Design for a large underground space

Y. Yuan, H.-L. Zhao and X.-F. BaoLarge underground spaces are required when constructing multifunctional buildings. These constitute one of the major engineering projects for the World Exp0 2010 in Shanghai, China. Restrictions caused by a building's function, and requirements for arranging municipal pipelines, mean that options for structural systems are limited. This paper presents a scheme design for a large underground space. To achieve a spacious underground passage and to resist the heavy load of the soil that holds pipelines, major structural systems are taken into consideration. Performance criteria for a scheme design involve safety, serviceability and economics. Subsequently, comparisons of a conventional reinforced concrete (RC) system, a prestressed RC system, and a steel-concrete composite (SCC) system are made with regard to these criteria. It is found that a conventional RC structure cannot provide adequate resistance to heavy load at a large column span. Correspondingly, prestressed RC slabs can replace conventional RC structures with the required structural capacity, but suffer drawbacks because construction space is confined. A composite system is desirable for an underground space with a large span and heavy load. Specifically, it has a convenient construction procedure and high economic efficiency which takes advantage ofindustrial manufacturing of steel members and reduction of concrete formwork.l. INTRODUCTIONUnderground space can be used for subways, passages and pipelines. These spaces are usually limited in their dimensions. A large underground space can meet requirements For expanding capacity for the smooth transfer of large quantities of people. There can, however, be major concerns for their structural systems when these are combined with other underground facilities.This paper presents the planning and design ror a portion oF an underground space. The space in question belongs to a multifunctional building, which is the main building for World Expo 2010 in Shanghai, China. The building consists of two storeys above ground and two underground levels, constructed along 'Expo Axis'. It will bc over 1000 m long and 100 m wide.The underground spaces will function as a transportation hub, and the above-ground section will serve as a sightseeing corridor. The project's distinctive features are that several roads cross the building, and municipal pipelines lie beneath the road surface.Moreover, underground space below the pipelines is needed for a large number of people to walk around. This situation meant that during the design stage for this underground structure, special consideration had to be given to safety and serviceability for bearing heavy load, an open space for passage, and a clear ceiling for ventilation.Usually, design of a structure includes two aspects: a horizontal structural system and a vertical structural system. Because the vertical structural system in this case only relates to the design of columns, the investigation of this paper focuses mainly on the horizontal structural system. There are several conventional horizontal structural systems used in underground construction, such as the waffle floor system, the girderless floor system, and the shallow-beam floor system. In local construction practices lhese are generally conducted with reinforced concrete (RC). As for the heavy Ioad-bearing underground structure with a large space, the limitation of these systems will result in enlargement of the cross-section, an incrcase in reinforcing ratios, and difhculty in avoiding cracks under service load combination.A steel-concrete composite (SCC) structural system offers improved performance as it combines the benefits of two construction materials. RC is inexpensive, massive and stiff, while steel members are strong,lightweight and easy to assemble. The steel and concrete components can act as an integral part to provide strength and stiffness to meet the design objectives. Furthermore, composite systemsfacilitate and accelerate construction through industrial manufacture of steel components and reduction of concreting formwork.During the past several decades, a wide range of research has promoted development of design guidelines for composite systems. Practical aspects of design and construction are included in standards and specifications, for example, Eurocode 4,2 load and resistance factor design specification (LRFD),3 AIJ-SRC code4 and Chinese codes.5,6 These design guidelines cover steel-reinforced concrete {SRC) beams, steel-concrete composite (SCC) beams connected by bonders, and various types of composite beam-column connections. The design for the large underground space of the multifunctional building is based on a contrast of a composite system with conventional ones. First, the comparison is conducted in terms of safety, especially load-bearing capacity. Second, serviceability, including deflection and concrete cracking, is considered. Finally, the comparison is made with respect to economic efficiency. The SCC structural system with high strength has a favourable performance appropriate for a large underground space in this case.2. ARCHITECTURAL