San Antionio Oral.doc

8/13/2019 San Antionio Oral.doc

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Revised manuscript for World Steel Bridge Symposium Nov. 2009 in San AntonioReferences and small corrections have been added. Some of the references will be in place later.The full text of the references can be found in the list of literature in The Network Arch on myhome pae! http!""pchome.rm.hia.no"#ptveit" $nclined characters are used when $ was supposed topoint at a slide.

1 Networ arc!" #angama!u. %y name is &er Tveit. $ come

from Norway. The title of this lecture is! 'enesis and developmentof the network arch.

The network arch is an extremely liht arch bride that $ came tothink of when $ was workin on my master(s thesis )* years ao. $am very thankful for this opportunity to tell you about it. $ have a lotto tell+ still there will be room for your ,uestions at the end of the

lecture.Network arches are arch brides where some haners cross otherhaners at least twice. Touchin on the development of networkarches+ $ want to tell you how network arches function+ about theiroptimal form and how they can be erected. Some examples of builtnetwork arches will be presented. %etric units will be used. $ amsorry if that is a problem.

2Nielsen Bridge.The forerunner of the networkarch was the Nielsen brides in Sweden. -Nielsen/0/1 Their chords were so stocky+ and the ratio oflive load to dead load was so small+ that they did not

need crossin haners to achieve small bendinmoments in the chords. $n a patent application from/02 Nielsen showed haners that crossed other hanersonce. $f built these brides would not have been networkarches. $n the brides that Nielsen built+ haners did relax

due to loads on part of the span. Please note that thereare no railings between the hangers and the traffic.
http://pchome.grm.hia.no/~ptveit/http://pchome.grm.hia.no/~ptveit/


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$ Beam Analogy. $ncreased loads andstroner materials make it advantaeous to usehaners that cross each other at least twice. Thenetwork arch can be seen as a simply supported

beam. The chords are the compressive and thetensile 3ones. The haners are a very liht web.

% &artial continuation 1. %ost of the shearforce is taken by the vertical component of theaxial force in the arch. The haners take some ofthe variation in the shear force. $ncreased rise of thearch could ive much smaller axial forces in thechords+ but it would detract from the ood looks ofthe bride.

' &artial continuation2.The arch shouldnormally be part of a circle. The nodes can beplaced e,uidistantly alon the arch or alonthe tie. The arch is well supported and can

efficiently utili3e hih strenth steel. That alsoapplies to all the other steel members. As lonas the haners remain in tension+ the bendinmoments in the chords are very small. 4hen

haners relax+ the bendin moments in thechords increase.


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( )oncrete ties wit! diagram of t!icnesses. $f thearches are less than ) m apart+ it is best that the tie is aconcrete slab with small ede beams.Normally the biggestbending moment in the tie is in the middle of the slab. The

lonitudinal bendin moment in the tie is normally smaller.The axial force in the tie is best taken by prestressingcables in the edge beams. 5or everyday loads theprestressin cables ive a lonitudinal prestress in theconcrete tie. This makes the tie more durable.

$t miht be a bit like swearin in church to defend the

concrete tie of network arches in this conference on steelbrides. $ humbly apoloi3e+ but the concrete tie is a cost6efficient solution when the arches are less than 7 to ) mapart. Then we do not need steel beams in the tie.

8onitudinal steel beams in the tie would lead to extrareinforcement to reduce the crack width in concrete thatrests on the elonatin lonitudinal steel beams.&restressin cables combined with lonitudinal steelbeams in the tie would introduce unfavourablecompressive stresses in the steel beams due to creep andshrinkae in the concrete slab.

8onitudinal steel beams in the tie reduce the bendinmoments in the arches and increase their bucklinstrenth+ but these effects would be sliht andunimportant.

8onitudinal steel beams in the tie can be veryuseful in many methods of erection.


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* +ransverse tension mem,ers. 4hen hih strenthconcrete is used to obtain better concrete durability+ we canmake the concrete slabs thinner and hope that the deflectionswill turn out as computed. $f we et too bi deflections+ they

can be counteracted by transverse tension members underthe tie.

- Wedges under tension mem,ers.Over the years, thesize of the deflections can be controlled by putting wedges

between the slab and transverse tension members. $f thebendin moments can be taken by the reinforcement in the

slab+ rupture of transverse tensile members will notendaner the bride.

$f the transverse prestressin consists of replaceable steel rodslike in 'ermany+ their stress can be altered to ad9ust the deflectionof the slab. Then the span of the slab can be over ) m.

