CONSTRUCTABILITY IMPROVEMENT OF BRIDGES USING STEPPING FORMWORK

Transcription

1 CONSTRUCTABILITY IMPROVEMENT OF BRIDGES USING STEPPING FORMWORK By Mohamed Emam Abd El-Razek, 1 and Ismail M. Basha 2 ABSTRACT: Many construction systems have been used to build several highway bridges and elevated roads in Egypt. Recently, the stepping shuttering system has been used in Egypt for cast-in-place concrete bridge. This system is most appropriate for concrete bridges of moderate spans ranging from 40 to 70 m. The stepping shuttering system provides appropriate means to satisfy local needs for concrete bridge construction in an urban environment, practically without infringing on traffic or property below. This paper presents an innovative method of construction that has been used in Egypt for the first time to build the 6th October Bridge extension. Issues regarding the constructability and innovation are highlighted, and the significant aspects during the bridge construction are documented. Also, the difficulties encountered and the lessons learned are thoroughly investigated. In addition, evaluation of the system is discussed in terms of construction cost, schedule, and flexibility. Finally, this paper presents an innovative method that offers substantial opportunity for enhancing constructability of concrete bridges using the stepping shuttering system. INTRODUCTION Constructability is the optimum use of construction knowledge and experience in planning, design, procurement, and field operations to achieve overall project objectives (Constructability 1986). O Connor and Davis (1988) concluded that the constructability is enhanced when innovative construction methods are used. They related the innovations in construction methods to innovations in sequencing of field tasks, temporary construction materials/systems, hand tools, construction equipment, constructor-optional assembly, temporary facilities directly supportive of field methods, or postbid constructor preferences. This paper presents an innovative method of construction of concrete bridges that offers substantial opportunity for enhancing constructability using the stepping shuttering system. Many construction systems have been applied in Egypt to build several highway bridges and high performance rates have been achieved. The construction systems most commonly used in Egypt were discussed in full detail by Gab-Allah (1989). The most recent system for bridge construction in Egypt is the stepping shuttering system that has been utilized to build the 6th October Bridge extension in Cairo. The stepping shuttering system enables the construction of bridges by spans, irrespective of the local site conditions. This system is used for cast-in-place reinforced or prestressed box-girder bridge types. Instead of using formwork and falsework supported from the ground, the stepping shuttering transmits the bridge weights into the ground through the bridge towers. Consequently, the construction process can be achieved without interrupting the traffic under the bridge. This system is most convenient for bridge spans varying from 40 to 70 m. For spans over 70 m, the system can be used to build concrete bridges section-by-section of the bridge deck in either direction. There is a need to document the method of construction to assist in achieving higher performance on future projects. The main objective of this paper is to document the significant 1 Assoc. Prof., Constr. and Build. Engrg. Dept., Coll. of Engrg. and Technol., Arab Acad. for Sci. and Technol. and Maritime Transport, P.O. Box 1029 Miami, Alexandria, Egypt. mohemam@aast.edu 2 Prof. and Head, Constr. Engrg. Dept., Facu. of Engrg., Zagazig Univ., Zagazig, Egypt. Note. Discussion open until November 1, To extend the closing date one month, a written request must be filed with the ASCE Manager of Journals. The manuscript for this paper was submitted for review and possible publication on August 1, 1997; revised July 25, This paper is part of the Journal of Construction Engineering and Management, Vol. 127, No. 3, May/June, ASCE, ISSN /01/ /$8.00 $.50 per page. Paper No aspects of construction and management of the stepping shuttering technique used to build the 6th October Bridge extension. Throughout this paper, issues regarding constructability and innovation are highlighted. The system is evaluated to provide recommendations for future work. Moreover, the encountered difficulties and the lessons learned are summarized. This can assist in achieving better performance on future projects. DESCRIPTION OF PROJECT Bridge Layout The length, span, and width of the bridge under study are 1,470, 42, and 18.4 m, respectively. The bridge design is a posttensioned box-girder type with top slab width of 18.4 m, web height of 2.45 m, and bottom slab width of 10 m. The bridge is constructed in an urban area, with a railway passing near the bridge. Under these circumstances, a construction system that is capable of building the bridge without infringing on traffic or property below is essentially needed. Contract and Project Team The owner of this project is the Cairo State Department of Roads and Bridges. The owner offered the contract to the Arab Contractors Co., one of the biggest construction companies in the Middle East, by direct order. In this type of contract negotiation is held between both parties, contractor and owner, until reaching a complete agreement regarding the construction method, cost, and schedule. It was first considered to construct the bridge using the incremental launching truss system, but the presence of several horizontal and vertical curves of the bridge deck ruled out this system. The contractor s decision makers decided to construct the bridge using the stepping shuttering system which was being used for the first time in Egypt. Cost The total direct cost for the bridge was about $13,500,000 ( E 45,000,000), based upon the unit-price contract awarded to the prime contractor. Fig. 1 shows a pie chart representing the bridge construction cost. The equipment cost was about E23,400,000. This included the initial cost of the stepping shuttering system, and equipment for placing and compacting concrete, cranes, and equipment for excavation and piles work. Labor costs involved about E8,000,000 going to the crew of the main contractor and the subcontractors. This included excavation and piles, steel-rebar erection, placing and compact- 206 / JOURNAL OF CONSTRUCTION ENGINEERING AND MANAGEMENT / MAY/JUNE 2001

