V. N. Heggade
Former Chief Executive Officer
STUP Consultants Pvt. Ltd.
Former Executive Director of Gammon

The literature study indicates that historically, there is no evidence of adopting scientific process for conceptual design. It is not that the term concept was not used but were being used loosely. Almost 3 decades ago, Billington introduced the terms efficiency, economy and elegance as a theme of approach to bridge design and promoted it as a conceptual design approach to the numerous anonymous, too frequent ugly utility structures based on mere technical and economic criteria.

There has been a longstanding concern by professionals to address the issue of conceptual design. It is generally felt that overall quality of many structures today leaves much to be desired. There has been a rapid technological progress but they do not reflect adequately the variety, beauty and sensitivity that are due, though, there are sporadic attempts like in the case of Signature Bridge at Delhi. Too often, structural engineers neglect the creative conceptual design phase by repeating standard designs and not sufficiently contributing with their own ideas to the fruitful collaboration with architects. The bridge engineers, thus, often waste the chance to create building culture.

Some of the unwarranted assumptions and preconceived notions that normally challenge the sound conceptual design are:
– Everybody knows that (steel plate girders, precast concrete girders, cast in place concrete) are the most economical structure for this location.
– Let’s use the same design as we did for other structure earlier.
– The enabling structures are readily available with the contractor and contractor is conversant with technology as such repeat the form.
– Regionally, steel structure or concrete structure is popular as such we will adopt the same.
– We have always done this way and it is time tested.
– We have not done in the past as such why take risks.
– In any case, cost will differentiate and determine the form, materials and technology as such why get in to so called conceptual design.
– Architects are nuisance in achieving constructability in structures, they don’t understand structural engineering and their structures are not cost effective as such avoid them in conceiving a structure.

We are at our disposal today, the most advanced codes that are the condensed extracts of highly updated knowledge available on Big Data, the powerful computation and analysis ability for any conceivable form by advanced computers, very high strength and light materials with huge options and variety of powerful technologies and equipment for construction. Despite these advancements, we see ugly looking, unsustainable and ill-conceived structures many times. Some of the reasons for ill conception are:
- Education system of structural engineers; presently there is no connect between university education and the professional structural engineers. There has to be system of professional structural engineer mentoring the students at university so that they can succeed professionally.
– Lot of research is going on in the world labs of universities and the knowledge is gathered. Again, there has to be system that the knowledge acquired in the university laboratories are passed on to professional structural engineers, codes and standards.
– The codes and standards should be written in such a way that they are not restricting the innovations and liberal thinking.
– The computers should become the slave of engineers not the other way around, especially for conceptual design. They should not imprison the minds of engineers from Imagineering getting bogged down by computer modelling. Computers should be mere tools of engineers to free them from the drudgery of computation. But they should not become the source of inspiration

Process Of Form Finding Of Structures
The present author defines the conceptual design as “a process of form finding by incorporating aesthetics while being germane to Socio-Cultural context” In the definition above, the terms Form finding, aesthetics and socio cultural context sum up the conceptual design.

While finding the forms for bridges by economy and efficiency that are achieved by dimensioning the structure adequately just enough to cater for the forces, but also making bridges elegant consciously started from engineer Thomas Telford who designed and built Craigellachie arch bridge (1814) and Menai suspension bridge (1819). Since then we see some outstanding and revolutionary bridges in that tradition. Gustava Eiffel’s Maria Pia Bridge (1876-1877), Roebling’s record breaking suspension bridges (1805-1869) are noteworthy in that tradition. However, Swiss engineers Robert Maillart (1872-1940) and Christian Menn (1927-2018) took the bridge design to the next level of structural art (Fig. 1) following that tradition.