SCHEME2.1. FunctionThe multifunctional building should be an important landmark within the Expo district. It was designed to function as a main sightseeing corridor for pedestrians, and a multiple traffic hub connecting mass transit railway with entrance-exit passageways to the Expo site. Four major Expo pavilions, the Chinese pavilion，the theme pavilion, the Expo centre and the entertainruent ventre, will be built along both sides of the Expo Boulevard. Most entrancc-exit passageways are linked with a bus stop, subway station or Maglev station. This building could also provide favourable solutions for the orderly organisation and channelling of the massive passenger flows through the underground and above ground. The multifunctional building is 1100 m long and 110 m wide, as shown in Fig.1. It consists of a three-storey above-ground architecture and a two-level underground space. These layers, with different elevations, as shown in Fig. 1 (b), have their special functions correspondingly.From -6.5 m to -1.0 m is the second underground level （UL-2). It connects with several subway stations and the basements of major pavilions. UL-2 functions mainly as a channel for visiting passenger flow. The first underground level （L-1) is from -1 m to 4.5 m. Municipal pipelines are located in this level, where therearc rour urban roads. Roads traverse the building at the top of UL-1 with an elevation of 4.5 m. The ground floor (GF) is at elevation 4.5 m and extends up to 10 m to provide semi-open space. The GF level serves as a main entrance passage and links with urban roads on the ground, where municipal pipelines are installed below. The first floor (FF), located at 10 m, connects directly with the elevated platforms of the theme pavilion, expo centre and entertainment centre. This level serves as an entrance passage and sightseeing corridor. At elevation of 18.5 m is the roof ofthe sightseeing corridor.2.2. Confinements to underground structureDue to the building's scale and multifunctionality, difficulties arise fo r structural designers fro m the specified structural confinements of the building. As 50% of visitors will use the building's underground levels, a large underground space must be designed to provide a clear ceiling for ventilation and expedite pedestrian flow. Therefore, specific confinements, such as a large span between columns and large net height from floor to ceiling, are set to the underground structure. On the other hand, to ensure the safe and efflcient use of municipal pipelines, they are installed beneath ground level and are surrounded by a substantial amount ofsoil, and this results in a heavy load on any large-span underground structure.The architectural requirements for large span can be seen in the investigated portion, ranging from axis Ato axis F and from axis Y-l to axis Y-11, as shown in Fig.2(a). The span from columns at axis Y-5 to those at axis Y-7 is up to 22.7 m; additionally, the span from columns at other Y axes to their adjacent axes is Approximately 11.3 m. Meanwhile, the spans between columns in the x direction are 8.3 m for the portion covering axes B, C, D and E where pipelines are located.In the elevated view ofaxis Y-5 shown in Fig. 2 (b), the height ofeach level is 5.5 m for both above-ground and underground storeys. To fulfil architectural requirements, the limitation of the nel height is 4 m for UL-2. Accordingly, the depth of the floor system for UL-1 can reasonably be expected to be no more than l.5 m.lt should be noted that the overburden filling in the UL-1 floor frame is about5 m, to hold municipal pipelines and to support urban roads.3. STRUCTURAL SYSTEMSUsually, schematic designs for a structure include two aspects: a horizontal stru ctural system and a vertical structural system. As in this case the vertical structural system relates only to column design, the investigation prescntcd focuses mainly on the horizontal structural system.3.1. Conventional structural systemsThere are scvcral conventional horizontal structural systems applied in underground construction, for example waffle floor, shallow-beam floor, and girderless floor. They are generally built from RC in local construction practices.3.1.1. Waffle floor system. A waffle floor system consists of decussate girders and thin two-way slabs. Girder depth increases sharply with span increment. Thus, if the girder span and the bearing load are large, it is difflcult to master the depth of a floor system within a desirable range. Generally, waffle floor systems adapt to modcrate-scale underground structures.3.1.2. Shalloul-beam system.Shallow-beam systems reduce the net height efficiently. They are also relatively flexible. In this system, beams are substantially wider than columns, but with quite a shallow depth near to or the same as the thickncss Of thc floor slab. The joint and hinge regions must have sufficient tie and stirrup reinforcement. In the casc of large span and heavy loads, excessivc deflection usually occurs andjoints may be hard to handle. This type or structural system is suitable for urderground structures with exacting demands for floor heights.3. 