Achieving constructability in structural design for building structures Jan A Wium Professor Stellenbosch University Stellenbosch, South Africa
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Jan Wium, born 1957, received a civil engineering degree from the University of Pretoria and worked as consulting engineer. In 2003 he joined Stellenbosch University and fills the Murray & Roberts Chair in Construction Engineering and Management, focussing on risk and design management.
Summary Constructability in structural design is investigated by using a case study of a 21m concrete roof structure as basis for the evaluation. The case study shows that even in a case where the designer paid special attention to constructability during the conceptual phase, the contractor may still have special preferences which differ from the concept design. Therefore, by just relying on lessons learned programmes and previous knowledge, a design may not be suitably constructible for a given contractor. It demonstrates that the conceptual design is a function of a number of factors which need to be considered in conjunction with each other, rather than a merely counting on previous experience and knowledge. In addition, it is shown how the designer should consider all possibilities, even beyond the direct environment of the specific structural context. Keywords: Concrete construction, constructability, roof construction
1.
Introduction
Constructability can be described by “the optimum use of construction knowledge and experience in planning, design, procurement, and field operations to achieve overall project objectives” [1](CII, 1986). This can be phrased as “the extent to which the design of a building facilitates ease of construction”, a formulation which intuitively flows from the concept. A lack of constructability in a design can cause serious repercussions in the construction of a project, and mainly arises when a designer does not implement sufficient construction knowledge in the preparation of design concepts and details. A lack of construction experience amongst the design professionals and insufficient guidelines or tools to assist the designers, are probably the reasons for a lack of constructability in many designs [2]. Although interaction often takes place between bridge designers and contractors, the design-bid-build procurement methods which are prevalent in building construction projects, often prevent the embodiment of constructability in design concepts. Such constructability knowledge is an aspect which needs to be applied during early project phases, in which the application of constructability knowledge is particularly desirable [3] [4]. This paper addresses the subject of constructability in structural design. The paper takes a critical look at the way in which designers need to find solutions for structural concepts and details. A case study of a 21m clear span concrete roof structure is used here as basis to explore the principals involved in achieving a constructible design. The designer and contractor considered several options for the roof which covers a 9m storey height. These options included composite steel and concrete construction, a post tensioned voided slab and other slab options. The physical environment imposed special constraints on the construction method. Similarly, the preferences of the client for a flat soffit could not be ignored.
Extensive input was obtained from a contractor for the roof concept during the pre-tender design phase. However, the contractor appointed after a tender process, decided to pursue another concept which was finally constructed and which also suited the client better than the original design. Based on this case study, the paper identifies the relevant considerations for constructability. Although it is often argued that experience and lessons learned programmes will assist designers in better designing for constructability, it is demonstrated that constructability is achieved through collaboration and design management processes, in addition to lessons learned programmes.
2.
Project description
2.1
Cellar description
The facility to be constructed was a modern wine cellar, consisting of four floor levels and a flat concrete roof. The footprint of the structure has dimensions of 21m x 65m and was constructed in the hills surrounding the picturesque university town of Stellenbosch, in the heart of the Cape wine lands in South Africa. The concrete structure of the wine cellar was constructed on the slope of a hill, with each successively higher floor having a larger floor area (Figure 1). The French owner of the facility required special concrete finishes for the exposed concrete structure, using pigmented concrete and giving it a distinct yellow/camel colour to blend with the soil colour of the surrounding area. To facilitate temperature control a flat concrete roof was desired, and for aesthetic purpose the architect also wanted a flat soffit for the roof. Figure 1: Frontal view of the cellar The roof span of 21m covered the fermentation tank hall with a clear height of 9m from floor to ceiling. A breath taking view from the tank hall onto the surrounding mountains resulted in the requirement from the client and architect for a 9m x 21m glass façade on the one end of the building, overlooking the valley towards the mountains (Figure 2). A concrete framed opening housing the glass façade provides lateral structural stability for the roof diaphragm at the one end of the building, while a reinforced concrete shear wall provides lateral stability at the other end. Apart from the fourth floor level which hosts the fermentation tanks, the structure consists of reinforced concrete beams on a column grid of 5m x 7m, supporting concrete floors. Side walls are 400mm – 500mm thick solid concrete. No columns are allowed in the fermentation tank hall, resulting in the 21m span for the roof, spanning from concrete wall to concrete wall with a 9m clear height. Figure 2: View from the tank hall 2.2
Procurement model
The private French owner of the wine estate appointed a professional team consisting of a French architect who collaborated with a local South African architect, local South African engineering
consultants and a local quantity surveyor. The professional team was appointed to prepare contract documentation and a preliminary bill of quantities to be used in an open tender process for selection of a contractor. The standard JBCC [5] contract document was used for the project, as is often the case on standard design-bid-construct building projects in South Africa.