*Fig. 1: Aesthetic Conception Taken to the Level of Structural Art*

The present author coins the following 3 phrases with abbreviations for the conception of structures through finding the forms.
– Form Finding by Following Nature (FFFN)
– Form Finding by Flow of Forces (FFFF)
– Form Finding by Forcing the Flow of Forces (FFFFF)

Form Finding By Following Nature (FFFN)
The concept of music, art and artefacts to be associated with nature can be found since ancient times. It was discerned that there has been some sort of rhythm all pervasive in nature in the petals of the flowers, leaves of the plants and trees, pine cones, shells, space and even the features of the human beings following some sort of a godly, divine rules. When there is a such a godly divine rule is inbuilt in the nature all around and the human psyche is used to this rhythm of this mathematical ratio, it is quite natural for humans to imbibe this very ratio in music, arts and artefacts lest they can be incorporated in to nature with harmony. Thus, the very rules derived from nature that has been used for finding the forms of the structures can be defined as ‘Form Finding by Following Nature’.

This godly, divine mathematical rule was called ‘golden section’ rule (Fig. 2). It may be the case that the ‘golden section rules’ were evolved studying the nature. There are many different names for the golden section rules, perhaps formulated in renaissance period; The Golden Mean, Phi, the Divine Section, the Golden Cut, the Golden Proportion, the Divine Proportion, Golden ratio.

The Golden ratio is a special number found by dividing a line into two parts so that the longer part divided by the smaller part is also equal to the whole length divided by the longer part. The same can be expressed in an equation form as a/b = (a+b)/a = 1.61803398874989… (etc.). This is also referred as ‘φ’, an irrational number. This irrational ratio ‘φ’ (Fig. 4) is inherent in some or the other way in rectangle, triangle, pentagram, spiral.etc. to make them divine.

This ratio has certain algebraic and geometric properties and is a transcendent number similar to ‘π’ (ratio of perimeter to diameter of a circle). The proportions are divisible infinitely where each subdivision retains its original proportion and is harmonically related not only to whole but to all subdivisions. For example, when a series of golden rectangles are assembled on each other and their outside corners are connected by a smooth curve as depicted in Fig. 4, the culmination is a golden spiral. Luca Pacoli friend of Leonardo da Vinci called this as ‘divine proportion’ and Kepler called the ratio 1.618 as ‘ one of the two jewels of geometry’ and expressed it as a positive root of x2 = x+1.Thus the golden section satisfies the relationships φ2 = φ + 1 = 2.618, φ3 = φ2 + φ = 4.236 and so on. Thus, it has a mathematical threshold of equipartition (symmetry), succession (order) and continuous proportion (regularity) which can give rhythm to any art and physical form.

*Fig. 2: Golden Section Rules Pervasive in Nature*

If a golden rectangle is drawn and a square is removed, the remaining rectangle is also a golden rectangle. Continuing this process and drawing circular arcs, the curve formed approximates the logarithmic spiral, which is a form found in nature. These numbers are encountered in many plants in the arrangement of leaves around the stem, pine cones, seed head packing, and flower petals owing to optimality conditions.

This can be a surmise and an inference by the classical art works like An Old man, The Vitruvian Man (The Man in Action), Mona-Risa by Leonardo Da Vinci and Sacrament of the Last Supper, by Salvador Dali etc. that they followed golden section rules.

Adolf Zeising (1854) was the first to propose the romantic idea that in the golden ratio “is contained the fundamental principle of all formation striving to beauty and totality in the realm of nature and in the field of the pictorial arts.” Two prominent 20th century architects, Ernst Neufert and Le Corbusier, used this ratio deliberately in their designs. More recently, many industrial designs (such as the dimensions of the ubiquitous credit card) have been based on it. The medieval builders of churches and cathedrals approached the design of their buildings in much the same way as the Greeks. They tried to connect geometry and art. Inside as well as outside, their buildings were intricate construction based on the golden section.

