segment struclural beam IQ. ThG beam 10 is furrned ~rom a plurAliq of tEansversely e~tending relati~ ely short pieces 12 of t~rnber such as waste off-cuts of low vaiue having a length D (see Figure 3). The ti~nbe~ pieces 12 are assembled facc to face with their Z~ resp~ctive relatively lon~ faces contiguous to and abuttin~ similar faces of ~djacent piec~s 12.
Thc pieces }2 are aIi of Simiiar length D and of sirnil ar width B as can be scerl in Figure 2. Further each piece 12 is of generaliy simi~ar thickncss T. The pieoes 12 do rlot have to be of the same thickrlcss but for ~be beam 10 to be linear, the abutting faces of each 25 piece 1~ do need to be l~ ~ parallel.
E!ach timber piece IZ is formed with 8 Elole 13 such as by driliin~ and the holes ~3 of tbe pieces 12 aEe aligneù in the beam 10 when the pieces 1~ gre placed in abutting relation.
A prestressing cshle 14 which may be made of steel is passed through the aiiE~n~d holes ~ 2176~0 from end to end of the beam 10.
The ~ cable 14 can be of any convenienl forrn such as a rod, wire, strand or c~tble. The cable 14 is anchored against the ends of the bcarn 10 by moans of anchols 16 which press a~ainst bearing plat~s 18. The bearln~ plates 18 can t2~e the forn~ of 5 U-shaped plates as shown in ~igures I to 3, or flat plates or rolled ho~o~ section bearin&
plates provided with access aperturos for the anchors 16, or any other convenient form.
In use, the pi~ces 12 arc placed betweer~ the bearing plates 18 and th~ cablc 14 is passed through the aiigned hol~s. The ar,chors 16 are then mounted to thG cable 14 m abuttirlg relation with the b~arin~ plates 18 so as to prGSS against the bearini~ plates 18. Thc cable 10 14 is then tensioned such as by rneans of an hydraulic jacl~ and attached to the anchors 16, iA ~nown manner. The timber rnay be subject ro Loss of dimer~s~on bccause ofshrinkage due to loss of r~oisture below f~bre saturatiOrl point and creep. Thelefore, it is prefcrred to use seasoned or dry tinnbor havine a moisture content of less than 15% by weight because this is less than fibre saturation point which is about 30~o by weight but lS above ~quilibriurn moistule content which is about 129to by weight in many clirnatcs.
The st~el used in the cablo 14 is generaUy of a high stTength and l~w relax2tjon matetial.
The bearing plates 18 gen~raily are SUCil JS iO be able to rransfer fotce io the structure at acoeptable pressure. Ihe anchols 1~ are generally such as to i~e ablc to hold the cable }4 after stT-cssing.
20 Thc distance from the top of tbe bcarn 10 to the centre of the cable 14 is 13 as shown in Figure 1. ~or the b~am 10, the distance E is prefelably greater thal~ Dl2.
With the beam shown in ~Igures I TO 3, bendiug cnm~rP~in7~ about the major axis in ~he uppel areas of the beam i6 ' ' b~ cross ;raln compression stlan~th within the tir~r and at the inter faces between the pie~es 12.
2~ Fwthf,r, bending tension about tie major a~cis in thc lower areas is a ' ~ by the comoosif.e action of the cable at~d the timber This ~ in bend~ng tension i6 a prirnary iimcTtiot~ ~f the cablc 14.
Another funcrion of thé p~stressing ~ble 14 is to sustain force across the face6 of the 21~6~5~
s timber plece6 12 such that intcrfncial friction betweetl the pieces 12 is able to transmit ~erdcal shear ~n the bear~s in the longitudihlal direction.
In most ~ orXing situaffons adhesivc does not rlecd to be ~pplied to the interfaoes of the timber picces 12 to assist ~ of Yertical 6hcar.
5 The relatiYely low moisture cantGnt rcduces the creep ~n the timber pieccs 12 urlder ~r~C~iAn and long tcrnn load does not lead to signifiGant loss of terlsion irl thc cablc which would impair the bearn . h_.~, ~,, j,l".~
In connection vith the beam of Figwes I ho 3, it has been found that timber across the glain, cspecially softwood, is very n, ~3;~1~ comparcd to tir~ber parallel to the grain.
10 This ~ . h ~i~.l;. of ~ y is mea6uled a6 the ratio of stress to strain arld ls CQtlCd Moduhls of Etasticlty ~E). E~ for dly drc~sed pirle parallel ho the ~raln may be G~00 but across the grain rnay be only 150-200, As sho yn in Figures 1 to 3, it is possible to affix rnetal strapping 19 such as hoop iror strapping, to the ends of the beam IO by bcnding o~er thè ends of the strapping 19 ar d 15 nailin~ thc bcnt over ends to the beam 10 by means of nails l9a. Also, the strapping lg can be n~iled at interYals along its length t~ the top of the bcam 10 by means of nails l~a.
The provision of the strappin~ 19 reduoes up~ard pre-carPber duriPg stsessmg of the sing~e cable 14 ~ocated towards the lo~ er end of the beam.
In Figures 4 and ~ of the - nl l~ Ing dlawings, there is shown a stsucturai beam ~0 ZO which is sinniiar to that shown in ~igums I to 3 and like reference numerals denote like parts.
Howe~er, in additio n to the lower cable 14 these is also provided an upi~ercabl~ 21 which mirror irnages the lower cable 14. The addition of the upper cablc 21 controls pre-calnbe~ and increases the stiffness of the bearn 20.
25 'rhus, a lensioncd cable 21 is being intro~iuced into the . , zonc of a sirnply supported beam irl bending. The L.. t~l~ .. ~L.~ in stiffness is sttributable to high f ~ A~t~tinn of the r~unber caused by rhe pre-strcssing. In effec~ sorne of the f~e~ibility of the timber is bein~ removed in the ~ v staee and before 1-- 2~7~5~