9 Sc!ulen,urg Bridge" cross section. :ven ifwe have transversal steel beams in the tie+ thelonitudinal steel beams in the tie can be avoided.;ow this can be done is shown in this slide of theSchulenbur r. 5rank Schanack+ who is present atthis conference. ;e is probably the man in the worldwho knows most about network arches. The

transversal steel beams in the tie are deeper than thelonitudinal concrete beams.


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10 Sc!ulen,urg Bridge" picture. This is apicture of the Schulenbur


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1$ Stein/er influence lines. This slide showsthe influence lines of the Steink9er network arch. -Tveit/221ou can see that the axial forces in the chords musthave small variations. The same applies to the

maximum bending moments in the tie. The slope of thehaners is stated in the top riht corner of the slide.

1% 200 m !anger arrangement. The hanerarranements in this slide were found by trial anderror in /?/. $f there are no transverse beams in

the tie+ then it is usually best to use a constantdistance between the nodal points in the arch. Thehaner arranement to the riht was used in Teichand 4endelin(s master(s thesis in 077.

1' vi Sound 2001. This slide shows the networkarch in Teich and 4endelin(s master(s thesis. -Teich and 4endelin0771 :B loads and codes were used. The tie is a concreteslab. The arches are made of universal columns with ayield strenth of */7 N"mm0. The universal columns aresupposed to come pre6bent from the steel works. Twoways of fastenin the windbracin are shown.

1( Steel weig!ts in various arc! ,ridges. $n this slidethe steel weiht in Teich and 4endelin(s network arch iscompared to steel weihts in some 'erman tied arch brideswith vertical haners.N meansthat there is no windbracin.

! meansthat the arches slope towards each other. The yearswhen the brides were built are indicated.


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The network arch has no steel beams in thetieC still it needs about the same amount ofreinforcement as the other brides. The slide

supports my claim that network arch bridescan save over D of the structural steel neededin other steel brides.

1* erog. This slide compares networkarches to other steel brides that have been built.

The other brides are from before /?E. -;er3o /?)1$ believe that today they would have used moresteel. Fptimal two6lane network arches would useslihtly less steel than indicated by this diaram.

1- &oints of importance. Gou all know that steelweiht is not the only tin that matters. This slideindicates other thins of importance. Network arches areslimmer. That looks ood. They have a thinner lowerchord. That makes ramps shorter. $t also makes it easier to

desin the roads leadin up to the bride.Fptimal network arches have shorter welds andsimpler details. Fther tied arch brides have much moresurface that needs corrosion protection. Fther concreteparts need much more maintenance than concrete slabswith a sliht prestress. :rection is often more expensive

with two to four times more steel to erect.


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%aybe the structural steel for Teich und4endelin(s network arch would not cost more pertonne than the steel in the 'erman tied archbrides. The steel for the arch is #H7 the weiht.

19. Steel (0 to 200 m. This slide ives a rouhestimate of steel weihts in two6lane network arches.The spans are between 27 and 077 m. 8oner spansmiht not be ,uite so competitive+ because the crossinhaners reduce the stresses due to live loads+ but not the

stresses due to permanent loads.The dots indicate the steel weihts from Teich and

4endelin(s Ikvik Sound


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The diaram can be used for comparin the steelweihts of two track railway bride to otheralternatives. The 'erman railway(s advisory boardfor bride desin favours network arches over arch

brides with vertical haners.-Schlaich et al+ 077H1. 5our networkarch railway brides have been built in 'ermany.

2% Sc!anac 1. 5rank Schanack has made two veryinterestin diarams that show the advantaes ofnetwork arches. -


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2' Sc!anac 2. $n this diaram Schanackstarts at the top left with a tied arch with verticalhaners and the maximum live load on half thebride as shown in the previous diaram. Thenthe anle between the arch and the haners isradually reduced. At the bottom left of the slidethe maximum bendin moment is only 2 of thestartin value.

2( ran4s !anger arrangement.$n 077E 5rankSchanack suested a constant anle between thearch and the haners+ except near the ends of thearches. This haner arranement has manyadvantaes.

2* Scaffolding for Bolstadstraumen.Nowto the erectionof network arches. Network arches have been built or are beinbuilt in 'ermany-'raKe 077?1+ &oland-Loltowski 077)1+ M3ech Republic+ -Sasek077)+ 07721+ Slovakia+ Norway+ Spain+ The Bnited States+ -Steere 077H1+-4ollmann 077H1+ &eru+ Arentina+ New Lealand -Mhan 077H1+ @apan+ -Nakai //)1Taiwan and maybe more places.

