IISE Paper - Integral Frames for Mumbai Monorail Guideway

Integral frame for M mbai -

onorai Guide ay

Milind Bhoot 1,

Elizer A rea2

Mumbai onorail is to be the l ongest in t he World fter Chon qing and Disney. The uideway Structures in the thr e projects belong to three diff rent generati ons – inte gral frame , simple span concr te and st el beams respective ly. A structur l enginee r may ap reciate th at Mumb i project adapted t e best of the prevaili g practice in integral frames an d then refi ed it. umbai adapted the most acc pted strad dle-beam type Mon rail syste ; in which v rtical and horizontal tires of th car to straddle (hol ) a single concrete b eam from th ee faces against ov erturning. The guid way bea forms th track fo the monorai l cars; wit h exposed concrete surface pr e-cast to t he perfect ion of co plex curve-s rface geo etry true t o the geo etric-desig n. It was the pioneering dvanceme nts of con truction te chniques i Kuala Lu pur project that made the integ al frames possible for Monor il. 5,6Uniq e constru ction operatio ns are des igned to p rfectly pos ition the be ams on cu rve geome ry of the g ideway. H wever, the method involved temporar clampin frames t beam nds; controll d by verti cal and in clined hyd raulic jack s from th pier-hea , which c ause second ry prestr ss effects and corr esponding time-dep ndent eff cts. So, it is essenti l to accou t the temporary clam ing effect ; in the m del of stru ture on bri dgespecial oftware, tr ue to the c onstruction statement s. utline det ils of Mon rail Guideway Structures are p ublished el sewhere 3,4 and this paper extends the sequ l; fulfilling structural engineer’s appetite; laborating vital features of monor il guidewa frame; di tinguished from conventional br idge-struct res. We rec mmend some back round rea ding on M onorail; m ostly avail ble online (for better u derstandi g of struct ural design concepts resented ere). e count o structural engineer’ tendency to underst nd the ne structure with referen e to his pr evious exp ertise – of the bridge structure. With such understan ding, this pa er compares Monor ail guidew ay structu res with bridges – ighlighting the distinguiishing feat res.

Introdu tion:

Benefit of Integral Frames are well known to a bridge eng ineer. The integral fr mes were adapted in Monorail Gui deway for ubstantial value addi tion with si ilar reaso ns.

The saving on reduced numbe of bearin gs and ex frames ields bigg er dividend s than the viaduct br idges - si beams eed massive bearing s and exp nsion joint . While th are co pletely eli inated fro m the fra e, the ca tilevered gaps ar reduced rastically i n numbers (to say 25 ). Journal of Indian Instit te of Structu al Engineers (IISE), Mumb i‐

ansion joi nts in mo orail ply as mo orail guid way torsion re straint bea rings teel finger s at expa sion

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Integral frames for Mumbai Monorail G ideway

Loads

System generated Desi n forces (by Monorail c r Manufacturer)

Axle loads

115 N, alternatively spac d @ 3.75

Impact factor

Concrete bea :

Braking & Traction

18

Hunting force

25

& 7.5m average.

I = 20/ (50+L) < 25%, wh re L =spa

& 20%.

Seismi forces: he durabili ty specific tions of Mumbai Mo orail for 1 for spe ific mix de sign requir ements. In addition t is reflecte d into upg design criteria fro Zone 3 to Zone ; accounti ng for hig her proba occurre ce over e tended life -span and ensuing sp ecial rebar details du

0 years called ade of sei smic ility of sei smic tility.

The struct ral engin er must correctly nderstand the late al forces and overturn ing actions on the gui deway beam caused in straddle monorail s ystem. e : Eccentric car positio (by manuf acture tolerance) Eccentric half live load

Lateral

e

CG

Loads

Balancing Rea tions on straddle Ty e

Basic Load Actions on Gui eway Bea s The pict ure above illustrates how the lat eral loads- hunting lo ads, centri fugal force s etc should ct on the Monorail uideway eams, a li ttle differe tly than th e normal s teeltrack rail cars. Sim ilarly, the ar-manufa cturer may explain e centricity f vertical l oads by mec anical an construct ion toleran ces. Half ar loaded on one si e of the b eam center ay be a cr itical condi ion, in seis mic and wind load co mbinations .