There is no evidence in the literature that golden section rules were consciously applied to the design of bridges. However, some of the elegant arch bridges seem to be satisfying the divine proportions. It is interesting to note that the span to rise ratio of Maillart’s famous arch bridges (Fig. 3) varied from 4.9 to 10.7 and were very close to golden section, like Stauffacher Bridge with span/rise ratio 10.7, Bridge at Zuoz with span/rise ratio 10.6, Schrahbach Bridge with span/rise ratio 7.2, Hombach Bridge with span by rise ratio 7.0, Salginatobel Bridge with span by rise ratio 6.92, Schwamback Bridge with span by rise ratio 6.23 and Luterstalden Bridge span/rise ratio of 4.9. The coronation bridge at West Bengal over river Teesta despite having span to rise ratio of 2.06 which is very low for an arch bridge seem to be deriving its aesthetic appeal by golden section rule.

*Fig. 3: Bridges Inadvertently Finding forms by Golden Section Rules*

In India during Rig Vedic age, the architectural concepts were highly developed pertaining to Mandirs (Temples), Mahals (Palaces & Forts) and houses. The testimony of the same is found in sixty chapters of Sanskrit manuscript, Kamikagaria. This explains the types of architecture including testing and preparation of soil and site selection etc.

In fact Manasara written in barbarous Sanskrit between 5th and 7th century AD is fairly comparable to Vitruvius of Roman architecture. The Salva Tantra and Kama Sutra of Vatsayana enumerate 64 arts including Taksanam (Carpentry), Vastuvidya (Architecture), Dhantuvidya (Metal making), Adarajnanam (Mining) and Chalitaka Yogah (Illusionary art).

The Hindu philosophy was among the first to relate the human figure as the basis of a system of proportion, which was years later demonstrated by Leonardo da Vinci and by Le Corbusier in Modular system of measurement. In Hindu philosophy the form of the purasha (human body) was made to suit the abstract idea of the square, as the supreme geometric form.

The basic form of the Vastupurashamandala (Fig. 4) is the square and square is the important and ideal geometric form in Hindu philosophy, which represents the earth. All the necessary forms like the triangle, hexagon, octagon and circle, etc. can be derived from the square. The four sides of the square represent the four cardinal directions. The square also symbolizes the order, the completeness of endless life and the perfectness of life and death. Similarly, the circle represents the universe and is considered as the perfect shape, without any beginning and end, suggesting timelessness and infinity, a typically heavenly feature.

The Vastupurashamandala, having all the geometrical, astronomical and human properties was the basis of plan for Hindu temples. The basic shape acquired by the temple plan is the outer most ring of square of the mandala forms the thickness of walls of main shrine. The central 4 squares acquire the place of the main deity and the inner ring of 12 squares form the walls of the garbhagriha (shrine) and the next 16 to 28 forms the pradkshina patha (holy circling path). These simple divisions of square with many permutations and combinations became the base for the complex structures of the temple; in the form of orthogonal and stellate plans of the temple Therefore the large squares of mandala were divided into thousand squares thus virtually forming a graph paper (Fig. 5) for the architect to facilitate him to add a unit at one side and setting back on the other. The superstructure consisting of plinths, columns, walls, ceilings and crowns depicted emotional poses and events.

Form Finding By Flow Of Forces (FFFF)
The economy and efficiency of the bridges are always achieved by providing the dimensions of the structural components just enough to cater to resist the forces define the concept of ‘Form Finding by flow of Forces’.

According to 25 BC Roman architect Vitruv, the structures should be strong enough to stand up (firmitas), should be robust and lasting (utilitas) and also elegant (venustas).

The architect, Volkwin Marg of Germany emphasizes on truth, goodness and beauty where intellectual truthfulness of structure and form merge, the beauty starts to shine.

As per Tadao Ando, an architect from Japan the ideal structure is one, which is minimized to such bareness that only utility and beauty are left.

David Billington’s concept of structural design was ‘efficiency, economy and elegance’. According to him, engineers’ job is to invent structures that are beautiful, cost less and consume minimum materials.

German engineer Jörg Schlaich advocates for lightweight bridges and structures that are ecological, social and cultural. They are highly sustainable as they use minimum resources, easily constructible and reusable.

