Preflex Girder

Bauhaus Summer School in Forecast Engineering: Global Climate change and the challenge for built environment 17-29 August 2014, Weimar, Germany

PREFLEX girders: Prefabricated composite bridges  PAP, Zsuzsa Borbála  Budapest University University of of Technology Technology and Economics Economics

Abstract The focus of this paper is the presentation of PREFLEX beam through the designing of the B398 underpass bridge at the Makó -Csanádpalota/Nagylak Csanádpalota/Nagylak section of the M43 motorway  in Hungary. As well as the comparison of this bridge type with the most commonly used pre-cast pre-stressed RC beam superstructure by this bridge type. The paper also presents the newest developments and an analytical calculation method of the PREFLEX  pre-stress girders. The pre-stressing girders are used usually at motorway projects. We show a handcalculating method via analysis a conventional motorway bridge, which is a traditional method based on stress-super positioning and stiffness-changing. stiffness-changing. Accordingly, two preliminary calculations are made: one with pre-cast, pre-stressed reinforced concrete  beam superstructure, superstructure, and one with with PREFLEX beam superstructure superstructure (Figure (Figure 1).

Figure 1: PREFLEX superstructure

PREFLEX bridges The pre-stressed, pre-cast reinforced concrete beams were developed by an Austrian engineer Ewald Hoyer in 1938. The PREFLEX technology was introduced early in the 1950’s by Abraham Lipski and Louis Bae in Brussels. Both types were invented during the period of post-war reconstruction. In the 60-s two significant building was made with the help of PREFLEX beams, the Tour the Midi (Figure 2), which is 138 m high, and the Berlaymount office building with 12 storeys.

Figure 2: The Tour the Midi in 1967, and nowadays

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While the Hoyer-beams were spread all over in Europe, the PREFLEX beams were gained ground mostly in Far East. There are only few PREFLEX structures in Europe. PREFLEX beam is a pre-cambered composite beam, made according to the following steps: While I-shaped steel plate-girder is bent under preflexion loads (four-point-bending), high-strength concrete is cast on its flange under tension. After the concrete hardens, usually three to seven days later, the loads are removed and then compressive prestresses are introduced in the concrete casing of the tensile flange as the beam regains a measure of its original shape. Then the whole structure is completed by transporting the PREFLEX beam to the site and pouring top concrete in situ. We can see the main stages of the construction in Figure 3.

Figure 3: Main steps of the construction

Stage 1 Fabrication of the campered steel girder (L/120). After this the beam is loaded with four-point  bending, and concrete is cast on its lower flange under this tension. Stage 2 After the concrete hardens, the PREFLEX load is removed from the girder.

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Stage 3 The PREFLEX beam is transported to the site and pouring top concrete and cross girders in situ. Stage 4 Other constructions (pavement, bars). Stage 5 The bridge is ready for the traffic.

The PREFLEX bridges have many advantages, for example the concrete slab increases the bending capacity and stiffness of the girder and under service load pre-compression stress in the concrete of the lower flange is reduced, but not totally, thus no cracking occurs. The concrete of the lower flange increases the flexural stiffness and reduces deflection. According to this the PREFLEX girders have a very high moment capacity, they are suited to the construction of bridges carrying heavy loads, in particular railway bridges. Due to these properties, prestressed girders are particularly suited to structures when the available construction depth is highly restricted. The slenderness ratio value (ratio of the span divided by the structural depth) may reach 45 for road bridges.

The development of PREFLEX bridges In South-Korea the PREFLEX beams are very widespread. The following types were developed by different Korean companies.

RPS  –  Represtressed PREFLEX beam The RPS beam is a double prestressed system; we can see the steps of the fabrication in Figure 4.

Figure 4: The fabrication steps of the RPS beam

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In this case the ratio of the height and length is about 1/25; the following table contains the exact geometry. Table 1. The geometry of RPS and PSI beam

length (m) height (m)

30 1,3

35 1,4

40 1,6

45 1,9

50 2,1

The strength of the lower concrete flange is 48MPa (~C50/60), the strength of the upper concrete flange is 27 MPa (~C30/37), the structural steel is SM520 according to the Korean standard (~S355).