working lcads are applied. The incre~sed str~fenin~ is ackicved ~y locatin~ at le~t tWQ
str~ssing cables each of ~set frorn the cenrreline of the bea~n 20. The cable 14 is locatcd bclow the ccntrelinc by a partioular distance and the cable ~1 is loca~ed above the cerltrellnc by a similar d;srance. The relative distaDces from the c~ntreline may vary depe~ding orl the prestress forces used and the beam desigr~ in ~neral.
Further, as can be seen in Figures 4 and 5, the beam 20 is provid~d with inlets ~2 for grout and outlets 23 for grout Th~ inlets 22 and the outlets 23 extend from the periphGry to the bearn ZO to the cables 14 and 21 arld enaole grout~ such as epoYy resin cr cement to be injected into the holes contair~ing the cables 14 and 21 so that the cables 14 and 21 10 arG hcld jD place in Ihe holes by the grout. 11lUS, for e2~a~ple, ~derload when t~.e top cable 21 becomes de-stressed, it can, when grouted to the timber, begin to Jesist bendlng ~ V. ~ .;VII or reinforce the beam. Thus, under load grouting erlhan~cs the cap~city and stiffness of the bcann 20 As seeD in Fi~ures 4 and S~ ~he bearn 2D is pro~ided ~ith ~ steel chanael anchorage ~4 lS at ~ach end. Aft~r ~routing the anchora~es 24 can be removed for rcusc which lowers beam cost without alterin~ the prestress s-~.~Li~ll~y.
Grouting also has the advantage over an ungrout~d beam in that if the ungrouted bearn is '~ ly cut~ bear~ failure might resu~t. Also, the presenoe of the grout reduces corrosion of the cables.
20 The bcain 20 of Figures 4 and 5 d- - ' an increase in stiffness relative to the bearrl of Figures 1 to 3, and a reduc~ion ia prc-cambe}. The bearn of Figuses 4 and 5 recovered tly to its origina~ nn after removal of load.
In the structural beam of the present int~ention, it is i~poltarlt that the ~ h cables are offset vertically cerltre~ine of the beam to achieve impro~ed stiff~ess i~ bending 25 situations. The sarne app~ies to slabs to be described hereinafteL
A centrally ~ocated cable has no influence on the stiffness cf the bea~n in bending. ~Ith a centrally locatec cab~e, the stiffne~s of the be~n is unaltered and remairls that of wcod.
In a modiflcation, ~he beam 20 of Figures 4 and 5 may be provided with cables ~4 and ~ 21~645~