1.3. Girderless floor system.A girderless floor system is also a convcntional type of RC floor in which a flat slab with substantially uniform thickness is supported directly upon column caps, without the intervention of girders. Investigation has shown that the tension stress distributed in the slab can cause severe concrete cracks when spacing between columns is relatively large.3.2. Novel structural systemsPrestressed RC structures and SCC structures are relatively new structural systems For underground construction. There is limited knowledge of their structural performance and implementation.3.2.1. Prestressed RC system.As for prestressed RC systems, the slab or bcam can be prestressed by means of stressing tendons extending longitudinally to create an upward camber to reduce the deformation and prevent the concrete cracking. This system has a favourable performance, especially for heavy loads and large-span structures. Relatively, theconstruction process is more complicated, especially for underground space. Adequate operational space must be provided for tensioning tendons; therefore, it is usually appropriate for shallow embedded structures.3.2.2. SCC floor system. SCC floor systems can serve as highly efficient horizontal structural systems, combining the benefits of structural steel (strength, ductility, favourable tensile performance, reducing deadweight) with those of concrete (stiffness, favourable compressive performance). Investigations have shown that concrete slabs provide lateralsupport for steel beams, preventing torsional buckling.8 In addition, concret slabs also increase the flexural stiffness for a constituted composite beam. Of such high strength and stiffness, a hybrid system would provide larger column-spacing flexibility and would improve floor-deflecting resistance under heavy loads.4. ASPECTS OF SCHEMA TIC DESIGNTo achieve a safe and economical large-span underground structure, designs should be made in the light of comprehensive comparisons. Both structural capacity and economic performance are taken as significant criteria in the comparison of these structural systems.4.1. Structural capacity4.1.1. Safety.Safety is the most fundamental factor affecting a structure's function. Safety corresponding to structural failure is associateed with the ultimate limit state. The major conccrns over structural safcty are static eqluilibrium (locally or for the structure as a whole), the resistance of individual cross-sections (strength and deformation) and structural member stability. The unique feature of the Expo underground space, and the combination of heavy loads and structural confinement, bring about special challenges to structural safety during the design process. Large structural resistance is needed f'or structural members with exacting confinements. The adequate load-bearing capacity of a structural member is the basic factor related to safety.There is an even more serious question concerning structural safety. Structures must be sufficiently robust to withstand extreme events such as earthquakes and other impacts. A large quantity of soil infills the limited part of UL-1; as a result, the non-uniformity of mass distribution leads to inertial forccs with great disparity for adjacent elements. Structural Failure may be induced or excessive displacements may endanger pipelines. Due to undesirable inertial effects, the structure requires reliable connections and stiffness.4. 1.2. Serviceabrlity. Serviceability generally corresponds to the normal operations of the structure associated with such characteristics as deflections and vibration, and visual effects such as cracking due to shrinkage or thermal effects which do not affect structural safety.A required level of serviceability is determined according to user needs in structural performance and related local specifications. In terms of the required performance, the crack width of concrete is expected to be no more than 0-3 mm; the limitation on the interstorey drift is l/800 times the storey height and the limitation on the maximum interstorey drift is 1-5 times the average interstorey drift; the deflection limits for girder are selected as: floor girders live Ioad deflection (L/500) and floor girder deflection (H/400). At service load levels, no plastic hinges are allowed,to avoid permanent deformations under service loads.4.2. Economical considerationEconomic efficiency is also a design consideration, especially for such a large-scale structure, The proposal scheme for this underground space should have a low cost based on ensuring adequate safety and serviceability capacity. Materials and construction cost are the most significant components of the total cost identified in the fmancial estimate.4.2.1. Materiat cost.Material cost varies considerably according to the type of material used in the underground structure. Materials can be decisive when determining the size and quantities of the structural members required for structural capacity, as well as for determining the processing method.There are three traditional materials: structural steel, concrete and steel strand rope. Concrete, which must be more or less associated with steel reinforcing bars, is the cheapest material. Usually, RC is extensively used in underground