Journal of Indian Instit te of Structu al Engineers (IISE), Mumb i‐

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Integral frames for Mumbai Monorail Guideway

Components of the Integral Frames for Monorail Guideway: Monorail integral frame comprises of precast guide-beams of 23 to 27m spans; to be ‘stitched’ in-situ with Y-piers of 12 to 20m height. The frame is finally post-tensioned by long continuity tendons; integrating the 3 to 5 spans. Similarly moment distribution in monorail frame gains bigger benefits than bridges, as the relative columns-beams flexibility permits more equitable distribution of forces in the frames. (See the comparison table below). Pile foundations: The typical two-pile arrangement compliments the integral frame arrangement very well. The moment-distribution becomes equi table and the added flexibility enhances the seismic response. Seismic shear stirrups in pile-caps : The seismic detailing stipulations of IRS code were followed in the detailing with reference to zone-4. However, the seismic detailing stipulations for pier-to-foundation connection are yet to be defined expressly in the present codes (though implied). This called for vertical seismic shear links in the body of pile-cap (around the pier cage). Though already implied in the present IS code, Railway’s seismic design guideline (7) now incorporates this with full expression.

External shear links around pier to arrest cracks

Stitch member (frame joint): is a vital element; integrating the monolithic frame. The stitch member is a 2.5m tall cast-in-situ stub projecting out of piers-head. The Stitch serves as a vital monolithic joint between the precast beams with column. Designer face a severe limitation here in the design of the stitch as the contact area available between the pier-cap and columns is limited to just a RC section of 800x1000mm – too short to accommodate high concentration of complex force combinations –lateral (cantilever) bending, out-of-plane bending and torsion (i.e. forces in all 6 degrees of freedom). Structural engineers often confront highly congested reinforcement in such joints, to be detailed with special seismic ductility requirements - with a meticulous rebar clash analysis, meshing beam dowels into the confined core concrete of the stitch members. The stitch interface with the precast beam is designed for shear-friction. Thanks to the prestress compression adds to the shear-friction capacity (well - only at the intermediate stitches, not end stitches).

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End Stitches: Compare the position of end stitches with the intermediate stitches in the picture above. Intermediate stitches are sandwiched between the two span, while the end stitches are left alone in a corner. As such; the dead-loads from two precast guideway beams get balanced on the intermediate stitch joints, while the end stitches remain unbalanced on dead loads of the end-span beams.

In addition, the intermediate stitches are boosted by the compression of continuity pre-stress for high resilience against concentrated forces, while the end stitches remain un-tensioned RCC members. (The constructability constraints continuity cables are terminated at the interface without intercepting the end stitches). The end stitches are further constrained by reduced sections in accommodating expansion joint fixtures. While the intermediate stitches enjoy the full 800mm wide beam section, the end stitches are left with only 500mm, in the top 1200 depth; under the finger joints. Torsion

Torsion

(Out-of plane BM)

Dispersion

Stitch Cantilever BM Stitch connector

Pier Cap Picture showing ‘stitch connector’ Dispersing ‘stitch’ forces

Connector

Stitch

Effective width of stitch after dispersion (per stitch)

Stitch connector: As shown in the picture above, the pair of vertical stitch members is tied by a horizontal connector 450x1000 projecting monolithically over the pier-cap. The stitch-connector is a part of the pier-cap cast in second stage simultaneously with the vertical stitches. The precast guideway beams are embedded 250mm in such a way that the beam-column force transfer are not concentrated on the narrow 800x1000 stitch members and are effectively funneled (diffused) into a bigger section 800x2200 contact area between the pier and connector. To ensure this dispersion, it is necessary to embed the beam-dowels into the stitch-core confined with the main stitch bars.


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With some deliberation; engineers may grasp the rationale of dispersion; an accepted resolution of a standard problem in Monorail integral frames. Diaphragms: horizontal diaphragm slabs (3 numbers per span) substitute the conventional vertical diaphragms in the bridges. See picture of computer model below connecting pair of guideway beam bottoms.