21 with '~ , increased cr~ e~!lo~l~l rlrea which increases the stiffness of the bcann ovcr a wider range of loads.
In Figure 6, there is illustratcd a variatioD on the b~ams of Figures 4 and 5, in which tne is ~hown a bearn 25 having two c~ables 26 in tho top and two cables 26 in the bottom of S the ~earn ~nd equally offset from thc centreline of ~he b~2nl 25.
This ~ may assist in controlling lateral straightness dDring pIestr~sslng 2nd providcs an incrcase in iatcral strffness.
In Figure î, there is illustrated a bearn 27 which is the salne a~ ~be beam of Figurcs 4 and S e~cept for the t~ ~ v~ of unstressed steel bars 2~ which are grouted to the timber so as to reinforce the bear~ and resist any tcndency to creep. The unstressed steel bars 28 are installed before prestress~ng and are grouted after stressrng of the prestressing c~tbles.
In Figule ~, there is shown a furtner variation of the beam of Figures 4 2nd 5, rn which a beam 2g is clamped in a straight position after fabrication by fixing a continuous member 30 such as by the use of nails or screws, to thc top or bot[om thereof or one or both sides. The cvntinuous member 30 rnay be made of such material aS wood, ply~vood or metal. Thls conf~rs increased lateral ~' bendlng capacity and sti~fness on the ~earn 29.
Ln Flgures 9 to 11, theK i9 s~ovJn a stluctural plate 40. Tlte plate 40 compdses a plura~ity of structural beams 20 which as shown in Figurc g, extend I ~ along the plate 40 parallel to one another.
In additlon, the beams 20 are inter-connected at right angles by a ~eries of timber segments 42 formed o~ tin~her pieces 12 disposed between the adjacent pairs of beams 20 and being prestressed by cables 44 anchored aga~nst s~des of the outer bea~ns Z0 by mertns of bearin$ plates 46 and anchors 48. The cables 44 cxtend throi~gh align~d holes in the segrnents 42 and the bea ns 20. The bearns 20 have an eccentdcity of E~ as discussed abo~e whereas thc segm~nts 42 bav~ an cccentri~ity E which ig less than 13 by at least 1.5 hole diameters to avoid interfeIence of cables.