Starting from Roman architect Vitruv of BC 25 to contemporary German engineer Jörg Schlaich, and also from Telford’s Craigellachie Bridge to Menn’s Sunniberg bridge ‘Form Finding By Flow Of Forces‘ (FFFF) seem to have been the way of conceiving bridges.

All typologies of bridges including Arch, Cantilever and Cable supported bridges are invariably conceived with the principle of FFFF where force flow is explicitly expressed by their form as depicted in Fig. 6 till the intervention of architects in designing of the bridges.

*Fig. 7: Bridges of Calatrava; Top left: Dallas, Top right: Samuel Beckett, Bot left: Sundial, Bot right: Alamillo*

Form Finding By Forcing Flow Of Forces (FFFFF)
In the urban surroundings, viewers continuously see many public utility structures like, airports, stadiums and museums, etc. from different angles. They demand certain ornamentations and symbolism to contribute to artistic enrichment of the structure by creativity, originality and imagination. These structures require expertise in arrangement of abstract and symbolic visual forms. Architects are well trained in these aspects and they follow the principle ‘form following the functions’. If required, to achieve their ‘awe’ factor by expression of their symbolic visual form, they have to ‘force the flow of forces’ in the structural form in an un-natural and circuitous way.

Thus, the collaboration between architects and engineers started in late nineties for ‘Signature’ or ‘Iconic’ bridges where the bridge forms were found by forcing the flow of forces. The author defines this process as ‘Form Finding by Forcing the Flow of Forces’ (FFFFF).

Some of the outstanding bridges (Fig. 7) by tweaking the flow of forces to the designer’s desire were conceived using this principle. These structures defying the natural flow of forces, despite being master pieces upon completion, acclaimed wide criticism as they invariably overshot the budget to a large extent, consumed much more time than envisaged and constructors were finding them difficult to construct.

The architectural penchant of abstraction and symbolization with visual forms are evident in these new breed of bridges.

*Fig. 8: Determinants of Structural Aesthetics*

Aesthetics Of Structures Defined
The aesthetics of structures are the interplay of 3 prerequisites, viz. structural form itself, surrounding environment and experience of beholders. Author calls these three prerequisites as ‘3 Determinants of Aesthetics’ (Fig. 8). While the structural form is in the manipulative control of designers, the surrounding environment is given and at the best, keeping in view the future development of the surrounding environment, the structural forms are conceived. On the other hand, historically many times, the very conception of structural forms were so powerful that the surrounding area began to be recognized and such structures are called Iconic or Signature structures. The third element ‘experience of beholders’ has a ‘subjectivity’ connotation attached to it as the saying goes ‘beauty lies in beholder’s eyes’. This determinant of aesthetics is generally achieved by visual means like, simplicity (harmony), symmetry (equipartition), order (succession) and regularity (continuous proportion).

The above key determinants are made up of attributes, such as landscape and environmental aspects, approaches and access, materials and their modes of assembly, functional and structural considerations, details, texture, scale, color, form, lighting and shading, ornamental features, drainage, noise barriers, long term appearance, maintenance and operational requirements.

The attributes and their relations are encapsulated in the form of a matrix shown in Fig. 9. All these attributes of 3 determinants of aesthetics can be seen in all typologies of bridges including arch, cantilever, extradosed and cable stayed bridges Fig. 10.

A Model Of 4 Determinants Of Conceptual Design For Structures
Billington’s theme of conceptual design was efficiency, economy and elegance to circumvent numerous anonymous, too frequent ugly utility structures based on mere technical and economic criteria. He coined the phrase ‘structural art’.

It has been the case since long many structures leave much to be desired. The rapid technological progress does not reflect adequately in their variety, beauty and sensitivity. Too often, structural engineers neglect the creative conceptual design phase by repeating standard designs and not sufficiently contributing with own ideas to the fruitful collaboration with architects. Engineers thus often waste the chance to create building culture.

As the bridges are viewed as public utility artifact, the owners are seeking the greater collaboration between architects and engineers for delivering elegance, especially for Signature bridges in the recent past.