The 300mm thick diaphragm-slabs of 3x3m size integrate the two beams against the lateral forces and torsion. The diaphragm adds lateral rigidity for enhanced performance of guideway beams (reduced lateral deflections). Lateral Load Lateral sway

Diaphragm Generates U-section effect composite of two beams for: Restrained beam rotation and sway. Resilience to loads vertical / horizontal. • •

Elimination of Diaphragms : Mumbai Project eliminated all the diaphragms from the integral frame. The rationale is same as elimination of diaphragms from the modern bridge structures – aimed at elimination of a tedious construction stage.

Lateral beam-sways were checked without the diaphragms on various curves; by rationalizing the lateral forces as per the design speed-limits on the particular curve raddi (instead of using standard straight line design-speed on all curves). After such verification of the sway; to be within the permissible limit of 3mm could the designers could eliminate the diaphragms. The increased beam forces (after removing diaphragms) mean extra beam-rebar. This increased cost offsets saving in diaphragm-costs. Yet the time and trouble eliminated in the construction stages are appreciable. Add a bonus – freedom from maintenance worries in Mumbai, accumulating pigeon-waste over the horizontal diaphragm slab.


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Stages of construction:

Fig.: Span & continuity post-tensioning arrangement

Stage- 1: Guide-Beams in Precast-Yard: Stress first two cables (bottom-dashed parabola marked in picture above) at 3 rd day of precasting the beam in M60 grade. Stage- 2: Beam Launching and alignment , after 14th days of pier casting in M40. Stage-3: Intermediate stitch concrete M60 – (gap between beams shaded in picture). Stage-4: Stress the four continuity cables (frame length) after 10 th day of Intermediate Stitches (continuous parabolic lines marked in the picture above). Stage-5: Cast end-stitch M60 after inserting finger joint sole plates. Stage-6: Construction of in-site diaphragms. Clamping stage – Secondary effects : Readers may re-visit the stage 3, 4 written above. The beams are clamped temporarily in lateral (and vertical) direction to the pier-heads, while jacking the prestress on continuity cables. The clamp-supports (primarily meant for alignment purpose) are necessary till the ‘stitch’ becomes strong enough to bear the continuity forces - with help of prestress ).

The temporary lateral clamp-supports (while pre-stressing) cause secondary lateral effects on the continuous beams. The secondary effects are only partially restored on removal of the clamps. Thus the structural designer must incorporate these temporary construction stages into the frame analysis model. Special bridge-purpose software calculates all the time-dependent effects accounting all micro-stages including such temporary construction effects in the frame. The structural design sequence must permit release of clamps from the intermediate supports; before completing the end stitches. The clamps on end stitches may be released only after installing sole-plates (fixture plates underneath expansion joint fingers). And it is reasonable to release the intermediate clamps for reuse at other frames for optimum use of construction-resources. The table below explains why the monorail structure is flexible in comparison to the normal viaduct bridges. Hence attracts smaller seismic forces such that wind force generally governs over seismic (though the seismic detailing is mandatory).


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Comparison of Integral Frames for Typical Viaduct in Mumbai 1. Deck

2. Colum

Mumbai Monorail

Stiff Deck 10x2m (Box or Tee beams)

Flexible beam 0.8x2.2m voided

Column Height : 6m typical Column size : 1x1.5m typical : Typical 4-pile foundation.

Colum Height Column size

: 10m typical (max 17m) : 1x1.5m typical. : Typical 2-pile foundation.

Two pile arrangement further increases slenderness of piers - improving the seismic response.

3. Frame action

4. Seismic Response

5. Diaphragms

The deck rigidity compared to column is so high; that the column-head is “fully fixed” against long forces.

The beam stiffness is comparable to column. Hence column head may rotate at the beam joint; in proportion to its stiffness. And attracts And column contribution in the moment fair contribution in the moment distribution. distribution at the deck joint is negligible. The small height and fixed head conditions make the column stiff, attracting higher seismic forces – to the tune of 10% as calculated with IRC stipulations.

The vertical diaphragms are meant for improved grid action between the long beams.


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The slender flexible head column attracts seismic forces – only say 4% as calculated with IRS-BR stipulation (IS 1893: 1994). Only horizontal diaphragms are permissible, avoiding the path of straddle tires. The diaphragms generate U-section effect mainly adding lateral stiffness.