~ 21~6~5~
g Thus, there ts formed a cross grid of pre-stressed beams irl which the top surfaces of all beams ar~ in the same plarle.
To ens~re go~i square o} rectr!ngular sLtape, con~enrional beam cross-bracing 52 may be applied to the ur~der slde of t~le plate 40 extending diagonally from comer to corr~er as shown iQ Figure 9.
It is intended that thc plate 40 be lifted from or supported at the corrlers but because o~
the upper and lower cable _ - t, some lirQited fle~ibility iQ lihirlg or supporting poriitions exisrs.
The platc struchuro ~0 shovm irl Figures 9 to 11, has an initiai pre-carQber aQd is very stiff. Thus, it is suitable for supporting large items ser~sitive to deflections SUCQ as houses iQcludin~ .Dpo,l~bl~ houses duriQ~ tlarlsport.
In ~ igurcs 12 to 14, thore is shown a structuraL bo~c besm 60. Bo~ bcaQQs Car~ provide a relatively hlgh lozd carryiQg capacity Tho box bea~t 60 comprises upper and l~wer, s~aced flanges 62 arld 64 respectively.
Each flangc 62 and 64 is sirniiar to the beam 10 having a plurality of timber pieces 12 pulled together by a pre stressirlg cable 14 passing through aligned holes 13. However, the flanges 62 and é'4 are i ' by a plur~ity of cross diaphra~~rQs 66 intcrposet~
at intenals in the lor~ihldiQaL directiot~s between tQQber pleces 12 and at &e ends o~ tQC
bo~ bearr~ 60. Also, thc ~lange 64 has a space~ pair of cables 14 whilst the flangc 62 has 2~ a sulg~e cable 1~. It should be noted t~lat the nur~1ber of cables 14 in tne f~ane~s 62 and 64 can vary.
Furt}~er, web rrlembers 68 are }ocaled at each side of th~ bGarn ~0 and flxed to the flanges 6~ and 64 by any convenient mcans sut h as nails, screws or coach St rews.
IQ use, the flanges 62 and 64 and rhc dlaphragrns 66 may be assembled first and a ~artia~
stless a~p~ied to &e f~an~7es 62 and 64, bcfore the webs 68 ar~ artached. Aftcr the webs 68 are attached additi~n~l stress may be applicd to the top and bottom flanics 62 aad 64 to t~l~ r~quired le~el for use.
Bending tension and ~ are ' by the composlte act~on of the 2 i 7~
cablcs 11, the tirnbet ,oiec~s 12 and tbe diaphtagms o6 througb which the cables 14 pass, and the webs 68.
Vertical sh~ar is ac ' ' ~ by the wcbs 68 whilst hori~ontsl transe-er of vertical shear ftorn point loads on the f~anges 62 arld 64 can occur via Etictivn from intcrfacl~l S p~essure bet ween tlmber pieces 12 or fi~ings connectirlg webs 68 to ehe flanges 62 and 64.
The diaphra~ms 66 which 2re stressed ineo the flange systern provide a shbilising mechanism for thè bo~ beam ov. Cross bvlts (not showr.) cat~ be provided ~ocatedcentrally in dle webs 68 an~i close to the diaphragnns to providc arlti-buc~linR restraules 10 in the webs 68.
In Figures 15 and 16, there is shown a bcatn and s~ab corlstruction 8v. The beam and slab ~,u.. 'i u~ LIvll 80 uses a pluraiity of ' ~ ' ~ ''y extend~ng beams 20 spaced apart and disposed parailel to one another. Betwcen each pair of adjacenl beams 20 there is disposed a plurality of random len~ timber pieces 8~ forn~ing slab segtnents 84 which 15 e~tend transversely betw~en the beams. ~e top surfaces ûf ail beams 2~ and tlab seg!nents 84 are at a common level.
A plu}ality of spaced transverse upper and lower prestressing cables 86 are passcd through aligned holes drilled transversely through the ocams 20 and the timber pieccs g2.
The cables 86 ar~ arlchûred against sides of thc outer beams 20 as described I ' ' ' .. .
20 The transverse cables 86 are located in pairs one above and one below the centreline of t~te slab ~gtnents 84 as can be scen in Eli~ure 16 to ensure composite action of the slab Spanning bet vcerl beams. Aiternatively, cablcs can be located alternately above and below the slab centre~ine at suitable hor~zontal spacir~gs to tbat ensure adequate pr~stress is maintained.
25 ~c transvcrse cables 86 are prestressed and cause ~ . ' between tirnber pieoes 8~ and bet veen slabs 84 and beams 20. I~s enables the connection of the slabs 84 to the bearns 20 and the, ' ' of vertical shear atld thc transverse bendi~g in the slab.