The conceptual design of structures can be modeled in matrix form with the following 4 determinants: – Inspiration – Corroboration – Creation – Actualization The author calls this model as ICCA model of 4 determinants for conceptual design of structures (Fig. 11).

In this ICCA model, within the matrix of 4 determinants, the tasks like Abstraction, Conception and Science has to be carried out for the final culmination to reality.

The ICCA model is inspired by R Gold’s ‘4 hats of creation’ and encapsulates certain skill sets required with a pattern. The top row of the model represents the ‘pursuit of reality’ by abstraction from socio cultural context and symbolising into visual forms. The bottom row represents ‘problem solving’ and ‘analytical’ skills, after data collection. Left column represents the strength of drawing skills, sketching and imagining ability. 3D modelling & 3D printing for conformance. The right column represents the computational, structural designing and engineering ability to find the truth.

*Fig. 12: Inspirational Projects for ICCA Model*

The ICCA model has been evolved on the basis of the conceptual design of the above four outstanding bridges in the Fig. 12 and validated through them. In fact, the model belongs to the category of artistic creativity where the idea generation need not be validated unlike scientific creativity where idea generation has to be validated.

The author had a once in a life time opportunity of leading the Joint Venture (Gammon-Cidade-Tensaccai) as a JV attorney for successfully constructing Signature Bridge at Delhi from inception to commissioning. This technically challenging feat was possible due to author’s experience both in design and construction. Thus the Signature Bridge at Delhi was used for illustration of ‘model of determinants of conceptual design’ in the webinar on conceptual design of bridges which is the premise for this paper. However, Sabarmati Pedestrian Bridge under construction is used for the illustration of the ICCA model in this paper.

Inspiration
The inspiration for conceptual design is the abstraction from structural heritage, history of structural engineering, emerging technologies & materials, education and comprehension of structural phenomena. It burgeons and gets actualized by the tenacity and commitment of the leadership of the day. The visual forms are inspired normally by the keen observation of the nature, culture and tradition of the locality when the architects are involved.

Sabarmati River has been an integral part in the life of Ahmedabad since the time the city was founded in 1411 along the river banks. There had been a long-standing acknowledgement that the Riverfront could be turned into a major urban asset from its undesirable state. Proposals to achieve the same had been made since the 1960s and this multi-dimensional project of Sabarmati Riverfront Development was undertaken in 1998.

Sabarmati Riverfront Development is developed in order to provide city with a meaningful waterfront environment along the banks of the river & redefine an identity of the city around the river. It also promotes rehabilitation and resettlement of riverbed dwellers and activities; creation of parks and public spaces; provision of sociocultural amenities for the city.

It has already become tourist attraction and in fact, Indian Prime Minister, Mr. Narendra Modi very famously hosted private dinner to the Chinese Premier, Xi Jinping. In continuation with this historic event further developments as a tourist attraction on either side of the banks was envisaged.

*Fig. 13: Inspired by Flying Colors & Kite Festival*

The events ground has been planned to continue the riverfront’s long historic tradition of hosting events and offer a wide range of venues for local as well as international events. The ground will provide the infrastructural facilities for holding events along the river in an organized manner. The garden has been envisioned as a permanent flower garden spread in approximately 45000 M2 where more than 330 native and exotic flower species can be displayed. An exhibition centre with a trade-fair facility is proposed on the eastern bank of the river. It has been envisioned as a multifaceted convention venue with adaptable spaces that will be equipped with the latest technological equipment and state-of-the-art facilities. Also, The Ahmedabad Urban Development Authority (Auda) is undertaking the development of the central business district (CBD) on Ashram road, the Sabarmati riverfront.

In view of the developments on either side of the river a Pedestrian Bridge joining the promenades of river Sabarmati between Sardar Bridge and Ellis Bridge is necessitated at the flower garden on the west promenade and the informal market on the east side. A pedestrian bridge connects two different places or identities, something that connects and acts as a transition space for people from one environment/activity to another. It also forms an integral part of the pedestrian street life. A pedestrian bridge structure being structurally light, opens many opportunities to introduce various forms and experimentation. Such bridges add to the cityscape and have opportunity of becoming an identity of the city.