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Frame-analysis: Monorail structures are fine examples of how integral frames are designed and skillfully optimised on the modern bridge design software. Such software allows auto generated moving train, post-tensioning forces, losses and tendon extensions etc. As explained above, the time dependent analysis for the creep and shrinkage effects accounts for construction stages of pre-stressing phases; changes in support conditions during handling stages. Analysing prestressed integral frames is no more a lengthy cumbersome exercise. The modern bridge software takes care of the construction stage analysis, phasing out all the time dependent effects like creep and shrinkage. Construction sequence drawings: The last paragraph explains very well, why construction method statements are completed in consultation with the structural engineer before the designs are finalized. Construction sequence drawings define the stages of constructions and corresponding assumptions of concrete ages. The structural engineer accounts for the pre-determined construction sequence for phasing out the time dependent effects in the same sequence. Once defined on the drawings; the sequence must be adhered at site; acknowledging the design implications. Any temporary clamping of elements involved in the enabling structures, must reflect into the construction sequence drawings. In short, the construction sequence drawings are to be respected as part of the structural design; for integral frames; as changes in construction stages means revision of design assumptions. Portal Frames with sliding bearing supports : Portal beams support the guideway beam with unidirectional sliding bearings (Pot-PTFE), releasing moments in all 3 directions. Release the guideway beam supports of long moments; means release of torsion reactions on the portal beams, (in perpendicular direction). As one of the beam support within the frames is released for lateral rotation, effective unsupported lateral span length of beam-top increases to two span lengths. For the given stiffness of beams, the increased lateral deflection of beam-top was crosschecked to be within 3mm limit (for monorail car manufacturer).


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Integral frames for Mumbai Monorail G ideway

Portal rames with torsio restrain bearing : In continuity of th e explana tions above, r eleasing tr ansverse r tation of t o consec tive guide way beam supports ould increas the later l unsuppor ted span t o 3 span l ngths and lateral sway of the b eam top will exceed th permissib le limits sti pulated by monorail ars manufacturer. ( hich means the lateral sway of th car will e xceed the permissibl limits). In addition, t here will be excessive torsion in the guide ay beam to acco modate i the stru tural designs . The Mumbai Monorail crossing of Eastern-Freeway re uired 8 co secutive p ortal frames. Providing oment release beari ngs at all portal su ports me nt unsupp rted lateral span longer than 200 . Thus tor ion restrai nt bearing were req ired to limi t the lateral unsupporte spans within 55m. s suggest d earlier, the torsion restraint earing; us ed historic ally in mo orail simple pans, were provided in Mumb i project f r only limi ted locatio ns. The to rsion restrain d bearing ; designed as per BS5400 in M norail, are comparab le to roller steel bearing , well kno wn in Indi n Railway for old s eel bridge . As we know, thes are massiv bearings 200x800x 500m weighing 3 ton s. Long tor sion restra int anchor- bolts clash with conges ted stitch ebar addi g to the roblems f handling at the ti e of constru tion. Conclu ions: Aft r the exp rience of onorail, I ndian engi neers are likely to d sign more in egral bridg es for rails and road . The imp ct of such value engi neering ha s far reachin implicatio ns on the ay we design and bu ild our stru ctures. Modern tools used in Monorail have p otential to develop th e cutting e dge capabi lities, com etitive wit the global standards.

1. 2. 3. 4. 5. 6. 7.

Elizer Abrea, Ph.D., Principal tructural Engineer, Louis Berger Group. Milind Bhoot, S nior Structural Engineer, Mumbai Monorail, Louis Berger Group

Elizer Abrea nd Milind Bhoot, on “Mumbai Monorail – Blending of syste s with Gui eway Beams,” Kons ruction Revi w” Sep 19, 011 (digital C. Shankarlin am, “Constr ction Methodologies for umbai Monorail Project, “Civil Engin ering & Constructio Review, April 2011 Vishnu Kumar and Milind hoot, “Design and Construction of M norail Guideway Beams,” IIBE Symposium, Hyderabad Jully 2011 Roland von Wölfel & Manfred Braun- T e Guideway System of onorail Kuala Lumpur– Design & Erection, IA SE Symposia, Shanghai 04 IITK-RDSO G idelines for eismic Desi ns of Railw y Bridges. Provisions wit commentary and explanatory E amples. Oct ber 2010.

Journal of Indian Instit te of Structu al Engineers (IISE), Mumb i‐

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IISE Paper - Integral Frames for Mumbai Monorail Guideway

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