~ 2176~5~
1~
The cablcs 86 are anchored on anchors bearing on flat beanng plates or an~les orcharmels 90 to the outer side of thc o~ter beams 10. I'he plateg gO and the like acting vith the outer beams dlstrlbtite anchorage forces more or less uniformly along thc longitu~oinal edges of the slabs.
5 It ~s ~referred that the timber pieces ~2 have a minimum length of 2.03 times th~ lateral spacing of the transverse cables 86. In the consttuction shown in Figures 15 & 16 no attempt has been made to disperse or tighdy close tho butt joints in the longitudinal pleces 82.
T ~ bending capacity of the System reflects the bcnding càpacity of the beams 10 acting on their own.
Iri the evcnt the lengths of the longitudinal pieces 82 ~9 mana~cd (as opposed to random selection of lerLgths) and end-30irits in lon~itudinal tirnberg are dispers~d arld close-butted the ~ , betwcen slabs 8$ and b~ar~s 20 and between pleces 82 enables the of axial or lon~itudinal , in the slabs 84 from the beams 20.
15 In thls cvGnt wlth the beam and slab construction of Fielires 15 to 16, thc longitudinal bending tnoment capacity of the system is enhanced by the axial ~ , capacity of th6 slabs (in additio~ to rhe cornpression zvnes of the beams 20). Tension capacity of the longitudinal stressing cables would be increased to raatch ~"r,l" ~-~ .., capacity of the system. I l ~ " ' bending capacity of the system would reflect the 20 ~apacity of beams and slabs cvrnbined and the vertical scparatlon of the ~
one and the l l- " ~ stressine cables of the boams 10 ~providing systern tensioncapacity).
Prestressin~ of bearns 20 may be carried out in stagos wlth the final stress applled after the stressine of the transve~se cables In thc sl~bs.
2~ l'he bottorns of the beams may be braced, if desired, such as by metal flats, an~les, plpes, or plywcvd shccts nailed or screwev. tv the bottom of the beams.
The bcam and slab, ~ iVII of Figurcs 15 and 16 can bc used for bridge decks arldbuildln~ slabs.