Ahmedabad is also known as the city of flying colors. The festival of Uttarayan is when the skies over most cities of the state fill with kites. Since 1989, the city of Ahmedabad has hosted the International Kite Festival (Fig. 13) as part of the official celebration of Uttarayan. Kites are known for its unique shape and vibrant colors. Taking inspiration from this shape, the frame has been derived from this form so that the city can be identified from this kite bridge.

Corroboration
At this stage of conceptual design, the inspired theme is corroborated by studying the existing structures in the region, deliberating with the locals, investigating the site context and studying the close resemblances anywhere in the world. This is also facilitated these days by the availability of data in abundance on Internet very easily as such this determinant is an added recent feature of the conceptual design.

The site investigation has to be studied in detail to ascertain the alignment of the bridge in relation to environmental clearance requirement. The study of the site context also generally involves the following:
– The materials availability and their cost;
– Availability of local craftsmanship, labour and their cost;
– Availability of nearby areas for site establishment, fabrication yard and casting yard;
– The studies required to be carried out for geotechnical data, meteorological data, ocean data, topographical data, bathymetrical data, climatological data, environmental data (earthquake, hurricanes);
– Material properties, accessibility and transport facilities;
– Availability of right of way & local construction rules; – Availability of working seasons & working hours;
– Embodied carbon footprint and embodied energy of materials landed at the site to assess sustainability impact;
– Local design codes and standards for the performance requirements like, Safety (Strength, Resilience, Redundancy, Robustness), Serviceability (Deflection, Vibration), Fatigue and Durability (to realize safety, serviceability and fatigue); Ahmedabad is one of the fastest growing cities in the world. This rapid growth has led to a boom in construction with the recent construction of skyscrapers that accompany the fusion of Persian and Hindu architecture of the Indo-Saracen style.

After India’s independence, Ahmedabad began to accumulate buildings commissioned by such great architects as Louis Kahn, who designed the IIM-A; Le Corbusier with the Shodhany Sarabhai villages, the Sanskar Kendra, the Mills Owners Association building and the Ahmedabad Museum; and Frank Lloyd Wright, who devised the administrative building of Calico Mills and Calico’s dome. Also worthy of mention are the works of B. V. Doshi, author of the School of Architecture, the Gandhi Institute of Labor and Charles Correa, among others. Ahmedabad impresses by the concentration of interesting buildings signed by true stars of architecture and by the characteristic local style of its ancient constructions.

Despite this rich heritage of outstanding architectural culture and also having a beautiful river front in the form of Sabarmati, the city was yearning for a bridge representing the tradition, culture and history of Ahmedabad.

In Sabarmati, as explained earlier a pedestrian bridge was envisaged between Ellis bridge and Sardar bridge (Fig. 14) connecting the promenades spaces at around 290 m of West and East banks. The flower garden and events ground is to be developed on West bank while the East bank will be housing exhibition centre and informal old and antique art market. The proposed bridge has to have facility of landing at two promenades levels (Fig. 15) for pedestrians crossing from one bank to another.

Ahmedabad has a hot, semi-arid climate with marginally less rain with an extremely dry climate.The weather is hot from March to June (40 °C- 27 °C). From November to February the climate is extremely dry (30 °C -15 °C). The southwest monsoon brings a humid climate from mid-June to mid-September. The average annual rainfall is about 800 millimeters but infrequent heavy torrential rains cause local rivers to flood and it is not uncommon for droughts.

These weather conditions warrant shading for pedestrian comfort in walking and also landscape will help to improve micro climate. High Flood Level (HFL) data of Ellis and Sardar Bridge can be used to interpolate HFL at proposed site. Proposed site is approximately 647m from Ellis Bridge and 568m from Sardar Bridge. Existing upper promenade level is about RL 47.35m which is almost same level as that of HFL of proposed site.