structures. Structural steel, including structural shapes, plates, steel joists, and steel deck, takes second place. The strength of steel can increase by almost an order of magnitude compared with concrete. The gain in volume and weight savings of structural members is one ofefficiency. Steel strand rope is the most expensive material and its processing is also complicated.4.2.2. Construction efficiency.Construction costs include the machinery and labour costs involved in excavating soil, constructing a foundation pit and configuring the structure. In this project, accounting for the scale of the structure, the amount of work needed to excavate the underground space, the support and bracing of the foundation pit and the configuration of the underground structure, is considerable. Accordingly, the cost for the underground structure's construction may dominate the overall construction cost. In order to improve economic efficiency, the proposed structural system must be easy to construct in an underground space. Meanwhile, shallow foundations are favourable to improve construction efilciency. Obviously, large external loads such as soil weight and vehicular load will be applied to the structure; therefore the structure's own weight should be limited wherever possible.5. COMPARISON BETWEEN STRUCTURAL SYSTEMSComparisons with respect to safety, serviceability and economy were conducted between the structural systems as given below. Hereby, it should be noted that very heavy large loads applied on the floor of UL-1 arise from the overburden of 5 m, as well as the loads transferring from the live load of the road surface and the dead load of the pavement. The total vertical equivalent uniform load acting on the floor is approximately 130 kN/m2. On the other hand, the floor frame system supporting such heavy loads must be designed as a large span structure with columns spacing of 22.7 m and 8.3 m in two directions respectively. The depth of slab and girders is limited to 1-2 m, excluding the height of ceiling equipment.For this project, six girders supported by columns at axis A to F are termed transversal girders. At the same time, girders supported by columns at axis Y are termed longitudinal girders.5.1. Conventional RC systemIn an RC floor system, the assembly of horizontal load-carrying units (consisting of concrete slabs, cross-coupling beams and the corresponding connections) are arranged according to flexible column spacing and load condition. Six transversal girders could be designed as upstanding RC girders with a large depth approximating to 5 m. The upstanding part could be inserted in the soil because transversal girders are parallel and do not intervene with pipelines. However, longitudinal girders at axis Y-5 and Y-7 could not provide the adequate resistance to load effects when they are designed as RC girders with a depth of1-2 m, even if compensated by excessive steel reinforcements. This does not satisfy safety requirements.When it comes to serviceability, the RC structural system has an inadequate performance. Concrete cracking arises easily in large concrete members due to serious shrinkage or thermal effects. Additionally, accounting for the large load, the width of concrete cracks is difficult to control in the limitation of 0.3 mm. Therefore, an RC floor system is not suitable for an underground space with large span and heavy load.5.2. Prestressed RC systemA prestressed RC system has improved structural capacity, particularly, a greater resistance to deformation and cracking. Its favourable structural performance has been proved when applied to structures with a certain span.A prestressed RC floor system could be applied in the area covering axis Y-l to Y-3 and Y-9 to Y- 11 (Jongitudinal side spans). In this area, besides the heavy load and large span of 11m, control of concrete cracking is also a significant concern with any design with large longitudinal length. Longitudinal girders are designed as post-tensioned prestressed RC girders; longitudinal prestressed tendons are installed in concrete slabs aiming to control the cracking. By means of theoretical analysis on crack-resistance, prestressed RC floor systems demonstratc that prestressing is effective in crack confinement.Consequently, when all the dimensions of girders are limited in the expected range, the prestressed RCfloor system not only provides adequate load-bearing capacity but also fulfils the requirements of serviceability. However, for an underground structure, the limitation is that the operating space for an construction is usually confined by surrounding soil strata.5.3. Composite systemGenerally, a composite steel and concrete floor system includes SRC composite beams, and SCC beams. SRC composite beams are shaped steel beams encased by RC. The almost fully composite interaction is achieved merely by chemical bonding and natural f'riction if the acting loads are no more than 80% of the ultimate capacity. SCC beams are composite flexural members formed by connecting bare steel beams and RC slabs. The interaction is achieved through the use of headed studs or other bonders along the steel girder-concrete slab interfaces.Composite floor systems could be applied in