~ 2:~6~
In Fi~ures 17 and 18, thcre is sbowrl a T-beam aud slab construction 100 comprising a pl~rality o~ l~\ng~n~ n~ y e~tendirl~ parallel beams 20.
D~spose~t between the beams 20 arld orl cach outer side are slab segments lO~ formed of a plurality of tirnber picces 104. rbe drnber pieres 104 are of varying lenglhs. The slab se~ments 102 are held together and to the b~ns 20 by means of pairs of transYers~ly extendiug upper and lower prestressin~ cables 106 pass~ng thlou2~h ali~ued holes in the slab se~ments 102 and th~ beams 20.
The outer slab scgments 102 are located externally of the outer beanns 20 and are thus cantilevered As with the ooustruction in ~igures lS and 16, the transverscly ext~nding 10 stressin2~ ca~lcs 106 aEe ~nchored such as on corltinuous charh~l shaped bearin~ plates 110, The plates 110 may be provided w~th ~ertical vveb stiffener adjacent tbe stressirlg cable anchorages.
Th~ trarlSvorse caboles 106 cause ~UII~yl~2~21iUII between timber piece~ 104 and bet~een 15 slabs 102 and beams 20. This enables the slabs to act 'y in ~he transverse di~ction In a limited vay irlcludiag ~ of the outer sections and the transverse I~, ..' 2- ~.1 of slab shear. 1~ a~so erlableg the connect~on of the slabs 102 to the bea~s 20.
Preferably, erldjounts in longitudinal timber pieces lO4 of the slab are close butted and 20 spaced at least 1.~ motres or thereabouts irl any direction 80 a~2 to ~chieve wide dispersal of end-jo~nts. Th~s enables transrnission of a~tial or longitudinal ~ , ' in the 2-la~s 102.
With th~ T-13cam ar~d slab oonstruction of Fig~res l~ arld le'', the ~ong~tudinal bendin~
moment capacity- of the system is enhanoed by the ax~al: , capacjty of the 2~ slabs (in additien to the ~ , ' zone~2 o~ the beams 20) coupled with ~he vertlcal boparat~on of the ~ 20nes an~ the longitudina~ stressing cab~es of the bea~us 20 (jprol~iding system tension capacity?.
Pres~r~ssin~ of i~ams 20 n3ay be carried out in stages with the final stresg appl~ed after ~ ~17G~
the stress~g of the transverse cablcs in thc sla~s.
It is also envisaged that a T-beam ssructu-e could h~v~ a I ~gi~ , e~tcndin~
structural beam 20 with slab scgments formed of timber piecs similar to the slabsegments 102, disposed on each side. Thc siab segrnerlts ar~ held togethcr an~ to the 5 beams 20 by rneaDs of at least one transversely eYtending preslressing cabl~ passing through alig~cd holcs in the slab segments and the beam, and the slab segments beins cantilevered.
In Figures 1~ and 20, therc is shown a structural box girder 120 comprising a plurality of spaced, parallel, ~ e~tendins beams 20. Th~ box ~irder 120 a~so compriscsuppe.slaos~gmeAts1'2whicharesirnilartotheslabsc~rnentS102o~Figwes 17 and l~. How~vcr, lhe eocentricity of the cables 14 in th~ bcams 20 is reduced to tlansYerse cables (see Figure 20) as will be described. Ihcre are also lowcr slab segrnents 124 which ~ - transversely lower ends of the beams 20.
The upper slab segments 122 are primarily in ~ ~ ~r whi~st the lower slab segments 15 124 are in tension.
Thc lowcr sla~ segments 124 haYc a plurality of pairs of transversely extendlng strsssirl~g cables 126 mounted against b~aring plate charmels 128 and which c~ctend below the cables 14 as shown In Flgure 20. The lower flange segments 1~4 are similar to the upper flan~e segtnents 122 e~cept that there are no Gantilever portions. Joints in the lower 20 ~ange segments 124 are preferably suitably spaced to - '; a reasonable amount of tension.
In this constructiorl, principal bending tensiorl is ~ the ~
direc~ion by stressirlg cables In direct tension and the lowor slab segmcnts 124. Princi~al bending ~V~ in the longitudinai di~ction is ~ ~ ' by the upper slab 25 scgments lZ2 and the beams 20.
The longitud~nai bending morrlent capacity of the ~ystem is enhanced by verticalseparation of the uppsr slab 124 se~ments from ths cables 14 and the lower slab segments 1~4.

217~A~

The lovfer slab strGssed cublcs 126 result ir~ a shea~ cannection of the lower slab segments 124 to the beams 20, The timber pieces in the slab segments 124 trarlsmit axi2i force frorn the beams ;20 and from one another and we~l separated butt joints in the timber pieces ~re acceptable because stress 't ~:L is achic~able because of interface 5 pressure.
In F}gures 21 to 2?7, there is shov~n a pole 140 such as a li~t pole, pa~er pole or flag pole. The pole 1413 comprises a metal base plate 142 secured to a concrete File 144 by means of anrhor bolls 14~ which indade a levelling lochmt ~4$.
The pole 140 compriscs a pair of sloping faces IS0 which may be formed of c~addin~
10 material such as plywood. ~hus, the pole 140 has a trapewidal apilearance from the side 2nd a rectangular appearance from the front. Furth~r, bet~ een the faces lS0 are disposed a pair of beams 20. ~hc beams 20 comprise a plurality of tirrlber pieces 12 located bot~veen the base plate 142 and a mdal top plate 152 arld strcsscd by t~vo pairs of cables 14 arld 21 anchored on the plates 1~2 and 152 ~lnd e~tending ~hrou ~h aligned hol7~s in the 15 timber pieces 12. The tirrlbcr pieces have ~nds whlch are tapered 9j~1 -r ' ~ to fit within the slopping faccs ~50.
The pole 140 may cuntain one or mole diaphragms 1~4 formed of, for exatnple, plywood is~scr~ed between adjacont timbcr piec~s at intervals.
In the directiorls of the timber pieces, bending and axial tension and ~ , are 20 : ' ' by tite composite action of wood, plyv~-ood and preSt~essed ste~l cables.
Horizontal shear is transmittcd ~y friction induoed by the prestress in the pol~.
Hor~zontal torsion is d_ ~ ' ' by the box structurc of the faces ~S0 conriected to the timber pieces 12 transmitted vertically by frictlon between timbcr pieces i2 by prcstress and the shear strength of plywood, 25 The timber pieces ured in the plesent imrelltlon may be offcuts flom sawmi~ling or plywood manufacture or particle board ' ., or derivatives of other wood products.
M~ and Yariations srlch as would bc apparent ~o a skilled addresser are dcem~d 217~Q