It is also necessary to come back to this stage of corroboration after the structure is conceptualised at the stage of creation, to explore as to whether, there is history of constructing similar structures or close resemblances ( Fig. 16 ). If there are any closely resembling structures are available, the collection of data regarding design and construction history, difficulties faced during and after construction, time and cost overruns, etc. will be handy for conceptualisation for avoiding potential bottlenecks on the basis of past experience.

Creation
This is the kernal of conceptual design. The creativity process adopted here is ‘artistic creativity’ where the process is idea generation only unlike ‘scientific creativity’ where idea validation is required in addition to idea generation. The process involves form finding and optimisation, parametric desgn and lately even artificial intellegence is used. Many tools are used in the process of creativity as enumerated below in fib model code 2010.

– Experience, background, feedback, database
– Feeling, sensibility
– Artificial intellegence, imagination
– Capacity of simultaneously analyzing and integrating all criteria and constraints with their respective weights
– Quick Pre-design methods
– Design by sketching (from rough free hand sketches to accurate drawings)
– Visualization tools

The process of creativity for Sabarmati Pedestrian Bridge is depicted in the Fig. 17. The form finding and the optimisation of the form was achieved through imagination and numerous multimedia artistic impressions with various kind of bridges finally narrating the story of kites in elevations, cross sections and colors.

During the converging process, four alternatives were considered for evaluations. The first proposal was multi span network arch bridge. This design option facilitates the smooth transition between the activities on either sides of the river bank by providing recreation on the bridge itself in the form of kiosks, seating, landscaped areas, etc.

By providing a distinctive form with cable stayed arches, it promotes the health of the river through an iconic yet functional space while enhancing the civic fabric. This design option transforms the function of the bridge from merely being a thoroughfare into a destination space due to its enhanced form and additive functions.

The second is a Vierendeel truss roller coaster bridge. The Sabarmati River front is a vibrant recreational and tourist spot. To add to this vibrancy, a roller coaster track was planned over the bridge arch. This Roller Coaster was intended add to the liveliness of the area and to prove to be an iconic structure or a landmark. It was also to add an interest to the existing skyline. There would be an introduction of ‘amusement’ activities along the riverfront making it livelier. It would provide a good panorama of the Sabarmati riverfront and the neighbouring context from a different perspective especially during festival time.

The third option was an extradosed bridge inspired by the theme of kites representing local culture. However, the fourth option was chosen for its grandeur and uniqueness truly representing the colourful kite tradition.

Dual level bridge merging in the central span connecting both the upper and lower promenade was sketched out (Fig. 18), making it more flexible and more people friendly. The entire bridge was conceived to be acting as a truss as the cross members are inspired from the shape of kites. Shading is necessary to provide shelter to the pedestrians from the harsh climate of Ahmedabad. Colorful Fabric panels are thought of as shading features to add to the vibrancy, taking inspiration from the colors of kites.

Initially, single span steel truss was proposed across river. The distance between two promenades is about 290m and hence span of the truss is about 300m. However during the final design stage,

it was realised that 2 more foundations were required by making the span arrangement (100m + 100m + 100m). Shape of truss is largely governed based on the architectural concept. The cross section of the truss is having rhombus shape (Fig. 19) like a kite and in elevation truss is looking like arch. Depth of truss i.e. distances between top and bottom chord is varying along the length. Steel truss is supported on spherical bearings art the two ends on substructure. Foundation consists of 1.2m dia. bored cast in situ piles with pile cap at river bed level.

Apart from the hand sketches, many multimedia visuals (Fig. 20) and even physical models were created before arriving at final concept.

Actualisation
In the actualisation phase of the conceptual design, the ambition to materialise creative ideas are well synthesised. Currently in vogue, the sustainability aspects of structural design as well as construction are addressed in this phase. The designer has to be experienced and updated with the latest construction technologies and development in material science.

