Findings of U-Shape Girder for Airport Rail Link Project

In general U-shape Girder has several advantages over box girder section that is used for airport rail link project in point of AECs structural engineer view as follows:

1. U- shape girder provides less clearance than box girder section as a result in less bending moment in pier column and loads in pile than box section.

2. U-shape girder that uses twin webs can also acts as noise barrier and derailment. 3. Cable trough can be attached to twin webs.

This is right but please note that there are other advantages such as :

- Lateral beams can be used as maintenance walkways, - Permanent Visual obstruction of the section will be much reduced (by half)

improving the integration in the environment, - Easy integration in the stations as top flanges of the lateral beams will be used as

part of the platform, - Overall reduction of the profile for a given minimum clearance, allowing to

reduce stations height (with all the consequences on the mechanical stairs, comfort of passengers, ) and reduction of structural components of the stations,,

- Simplification of the construction and quality control, - Possible increase of the construction speed, - Strong reduction of overall costs,

However, several unclear and doubtful questions are needed as follows:

1. Both structural sections shall be designed and compared under the same loading conditions and design code. Please note that in all our projects using U shape concepts, we have been challenged and compared first with standard designs such as box girders. In all cases, the main selection was done based firstly on overall construction costs. This is the case, for instance, of Dehli Metro line 3, Santiago de Chile Line 4, Santiago de Chile Line 5, Duba Rail and Taipei Neihu Line. For your information and your reference, in the recent construction tender for the Duba Rail, it was opened to alternatives. Only one contractor proposed also an alternative in box girder but the cost was the highest. In the case of the Santiago de Chile Line 5, the project could only be done at that time thanks to the construction cost savings that were brought by the concept. In the presentation of the 20th of May, we showed a comparison based on Bangkok Airport Link loading and design codes. We have designed many U shape sections also with higher loads that the airport link and with more stringent design criteria (such as the Indian regulations, for example). In all cases, strong cost reductions have been performed.

Hypothesis for the comparison are given in attachment 1. We have been working with all types of codes worldwide and all this have resulted in same results. As a summary, the design hypothesis are as follows :

GENERAL DESIGN SPECIFICATION OF ELEVATED TRACK STRUCTURE

Materials : concrete Deck 40 Mpa, Pier 35 Mpa, Footing 35 Mpa, Piles 30 Mpa.

SIDL 27,75 KN/lm per track (including track). Live Load 16 t/axle and 160km/H

The structures were also checked with standard gauge as required by the Terms Of Reference. Compliance with the following codes were also checked :

AASHTO (17th edition); ACI 318 and 358; CEB-FIP 1990; Eurocode 1990;

We have taken into account 35.5m spans.

2. From the previous design of airport rail link project, barrier is concrete barrier

which is quite heavy and unnecessary. Barrier for airport rail link extension in missing link project, on the other hands, uses light material such as GRP panel as same as BTS project. U-shape girder also uses light barrier. Based on BTS experience, we think that the solution proposed on the airport link is much more reasonable. In deed, on the BTS, the panels are constantly falling and a maintenance team is permanently required to change the panels. It also brings to very costly maintenance. In our case, with 160 km/h and related pressures (suction and outer pressure), the panels will have to be very much reinforced. Finally, please note that light panels are very costly items.

20 m

11.7 2.1 m 4.1 m 11.7

The U shape does not need any panel. Very simple and primitive handrail can be installed in case that the evacuation is to be done laterally which is not the case, according to our knowledge, for the Bangkok airport link train system which has an evacuation from the front. For the U shape, in case of front evacuation, no barriers are required.

3. It is doubtful that structural depth for U-shape girder requires less depth than box girder. Center of gravity for U-shape is quite close to bottom slab. Prestress forces will then be required with the greater amount than in box girder section to resist bending moment at midspan. The height of the U shape is mainly governed by the car platform height when side evacuation is needed. The total depth is similar in case of box and U shape. For instance, considering airport link loading, the small (single track) U shape would have a height of about 1.80m and the big (double track) U shape would have a total height of about 1.95m. Please also keep in mind that all dead loads are significantly reduced compared to the box solution. In case of the single track U shape structure, we use pre-tensioning which have an improved efficiency compared to post-tensioning leading to additional reduction of prestressing quantities. We have designed many U shape sections also with higher loads that the airport link and with more stringent design criteria (such as the Indian regulations, for example). In all cases, strong cost reductions have been performed. As shown during the presentation we have verified that the structures under static and dynamic loadings comply with the specifications. The main results are as follows :

Double track U : Service Load design

Allowable tension stresses : 0,82 > 0Mpa O.K. Allowable compression : 16,90Mpa < 0,6 x fc

O.K. Deformation checks :

- static : - vertical deformation 18 < L/1200 O.K.

- warping 0,0004m/3m < 0,003m/3m N.A. - end rotation 10mm O.K.