within the scope of the present invcntion. For e~arnple, th~ tlrnber pieces could be r~plac~d by d~ o~ shape :I plastlcs material picces.

Claims

1. A structural beam comprising a plurality of transversely extending discrete timber pieces arranged in alignment, each timber piece having opposed transversely extending parallel faces which abut with equivalent faces of adjacent pieces, a respective bearing plate at each end of the beam, an aperture formed in each piece such that the respective apertures in the beam are aligned and a longitudinally extending prestressing cable passes through the aligned aperture and is anchored on the bearing plates under tension so as to press the aligned pieces together with the transversely extending parallel faces in abutting relation with equivalent faces of adjacent pieces.

2. A structural beam according to claim 1, in which the prestressing cable is offset from a centreline of the beam.

3. A structural beam according to claim 2, in which there is provided at least two, spaced apart longitudinal extending prestressing cables passing through aligned apertures and anchored on bearing plates under tension, the prestressing cables being located on opposite sides of the centreline and offset therefrom.

4. A structural beam according to claims 1, in which grout composition is introduced into the aligned holes after the or each cable has been tensioned so that the cable is essentially retained in place by the grout composition after it has cured.

5. A structural beam according to claim 4, in which the bearing plates have beenremoved after the grout composition has cured.

6. A structural plate including at least two structural beams according to claim 1, in which the beams are disposed parallel to one another and are interconnected by transversely extending timber segments formed of timber pieces, the timber segments also having one or more tensioned cables extending therethrough.

7. A structural box including at least two beams according to claim 1, in which the beams are disposed parallel to one another and are interconnected by transversely extending cross diaphragms.

8. A beam and slab construction including at least two structural beams according to claim 1, in which the beams are disposed parallel and are interconnected by slabsegments formed of timber pieces, the slab elements also having one or more tensioned cables extending therethrough, the beams and slab segments forming a flat surface.

9. A structural box girder including at least two structural beams according to claim 1, in which the beams are disposed parallel and are interconnected at one end of the slab segments formed of timer pieces, the slab segments also having one or more tensioned cables extending therethrough, the beams and slab segments forming a flat surface, and also having further slab segments formed of timber pieces at another end, the further slab segments also having one or more tensioned cables extending therethrough.

10. A pole including a structural beam according to claim 1, in which at least one beam is mounted uprightly on a base.

11. A T-beam and slab construction comprising a plurality of longitudinally extending structural beams according to claims 1, with slab segments formed of timber pieces disposed between the beams and on each outer side, the slab segment being held together and to the beams by means of at least one transversely extending prestressing cable passing through aligned holes in the slab segments and the beams, and the outer slab segments being cantilevered.

12. A T-beam comprising a longitudinally extending structural beam according to claim 1, with slab segments formed of timber pieces disposed on each side, the slab segments being held together and to the beam by means of at least one transversely extending prestressing cable passing through aligned holes in the slab segments and the beams, and the slab segments being cantilevered.