:rection is done in many ways. The two Norweiannetwork arches were erected on a timber structure restin onpiles in the river bed. The slide shows the scaffoldin for the


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2- +emporary lower c!ord. This temporary tie canbe used for the erection of network arches. Mombinedwith arches and haners it makes a stiff steel skeleton thatcan be moved. $t can carry the castin of the tie. 5irst the

concrete at the ends of the span must be cast. Then theede beams. They miht have to be cast from both endsto avoid excessive relaxation of haners. 4hen theede beams have been cast+ they take the bendin in thetie while the concrete lane is cast. $ could have said that this is the steel tie of anetwork arch. $t can be removed when the concrete can stand by itself. Then corrosion protection for the steel tie can beomitted.O

29 5ifting t!e steel seleton. The steelskeleton of the network arch from Teich and4endelin(s master(s thesis can be lifted by Norway(sbiest floatin crane. The span was E) m.

The liftin capacity of the crane is 277 t. The steelskeleton weihs around 0E7 t+ but it is practical to put insome of the wooden form and the reinforcement beforethe skeleton is lifted to the pillars.

$0 Removing t!e temporary c!ord. This waon

can be used for removin the temporary tie and theformwork. Part of the deck in the wagon has been ahorizontal part of the form for casting the concrete tie.The waon rolls alon the ede of the finished tie. $f thereis transverse prestressin of the tie+ the stressin can bedone from the waon. The same temporary lower chordcan be used for brides with various spans and widths.


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$1 Sew Bridge. This slide shows the first stae inthe erection of a network arch across a canal. The spanis a hundred metres. $n order to reduce the necessarythickness of the concrete tie+ the bride has three

arches.The structural steel supplemented by a temporary

lower chord is erected at the side6spans on one side of thecanal. $f the shape of the steel skeleton is riht+ then noad9ustment of the haners will be necessary later.

4hile the steel beams on top of the pontoon are tied tothe abutment+ the steel skeleton is rolled to the middle ofthe pontoon. Then the pontoon is pulled across the canal.5inally the steel skeleton is rolled onto the abutment onboth sides of the canal. Then the tie is cast.

$2 #angama!u.This slide tells about the erection ofthe %anamahu


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$$ Bec!yne p!oto. This slide shows a network arch that wasbuilt in the M3ech Republic in 077*.-Sasek 077)+ 07721$t has a span of * m.$t replaced an old bride with too little room for the 77 year flood.

The existin roads on both sides of the bride needed onlysmall alterations. The M3ech enineer 8adislav Sasek heard

about the network arch. Then he consulted my home pae anddesined this bride.

$% Bec!yne )ross6section. This is a cross6section of the


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$9 Blenner!assett erection.The method of erectionof the


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%1 )onclusions after '% years. &art 2. Networkarches are e,ually well suited for road and rail brides.All their members efficiently carry forces that can notbe avoided in any simply supported beam.

Tie and haners ive the arch ood support and hihbucklin strenth in the plane of the arch. Tension ispredominant in tie and haners. ;6beams with hori3ontalwebs Bniversal columns or American wide flane beamsO have a favourabledistribution of stiffness and ive simple details. All

haners can have nearly the same cross6section. :fficientmethods of erection are available. :rection can be doneusin a temporary lower chord which+ combined with thestructural steel+ has enouh strenth and stiffness to carrythe castin of the concrete tie.

%2 )onclusions after '% years. &art $.Network arches are not sensitiveto uneven settlements in the foundations. ;ih strenth and low weiht ivethe network arch ood resistance to earth,uakes.

%ost concrete parts need more maintenance than a concrete slab with asliht prestress. Network arches have small surfaces and need littlecorrosion protection. ;ih strenth steels are well utilised.

$f thins o well+ the network arch can save up to *7 of the cost and?7 of the structural steel. Ted Loli saved more than )7 of the cost byusin two network arches instead of a cable stayed bride footbride in San@ose. The spans were the same.O A hih percentae of the cost will be labour.Monservatism and lack of time are important obstacles to the buildin ofnetwork arches.

Thank you all for listenin. $ look forward to your ,uestions now anddurin the whole conress. $f you want me to+ $ can lead you to the slides andthe manuscript for this lecture. 6 Thank you very much.

This manuscript+ the slides and my contribution to the 077/ 4orld Steel

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San Antionio Oral.doc