the area from axis Y-4 to Y-8. in this area, the major concern is that the transversal spacing between columns is large, up to 22.7 m. High structural capacity is neccssary. Transversal girders are also designed as upstanding SRC girders. Longitudinal girders are also designed as SRC girders. Moreover,longitudinal secondary SCC beams could be arranged between transversal girders with a distance of 22 m. The shaped steelin SRC and SCC girders shares strength resistance and obtains great improvement for the entire strength of a composite girder. With the limited dimensions, composite structural members could provide adequate strength. Additionally, reliable connections between composite girders can be processed successfully to achieve the required stiffness and strength for the structural system. A composite floor system is found to be compliant with safety requirements.Accounting for the composite effect, concrete slabs act as an integral part of the beam and lead to an increase in stiffness of composite girders. The deflection under service load is in an allowable range. Furthermore, the concrete usage decreases and shaped steel shares in resisting loads, meaning the concrete cracking could be controlled efficiently. A composite system could fulfil serviceability requirements.As mentioned above, a composite structure is capable of ensuring system stiffness, strength and structural integrity, and providing desirable structural capacity.5.4. Economic efficiencyAs for economic efficiency, a massive RC structural system will not exactly be economical. Due to enlargement of member size, the cost of material including concrete and steel reinforcements, forming work, casting and concrete maintenance would increase considerably. Additionally, heavy structuraJ self-weight and beam depths for an RC system raise the foundation cost considerably.A prestressed RC system is a comparatively light-framed system, with relatively small structural members, even for a large span. However, its construction process is complicated. The cost for stressing tendons and construction, especially in an underground space, would be great.Composite systems have high economic eftlciency. Cross-sections can be small for large-span girders; the foundation cost can be low for light self-weight. The steel frame erected may first be used to anchor the concrete formwork instead of special formwork, thus enabling speedy construction. Furthermore, steel members are usually manufactured industrially, which lowers construction costs.The consumption of several structural systems for underground structures, shown in Fig. 3, is compared with the same overburden and different spans. The SCC system has the highest economic efficiency with a span larger than 10 m. Therefore, the composite system has the most favourable economic performance for an unclerground space of a multifunctional building.6. CONCLUSIONSThis paper investigates a schematic design for the underground space of a multifunctional building in the World Exp0 2010 project. According to thearchitectural function and structural confinements, floor depth is limited and span between columns is large to attain efficient use of space. The more serious challenge is that this large underground space must be subject to heavy loads.In order to propose a desirable structural system whichcould overcome challenges, performance criteria involving safety, serviceability and economics are given specifically, and then several structural systems including a conventional RC system, a prestressed RC system and an SCC system are compared with respect to these criteria.A conventional RC system is not suitable for a large-span underground space. Due to the floor depth confmement, an RC structural system does not have adequate strength and cannot satisfy safety requirements. Meanwhile, the concrete crack is difficult to control within acceptable serviceability limits.A prestressed RC system, within the confinements, could provide the required structural capacity. In particular, concrete cracking can be controlled effectively by means of the prestressing. Its construction process is complicated however, and operating space is limited in an underground space. These factors may result in undesirable economic performance. In the structural scheme, it is applied in the area oflongitudinal side spans.An SCC structural system is desirable and feasible. It has the potential to offer adequate resistance for safety, and satisfies serviceability requirements. Moreover, a composite system is economical, especially when employed in an underground structure with a deep overburden and large span.ACKNOWLEDGEMENTSThe authors appreciate the very constructive comments from the reviewers. The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (grant No. 50678133). The authors also appreciate the support of the Bureau of the Shanghai World Expo Coordination.REFERENCES1. VIEST l. M., COLACO J. P., FURL,ONO R. W. and GRIFFrS L. G.Composite Construction: Design for Buildings. McGraw-Hill,New York, 1997.2. EUROPEAN COMM丌TrEE FOR STANDARDIZAnON. Eurocode 4: Design ofComposite Steel and Concrete Structures-Part l.1: GeneralRules and Rules.for Buildinqs. CEN, Brussels, 1994.。