- dynamic :

- deflection 23,3 < L/1200 O.K. - vertical acceleration of deck 1,1m/s O.K.

Load factor design : Shear and flexural strenght O.K.

Single track U :

Service Load design Allowable tension stresses : 0,98MPa < 0,25 x

SQR(fc) O.K. Allowable compression : 16,5Mpa < 0,6 x fc O.K. Deformation checks :

- static : vertical deformation 20 < L/1200 O.K. warping 0,003m/3m N.A. end rotation 10mm O.K. - dynamic : deflection 27 < L/1200 O.K. vertical acceleration of deck 2,2m/s O.K.

Load factor design : Shear and flexural strenght O.K.

4. Most Prestress tendon for U-shape section is in bottom slab which creates tension stress at top surface of bottom slab. In order to reduce the effect of this tension stress the stiffener at this point shall be required. The prestressing is globally centered and no tension is allowed in the segmental section. We have verified that the stresses are within the allowable limits. As shown in the presentation of the 20th an integrated pier cap is used in the case of the double track U shape. They allow a good transmission of the loads to the substructures as well as a good anchorage of the post tensioning tendons with all the related prestressing transfer reinforcement.

For the evaluation, SYSTRA performed a modeling of the structure. The tendon layout is shown here-below.

In the case of the single track U, since the structure is done in full span and pre-tensioning is used, some slight tensions are allowed by codes. We also use limited sheathing of the strands of some strands to eliminate the tensions. For the anchorages of the pre-tensioning strands only very limited reinforcement is required to ensure the transfer of the strands loads to the concrete.

5. U-shape using pre-tensioned tendon is not applicable for this project. As

informed by speaker, the total weight of 130 tons with the overall width of 10 meter and 35 m length cannot be transported to site. Reducing span length will cause plenty amount of pier column. The small U shape (single track) will be about 4m at the base and total around 5 to 5.5m at the top.

There a various solutions to transport the elements : - at grade along the project : The SRT land seems wide and sufficient to transport the element. More investigations could be carried out with this respect. Then the element can be erected by using mobile cranes or by using a conventional re-used launching girder adapted for the purpose. Please note that this is the solution being used for the Neihu Line in Taipei which is being constructed in a dense urban environment.

- from the top : In that case the elements can be built in line with the viaduct. As the elements and fabricated and erected on fast track basis, only a very limited precast yard will be necessary. The space dedicated to some stations could be used, for example. The elements are then brought from the top, span by span, using a carrier. They are placed

Based on our experience, 25m spans with small U shape pre-tensioned are generally the cheapest scheme eventhough there are more piers. Additional piers will be very small. In addition, the deck height will be smaller and overall visual obstruction reduced. Anyhow, we propose to study both alternatives and propose them to the client for selection based on environmental, site conditions and economical studies.

6. Speaker informs that bottom slab thickness is about 350 mm for double tracks and

requires no transverse prestressing tendon. It is in doubt since bottom slab width is about 9-10 meter span transversely and creates a great amount of transverse bending moment. If transverse prestressing is not required, a large percentage of reinforcement shall be required. The 350mm is an average thickness of the slab. In case of the airport link, the slab would be 375mm at mid span and 300mm at the lowest point, allowing also to have the lateral drainage. It is true that the slab will be more reinforced that a prestressed slab or a standard box but it would be cheaper as an overall. Rebar are made of simple transverse bars, simple to install and creating no difficulty for the construction. We remind that the U shape is very simple to concrete since there is a wide access to the slab making concrete pouring and vibrating extremely easy, as compared to a box.

Please note that in case of small U (single track) the slab can be reduced further since the distance between the webs will be in a range of 4m-4.5m. In that case slab mimimum thickness will be around 250mm. In case of Canton Line 2, the system was a very heavy mass transit system carrying 16t/axle car vehicles. The minimum thickness was 250mm.

7. Speaker informs that for 35 meter span requires 1.8 meter structural depth. With

the same live load as using in airport rail link project, U- shape section has less concrete compression area than box girder. Greater moment arm shall then be required for U-shape section. As a result greater structural depth will be required.

The total depth is similar in case of box and U shape. For instance, considering airport link loading, the small (single track) U shape would have a height of about 1.80m and the big (double track) U shape would have a total height of about 1.95m. Please also keep in mind that all dead loads are significantly reduced compared to the box solution. In case of the single track U shape structure, we use pre-tensioning which have an improved efficiency compared to post-tensioning leading to additional reduction of prestressing quantities. Please note that SYSTRA has also verified the dynamic behavior of the structure under moving loads and the results are found adequate. Please also refer to 3. and to the following calculation results.

Fig. Vertical acceleration at mid span of the deck Big U with 2 tracks.