Having concieved the structure in line with the other determinants of conceptual design, some of the questions to be answered are:
– How it is going to be built?
– What are the materials to be used for construction?
– Whether those materials are sustainable from the point of view of embodied energy and carbon footprint?
– Whether the structure is sustainable from the point of view of maintenance and replaceability of those components whose shelf life is lesser than structure life?
– Can the construction technology adopted employ local labour to improve the living standards of the local area?
– Can the structure be built without disturbing local life style?
– Can the foundations in the river or sea be built without disturbing marine life etc.?

Sabarmati pedestrian bridge is an unconventional bridge of its kind where form is found by forcing the flow of forces through truss action to get the inspired visual form. The meticulous planning for fabrication of steel tubular joints (Fig. 21) and their erection has to be done keeping in mind the minimum welding and also transportation of the segments to site from fabrication yard.

There are not enough guidelines for multimember nodal joint analysis and design, also where multimembers are joining at a node, the welding has to be done with utmost quality consciousness as these joints are fatigue hot spots. Each Junction was modelled in Revit to check the junction detail and feasibility of construction.

Actual issue of modelling pedestrian movement and structural response is a very complex matter. The overall response depends upon the frequency of structure and the pedestrian movements like walking running, jumping etc.

Fig. 20: Multimedia Visuals and Physical 3D Model

Fig. 21: Actualisation of Multi-tubular Joints

Fig. 22: Joints Planning by Bolting During Erection

Fig. 23: Erection Planning on the River Bed

Fig. 24: Fabrication and Transportation of Members Before Being Erected at Site

The bolted Splicing are proposed to ensure that during erection (Fig. 22) there is less involvement of heavy pieces. The Splice in main chords are provided at 20 m c/c. The chords are welded with Small pieces of Diagonals at nodal location in workshop. The diagonal (X-member) are erected and connected with bolts at the site with main chords. The Joint is masked with 10mm thick steel plates welded in junction with the original pipes to hide the joints.

Underslung erection system (Fig. 23) using the trestle supports on riverbed is being adopted. Temporary steel liners are driven and trestles used to support at every 20m. Fabrication of each 20m length chord and cross members done at shop and transported to site. Special arrangement is made to support the chords and adjust in the position. Middle chords are erected first, then bottom chord is erected. The diagonals are connected between the chords.

Then cross girders are erected connecting middle chords. Then diagonals between bottom chord and op chords are erected and finally the top chord.

As the bridge was unique, the conceptual design for the process of actualisation (Fig. 24) warranted numerous new technologies and methods of construction that are not time tested, were to be evolved for, fabrication, trial assembly and erection before finally gets culminated into a reality as a master piece. At the time of writing this paper, the complete trussed arch erection was over, the concrete deck and other architectural finishing are underway.

Conclusions
In the paper a definition is given to Conceptual design of the structures as “a process of Form Finding by incorporating Aesthetics while being germane to Socio Cultural Context”. The process of form finding has been classified in to 3 categories, viz. by following nature, by following forces and by following forced flow of forces. Author also has tried to give his own model of aesthetics of structures as well as conceptual design of structures. The ICCA model of conceptual design of structures has been validated by the author through illustration of an outstanding pedestrian bridge.

References
1. V.N.Heggade, Conceptualisation of long span bridges for design and construction-some issues, Workshop on “Long Span Bridge Construction by ING-IABSE, Venue: National Academy of Construction Hyderabad (Telangana), Jan 31 – Feb 1, 2020.
2. Heggade VN. Conceptual design of structures. Structural Concrete. 2020; 21:2207–2219. https://doi.org/10.1002/suco.202000399.
3. Dhanajay Patil, Architecture of pedestrian Bridges, STUP inhouse webinar series, 3rd October 2020.
4. V N Heggade, Webinar on Conceptual Design of Bridges, Organised by Indian Association of Structural Engineers, 25th June 2021
5. Y Waigankar, Case study of a design concept of an iconic steel pedestrian bridge with steel tubular sections, Refresher course on Design & Construction of steel Bridges, Indian Association of Structural Engineers, 21st August 2021.

A Vision to Conceptual Design of Bridges

Load Testing of Bridges – Global State-of-Practice