3. Design process 3.1
Conceptual design
The architectural team developed a concept for the building layout in collaboration with the client, whilst the structural engineer provided input on structural element sizes and lateral stability requirements. One of the dominant load conditions for lateral stability is seismic activity since Stellenbosch is located in a zone with moderate earthquake magnitude. An item that received particular attention by the professional team is the concept for the roof structure. As mentioned above, the roof has a 21m clear span covering a tank hall with a height of 9m from floor to ceiling. For temperature control purposes it needs a concrete roof slab and both the client and architect expressed a preference for a flat soffit. An issue which the professional team considered to be of particular relevance for the construction of the roof is the height of the roof above floor level (9m). If formwork was to be supported on false work it would potentially result in a costly solution. Apart from the height, the fact that the building is located on the side of a hill would render access from the side of the building particularly difficult. Two options were considered. The one consisted of post tensioned concrete beams at 5m spacing supporting an in-situ concrete roof slab. The other consisted of steel girders at 5m spacing with precast floor planks. The structural engineer convinced the client and architect of the latter solution where steel girders would be spanning 21m and supporting prefabricated concrete hollow core floor panels which would be covered with a structural topping. Although a flat concrete soffit would not be achieved, placement of the floor panels on the steel girders would eliminate the costly false work and post tensioning of the alternative option. Options were considered to provide an architectural ceiling to create a flat soffit view. Although the project quantity surveyor had no preference from a cost point of view, the main advantage of the steel girder option was the potential time saving due to the use of prefabricated steel girders and hollow core concrete roof panels which could be fabricated off site, independent of the progress of the structure. Considering a tight time schedule, this became the option of choice. 3.2
Confirmation of the chosen option
The structural engineer was adamant that the solution for the roof should be a constructible option. Prior to tender, he therefore approached an experienced contractor to discuss with him the erection procedure and potential risks. Items that were of particular relevance included: - Fabrication of the 1.35m deep plate girder and available welding expertise in fabrication yards in the region; - Transportation of the girders, which required splicing of the members at 7m lengths; - Placement of the girders which could : o either be bolted together on the ground and lifted into place (5t total), o or be held in place by up to three cranes at a time (1.65t each), o or be supported on false work from the floor; - Exposing workers to working at considerable heights, - For the placement of the elements, mobile cranes could not access the sides of the building and would require a long and high reach.
The advice from the independent contractor was that this was a workable solution that would certainly save time when compared to the all-concrete option. On advice from the contractor allowance was subsequently made for additional crane costs in the quantity surveyor’s cost estimate. The professional team proceeded to prepare tender information based on the option with steel girders and prefabricated hollow core floor elements.
4. Appointment of contractor and construction phase Following an open tender phase, for which a number of experienced building contractors were specifically invited to tender, the contract was awarded to a different contractor from the one who had given advice on the roof concept. As construction started the appointed contractor approached the client and professional team with a request to reconsider the concept for the roof. He offered to consider alternative options within the cost and schedule constraints. Specific problems that he had with the conceptual design included:
Figure 3 : Void formers before concrete placement - unfamiliarity with erection of large steel plate girders, - quality management on a concept that he was not familiar with. The contractor offered an alternative solution of using a floor system consisting of a reinforced concrete slab with spherical polystyrene formers. The structural engineer was not content with this option due to potential deflection and shear capacity problems. However, as a counter suggestion the concept of using an 800mm deep post tensioned concrete slab with 550mm diameter pipe-like void formers was put forward, an option not considered previously (Figure 3). This concept is often used in road over road bridges, and not in building construction. Although the building contractor had no experience with the concept, he was prepared to accept it due to his experience with reinforced and post tensioned concrete slabs. Although the concept would require high false work to support the formwork, the contractor was able to programme the works in such a way that the original schedule could still be met, thereby obviating the need for the time saving offered through the original concept (steel girders). The proposed solution also satisfied the client and architect who were both keen on the resulting flat soffit and limited depth in comparison with the steel girders.