Fig. Vertical acceleration at mid span of the single track U shape

8. Twin webs provided in U-shape section shall be thicker than bottom slab

thickness. Load from train acted on bottom slab will be transferred to these twin webs. Torsional moment shall also distribute to these webs and may cause web buckling.

Please note that the slab can rotate and deformate (within the allowable limits) and that therefore it is note necessary to have the webs and the slab with the same thickness. Nevertheless, in our proposed design, the webs and the connexion with the bottom slab would be 300m thick in case of big U (double track). In case of small U (single track), the thickness of the webs would be 260mm thick and the slab 240mm. Since it is an open channel the torsional rigidity is not high. The excentric loads are transmitted to the webs by differential bending of the webs. This is a common behavior also valid for I beams structures. Regarding the web buckling, please note that we have done the necessary analysis to verify the non buckling of the webs. In addition, we have carried out full scale and ultimate cases as follows : - small U (single track) : The structure was loaded up to 6 times the service live load. The rupture was reached by compression in the top flange without showing

any secondary effect. Absolutely no buckling effect was observed and the detailed calculations verified.

Fig. Loading of the structure.

Fig. Rupture of the structure by full compression of the top flange.

- Big U (double track) : we made a full scale test in India, for the line 3 of the Dehli metro (involving very heavy train loads of 19.5 tons/axle, higher than the Airport Link Loads). We went up to 3 times the live load without observing any

damage of the structure. The structure was loaded up to 5 times the live load without reaching the ultimate state. The tests were very successful and absolutely no secondary effect was observed. The transverse derailment loads were alson tested successfully.

Fig. Dehli Line 3 Full Sc. Test.

Fig. Dehli Line 3 Derailment loading Test.

9. As explained in (5), post tensioned system shall then be necessary with segmental

construction. Normally one or two spared tendon duct are provided in box girder section for future use. U-shape section, on the hands can not provide spared tendon duct. Therefore, prestressing process shall be carefully operated. It is generally not required by international codes and practice to have spare tendons for simple spans structures. In the past, the spare tendons were generally for continuous bridges in order to solve some effects due long terms deformations (creep) which were difficult to calculate and estimate. This is not the case of simple spans and even continuous span these days. Nevertheless, it is perfectly possible to install spare ducts in the U shape structure, should it be required by the client.

10. During maintenance of utility duct, U-shape section requires to stop train

operation while box section that has utility system in cable trough can be checked while train still can operate.

Please note that the line will be operated at 160km/hours requiring safety procedures much higher than conventional MRT systems. As for such high speed line are generally maintained at nights. Should there be an incident, it is perfectly possible to inspect on one track while the other is under operation.

11. Material comparison between two sections is also in question. For example,

hollow pier proposed in U-shape structure can be also used in box girder section. However, a large amount of formwork also requires in hollow section.

Please note that as mentioned in your introduction, the horizontal loads will be significantly reduced. In addition, thanks to important reduction of the dead loads, the vertical loads applied to the substructures will be smaller. These important reduction of loads will allow for optimization of substructures. As far as hollow box are concerned there are various solutions : - use of precast segmental piers : this will help to reduce a lot the construction time, if necessary. In that case, the fact of using an hollow pier has no influence on the cost. -In case of cast in place piers, please note that according to our investigations in Thaland and experience on other projects, the difference of price is not important compared to the saving, in a range of about 5-8% higher per cubic meter of concrete.

Attachment 1 Hypothesis for preliminary calculations based on

airport Link.

1. HYPOTHESIS FOR PRELIMINARY STUDIES BASED ON AIRPORT LINK PROJECT.

The following hypothesis have been considered in the preliminary design..

1.1 Design hypothesis

1.1.1 Geometry The guideway, Single U carrying 2 tracks, cross-section was designed according to these geometric hypotheses:

Distance between tracks axis : 4.00 m; Distance between track axis and edge of structure : 2.00 m; Height between rail level and walkway : 1.10 m; Height between top slab and top rail : 0.54m; Walkway width: 1.40 m (including provisions for the catenary fixation); Web thickness is 0.30 m.

1.1.2 Codes of practice: The design is in accordance with the following standards and codes:

AASHTO (17th edition); ACI 318 and 358; CEB-FIP 1990; Eurocode 1990;

1.1.3 Materials: The specified compressive strength (on cylinder) for bridge structures is detailed hereafter:

Deck 40 Mpa; Pier 35 Mpa; Footing 35 Mpa; Piles 30 Mpa.

Reinforcement yielding strength is taken 390 MPa or 490 MPa (depending on rebar size). Low relaxation strands (ASTM A416 Grade 270) were used with an ultimate strength of 1860 Mpa.

Strand stress immediately prior to transfer = 0.80 fs; Stress losses are calculated using CEB-FIP code.