PSI PREFLEX-beam The difference in this case that the beam gets the PREFLEX load in three points, and the middle load is removed later than the other two, as we can see in Figure 5.

Figure 5: The PREFLEX loads of the ordinary (left) and PSI (right) PREFLEX beam

A-PREFLEX  –  Advanced PREFLEX beam This type is developed by the Sampyo Engineering & Construction Co.. The PREFLEX loads are replaced to the one third of the beams, and two girders are prestressed in the same time (Figure 6).

Figure 6: The fabrication of the A-PREFLEX beam

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D-PREFLEX (Division-Retension) beam The D-PREFLEX beam is the developed version of the RPF girder. The first difference is that the web of the cambered steel girders is ribbed till the hardening of the concrete on the lower flange. Furthermore the  prestressing of the cables is made in two steps. With this the losses due to shrinkage, creep and relaxation can be decreased.

Figure 7: The decreasing of the loss of prestress vertical axis: loss (%), horizontal axis: time, green: creep, red: shrinkage, blue: relaxation

Flexstress beam The Flexstress beam (Figure 8) is developed by the Belgian Ronveaux Company. Actually it’s almost the same as the RPS beam, so there are prestressed cables in the lower concrete flange.

Figure 8: The cross section of the Flexstress beam

Design and calculation In my diploma work the behaviour, advantages and disadvantages of the PREFLEX beam through an approximate, analytical and a detailed static calculation can be followed. Furthermore the calculation of a  pre-cast, pre-stressed reinforced concrete bridge (FCI-90 beams) was also made to compare with the PREFLEX superstructure. Here in this paper I briefly summarize my results.

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There is no Eurocode standard for the calculation of the PREFLEX superstructure; therefore I used an American standard for the design: US Army Corps of Engineers - Engineer Research and Development Center: Analysis and Load rating of Pre-flex Composite Beams (ERDC/GSL TR-11-33) According to this code in Stage 1 the maximum stress in the steel beam cannot exceed the 80% of the characteristic strength of the steel (according to this we can calculate the magnitude of the preflexion loads). In Stage 2 the maximum stress of the bottom concrete flange has to be smaller than the 40% of the compressive strength of the concrete (because after 3-7 days the concrete is still hardening). In Stage 3 there can’t be any tension in the bottom concrete flange, which means  that the concrete cannot crack. The concrete of the pre-cast, pre-stressed beam is C40/50. The bottom concrete flange of the PREFLEX  beam is C50/60, the top concrete is C35/45 and the steel is S355. The B398 underpass bridge at the Makó -Csanádpalota/Nagylak section of the M43 motorway is a three span bridge, and each span is 24.05 m length. The width of the superstructure is 11.63 m, the height is 90 cm in both cases as we can see in Figure 9.

Figure 9: The cross section of the pre-cast, pre-stressed reinforced concrete and the PREFLEX beam

In case of the hand-calculating, analytic method, we have to sum the stresses after every stage, and we have to make simplifications in the calculation of the creep and shrinkage. In the analytical solution when we remove the preflexion loads, we have to put it on the girder in opposite direction, as we can see in Figure 10 Stage 2. Figure 10 shows the results of the hand-calculation.

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Figure 10: The stress results of hand-calculation

According to the approximate calculations 10 FCI beams are needed in one span in case of the pre-cast,  pre-stressed reinforced concrete bridge but only 5 PREFLEX beams are necessary. The program called SOFiSTiK is used for the finite element calculation. This program is chosen because it can handle the time-dependant properties of concrete (creep, shrinkage). It was especially important in this case, because there are two concrete parts connected to the upper and lower flanges of the steel beam. These two concrete parts are poured and hardened (take part in the load bearing) at different times. The other advantage of the SOFiSTiK is that it can manage construction stages (CS), so the steps of the manufacturing process  –   that influences the further behaviour - could be accurately modelled. The following figure shows the finite element beam model made in this program.