5. Observations from the process Designing for constructability is a principle attributed to, or expected from, an experienced designer. In this case study the designer recognized that he may need more input and made a point of obtaining the opinion of an experienced contractor in shaping the roof concept prior to tender stage. On the face of it, it would thus seem that the designer followed the necessary steps to achieve a constructible concept. Nevertheless, regardless of this ideal procedure, the appointed contractor chose to investigate other options. Very often, construction projects for building use design-bid-build procurement processes, which do not allow collaboration between designers and appointed contractors at an early stage. In this case, sufficient time was available for the project team to reconsider the roof concept after award of contract. This may not always be the case. Here, the excellent collaboration between team members resulted in a solution which suited everyone. The principle reason for the appointed contractor to seek an alternative concept was his lack of experience in steel and composite construction. Considering the fact that the designers had originally identified the difficulties with the erection of the steel beams (height and access), and the fact that the project would most probably have been tendered by building contractors who, as a rule, construct mostly in reinforced concrete in the region, should perhaps have guided the designers away from the steel girder option. Although this expertise is available from contractors who predominantly operate in the industrial structures market, composite construction is not common in building construction in South Africa. Another observation relates to the structural concepts chosen by the designer. The designer originally only considered two structural concepts for the roof, an option with steel girders and precast floor elements, and an option with in-situ post tensioned concrete beams with a concrete roof slab. He was not familiar with the appointed contractor’s proposed method of a concrete slab with spherical void formers, a structural system new in South Africa at the time. Although he was familiar with the pipe-like void formers, he did not associate this concept, often used in bridges, with a building structure. It was only when he was confronted with the spherical formers that he realised that the pipe-like void formers could also be an option.
6. Lessons and conclusions Several factors play a role when a constructible design is to be chosen. There could be others (such as maintenance, community preferences, etc.) but factors that may play a role as identified in this case study include: - Previous experience of the particular contractor and professional team, - Time constraints on the construction schedule, - Cost of the structure, - Site access and surrounding structural configuration, - Safety of erection, - Available skills, - Client requirements including aesthetic considerations. From this case study it can be deducted that conceptualizing a constructible design is a multifacetted process. It is not a one way process, but one where the above factors need to be considered in relation to each other. These factors do not stand in isolation, and it may not be possible to lay down quantifiable rules to determine the constructability of a design without considering the factors as part of a system. In other words, a structural concept which applies for one project, a specific client and professional team, and a chosen contractor, may not be the solution for a similar project in another environment and with other team players. It does not mean to say that the conceptual design can only be determined in collaboration with the appointed contractor, but it does mean that the professional team need to take into consideration the
known characteristics of the specific environment when the concept is developed in collaboration with the client. The specifics could include the preferences, knowledge, expertise, and risk attitude of potential contractors. In addition, another important lesson learned from this particular project is that no party can take the decision for constructability in isolation. Project participants may each have different preferences and priorities, which are not only dependent on the role that they play (client, consultant or contractor), but constructability is also a function of their individual characteristics and experience. Moreover, in order to consider all viable options, designers should challenge themselves to consider structural concepts and options that extend beyond the norm of the environment that they are dealing with. In this case study, a concept normally used for bridge structures could be adopted for a building structure, but only after the train of thought of the engineer had been prompted by other considerations.
7. References [1]
Construction Industry Institute (CII). 1986. Constructability: A Primer. Texas, Austin, University of Texas.
[2]
GAMBATESE, J. A., DUNSTON, P.S., & POCOCK J. B. 2007. “Introduction” of Constructability concepts and practice. ASCE. Virginia, Reston.
[3]
POCOCK, J.B., KUENNEN, S.T., GAMBATESE, J., & RAUSCHKOLB, J. 2006. Constructability State of Practice Report. Journal of Construction Engineering and Management, 132(4): 373-383. ASCE. SONG, L., MOHAMED, Y., & ABOURIZK, S. M. 2009. Early contractor involvement in design and its impact on construction schedule performance. Journal of Management in Engineering, 25(1): 12–20. ASCE. JBCC 2000. Joint Building Contracts Committee Inc., Johannesburg, South Africa
[4]
[5]