1.1.4 Loads Dead loads [DL] : Dead load is the vertical load due to the self weight of the entire structures. The weight of reinforced concrete was taken equal to 24.5 kN/m3. Superimposed dead loads [SIDL] : In addition to guideways dead load, we have assumed an imposed dead load of 55 kN/m. Please note that parapet wall and derailment walls are included in the structures. Live load [LL] : The design was made considering the following configuration with 16t/axle (more stringent than the present loading in the specifications for the airport link extension).

The structures were also checked with standard gauge as required by the Terms Of Reference. The maximum operating speed is 160 Km/hr. Vertical static loads were increased by a dynamic coefficient (for static calculations) equal to (according to Eurocode 1 code Section 6.4.3): Where:

L is span length [m]. Haunting, braking and acceleration forces were also considered. Other loads: The following loads were also considered:

Wind load on live load; Wind load on structure; Earthquake loads

11.7 m

2.1 m

4.1 m

11.7 m

73.020.0

16.2 +=

L

20 m

1.2 Methodology

1.2.1 Service load design Combinations: The following combinations were used:

S1 = D+L+I+PS+LFn+DS+SH+CR+0.5DT+T+HF; S2 = S1+0.3 (WL+WS)

Where,

D: Dead load + superimposed dead load; L: Live load; SH: Shrinkage; CR: Creep; DS: Differential settlement; PS: Force and effect due to prestressing; WL: Wind load on live load; WS: Wind load on structure; LFn: Normal longitudinal braking; DT : Differential temperature (superstructure only); T: Temperature ; CF: Centrifugal loads.

Displacements: The following criteria were verified:

Maximum vertical deflection under live loads plus vertical theoretical impact for one loaded track;

Longitudinal displacement at rail level due to deck rotation; Deck twist; Vertical deck acceleration.

Stresses in the deck : No tensile stress is allowed during service stage. Compressive strength is limited to 24 MPa after all prestress losses.

1.2.2 Load factor design Combinations : The following combinations were used:

U0= 1.3D + 1.7 (L+I) +1.3 SH (or CR) + 1.0 PS

U1 = 1.3 D + 1.4 (L+I) + 1.3 SH (or CR) +1.3 DS +1.0 PS +1.5 (WL+WS); U3 = 1.3 D + 1.4 L + 1.3 SH (or CR) +1.3 DS +1.0 PS +1.5 EQ+1.4 LFe.

Where,

EQ: Earthquake LFe: Emergency longitudinal force

Shear verification : Members subject to shear were designed so that: ( )scu VVV + Where:

Vu: factored shear force including reduction due to prestress; Vc: nominal shear stress provided by concrete; Vs: nominal shear stress provided by web reinforcement; : strength capacity reduction = 0.9 for shear.

Shear strength provided by concrete shall be taken equal to:

dbfV cc ''1411.0=

fc specified compressive strength of concrete (40 MPa); b width of the webs ; d distance from extreme compressive to centroid of prestressing steel.

Shear strength provided by web reinforcement.

sdfA

V syvs = Where:

Av: area of web reinforcement within a distance s (s=0.2 m); fsy: yield stress of non-prestressed conventional reinforcement in tension.

Vs shall not be taken greater than:

dbfV csMAX ''664.0= . Flexure verification : Steel stress was computed according to AASHTO. For bonded members with prestressing only:

=c

sssu f

fpff'

'1'

*

1

**

f*su average stress in prestressing steel at ultimate load; fs ultimate stress of prestressing (1860 Mpa); fc compressive strength of concrete at 28 days (40 Mpa);

* = 0,28 for low relaxation steel; 1 = 0,75 for fc = 40 MPa ; p* ratio of prestressing steel.

We assumed that our section is a flanged section having prestressing steel only, in which depth of the equivalent rectangular stress block, defined as (Asrf*su)/(0.85fcb) is greater than the compression flange thickness t. With:

A*s area of prestressing steel; Asr steel area required to develop the compressive strength of the web of a flanged

section; Asr = A*s - Asf Asf steel area required to develop the ultimate compressive strength of the overhanging

portions of the flange; Asf = 0.85fc(b-b)t/ f*su b width of flange; b width of the web.

The design flexural strength shall be assumed as:

+

= )5.0()'('85.0

''6,01

** tdtbbf

dfbfAdfAM c

c

susrsusr

With:

d distance from extreme compressive fiber to centroid of prestressing force. For factory produced precast prestressed concrete members, the strength capacity reduction is equal to 1. Ductility limits Prestressed concrete members shall be designed so that the steel is yielding as ultimate capacity is approached. In general, the reinforcement index shall be for flanged section such that:

1

*

36.0''

c

susr

dfbfA

For members with reinforcement indices greater than 0,36 1 the design flexural strength shall be assumed not greater than :

( )[ ])5.0()'('85.0''08,036,0 2211 tdtbbfdbfM ccn +=