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Figure 11: The finite element model

Table 2 shows the construction stages (CS) which were made in SOFiSTiK. Table 2. Construction stages

CS

Name of the stage

10

Steel beam

15

Prestressing

20 21 22 25

PREFLEX flange PREFLEX flange active PREFLEX flange-7 day Unloading

26

g1 loads

30

Creeping till transporting Upper shear connection

31

g2 loads

40

In situ concrete

27

Time of the stage (for the calculation of creep and shrinkage)

7 days

28 days

 Note

Simply supported, steel, I-girders.  PREFLEX loading: concentrated loads on the first quarter and the third quarter  points. Create the bottom concrete flange.  Activate the bottom concrete flange. The bottom concrete flange is hardening.  Remove the PREFLEX loads.  Activate the g1 loads (the selfweight if the steel and bottom concrete flange). The bottom flange is still creeping.  Activate the upper shear connections.  Activate the g2 loads (upper concrete  slab). Creating the in situ concrete parts (top

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41

In situ concrete active

50 55

Creep 3 g3 loads

60-69

Creep 4

30 days (t=0) 3500 days (t=∞)

 slab, cross girder).  Activate the in situ concrete parts. The bridge is a three span bridge, and the beams are working together. The concrete parts are creeping.  Activate the g3 loads ( bar, pavement…). The concrete parts are creeping.

Figure 12 shows the cross section resulted by the detailed calculation. In this case the PREFLEX load is 864 kN (the maximum stress in the steel beam cannot exceed the 80% of the characteristic strength of the steel under the preflexion loads), and only five beams are enough in each span.

Figure 12: Cross section of the PREFLEX beam

Summary Table 3. The comparison of pre-cast, pre-stressed reinforced concrete and PREFLEX superstructure

Standard Weight of one beam  Number of beams Total weight Deflection (quasi-permanent combination) Technology Concrete

Pre-cast, pre-stressed reinforced concrete bridge (FCI-90) EC 725 kg/m 2x10 345t

PREFLEX bridge (height 90 cm) - (American) 930 kg/m 2x5 221t

45mm

0mm

concrete, cables, prestressing C40/50

steel, concrete, bending C50/60

Table 3 contains the comparison of the pre-cast, pre-stressed reinforced concrete and the PREFLEX  bridge. One PREFLEX beam is heavier than an FCI-90 beam, but we need only half of them. This results that the PREFLEX superstructure is much lighter, which is beneficial for the substructure. In case of the PREFLEX system the deflection is very small, for example in this case I calculated the  precamber of the steel girder that the deflection in quasi-permanent combination would be 0 mm. So we can see that this bridge type is very advantageous, if there is a limit in the deflection or when the available construction depth is highly restricted.

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The disadvantage of the PREFLEX system, that there is no Europian standard for the calculation and the technology is unfamiliar for the factories, manufacturers, but these problems can be easily fixed in the future.

References Ákos K., Nauzika K. (2012): PREFLEX girders: Prefabrication, erection and static, “MAGÉSZ” the  journal of the Hungarian Steel Association 2012 (3): 30-39. http://www.iblenc.co.kr/sub-21.html http://www.mansecorea.com/ http://PREFLEXbeam.co.kr/PREFLEX/a_principle.htm http://www.rpfbeam.com/rpf_html/set_html/set1.html http://www.sampyoenc.com/korean/index.asp http://xn--9d0b5q.com/subPage.php?pgCode=4/5 Oliver S., Tina K. (2011). Die Straßenbahnbrücke „Messe Dresden” - Brückenbau mit PREFLEX®Trägern, Brückenbauwerke

Stéphanie S., Guy R., Henri D., Bernard E. (2004). Innovative Composite Precast Prestressed Precambered U-Shaped Concrete Deck for Belgium’s Hi gh Speed Railway Trains, PCI Journal  Genock P., Ulises B. , José A. (2011). Analysis and Load Rating of Pre-flex Composite Beams (ERDC/GSL TR-11-33), US Army Corps of Engineers - Engineer Research and Development Center Zsuzsa P., Ákos K . (2013): PREFLEX girders: Prefabricated composite bridges, “MAGÉSZ” the journal of the Hungarian Steel Association 2013 (4): 72-78.