2 FIG. 1. Project Costs for 6th October Bridge ing concrete, prestressing operations, and operating the stepping shuttering technique. Material costs represented about E10,400,000. This included purchasing materials for both substructure and superstructure. The remaining cost accounted for other miscellaneous items. CONSTRUCTION METHODS AND EQUIPMENT The stepping shuttering system was being used for the first time in Egypt. Lack of experience combined with the tight construction schedule made attention to the construction methods and equipment essential. The following sections describe the construction sequence, the main components of the stepping shuttering, and their impact on the construction methods. FIG. 3. Stepping Shuttering during Advancing Phase Construction Sequences and Time The construction of concrete bridges using the stepping shuttering is performed through two major phases; namely, the concreting and advancing phases. Figs. 2 and 3 show the stepping shuttering system during both concreting and advancing phases, respectively. The main steps for both concreting and advancing phases are discussed as follows: Step 1. The shuttering was assembled on an existing bridge box girder constructed with the traditional formwork Step 2. The shuttering was hydraulically pushed until supported on the existing bridge box girder and Pier A, as shown in Stage 1 of Fig. 4. Step 3. The concreting phase was started by casting the bottom slab of the bridge box girder. Step 4. The bottom slab was cured for 24 h, and then a formwork was laid down on the bottom slab to cast the web and the top slab of the bridge box girder. Step 5. The web and the top slab was cast, and then the FIG. 4. Stages of Systematic Construction prestressing operation was carried out using a prestressing jack of 50 tons with a stroke of 100 mm. The prestressing process was started 3 days after casting the top slab and was carried out for the whole box girder at the same time. Step 6. The stepping shuttering was opened and advanced using groups of hydraulic jacks. The advancing process was performed in several increments until the system was supported on Piers A and B, as shown in Stage 2 of Fig. 4. Step 7. The stepping shuttering was prepared to cast the next bridge span, and so on. Table 1 indicates the main activities required for both concreting and advancing phases, and the average time required to perform each activity. The average construction time for each bridge span constructed with the stepping shuttering system was about 21 days. FIG. 2. Stepping Shuttering during Concreting Phase Stepping Shuttering Components Figs show the main components of the stepping shuttering system during both concreting and advancing phases. The main components, main truss, frames, formworks, and accessories, are discussed in detail in the following sections. JOURNAL OF CONSTRUCTION ENGINEERING AND MANAGEMENT / MAY/JUNE 2001 / 207

3 TABLE 1. Construction Sequence of Concrete Bridge Using Stepping Shuttering Step Activity Time (days) 1 Advancing the stepping shuttering into new position 1 2 Adjusting the stepping shuttering 2 3 Inserting the reinforcement and prestressing cables for 4 the bottom slab and the web 4 Concrete pouring the bottom slab of the bridge box 1 girder 5 Curing the bottom slab of the bridge box girder 1 6 Erecting the formwork for the web and the top slab 1 7 Inserting the reinforcement and prestressing cables for 4 the top slab 8 Concrete pouring for the web and the top slab 1 9 Period for achieving the required strength of the 3 bridge box girder 10 Prestressing operation 2 11 Preparing the stepping shuttering for the advancing phase 1 Main Truss As can be seen in Fig. 5, the main truss has three different parts; namely, main girder, launching truss, and launching nose. The function of the main girder is to support the system during concreting phase. As shown in Fig. 6, the main girder section consists of two web plate girders, that are connected by standard bracings and transverse girders to form a torsionproof box-type section. The main girder was assembled from 26 segments; each segment was 2 m in length and 3.45 m in height. The total length of the main girder was 52 m. This length was sufficient to cast the 42 m in length bridge span, in addition to another 10 m of the next span to avoid the formation of construction joints at the piers. It is important to note that this shuttering can be reused for bridges with different spans by changing the number of the assembled segments for the main girder. The launching truss and launching nose consist of light truss members to decrease the overturning moment during the advancing phase. Frames The main frames used in the shuttering are the support and the hanger frames. As shown in Figs. 6 and 7, each support frame consists of two legs and a horizontal beam. This frame supports the main truss and transmits its load into the piers. The horizontal beam of the support frame is a double cantilever to accommodate the transverse movement of the main truss in order to achieve horizontal curves through the bridge path. As shown in Fig. 6, the hanger frames are series of frames repeated every 6 m. Each frame consists of a cross beam and an L-frame. The cross beam passes through openings in the main girder, and is supported on jacks located inside the main girder. The L-frames support the whole bridge box girder during concreting phase and transmit its weight into the main girder through the cross beams. Formworks Three formworks; namely, base slab, outer, and inner formworks, are used to cast the bridge box girder. Fig. 7 shows a section of the base slab and outer formworks during both concreting and advancing phases. Fig. 8 shows a photo of the base slab formwork during the concreting phase. Fig. 9 shows the inner formwork during both concreting and advancing phases. Fig. 10 shows a photo of the inner formwork before casting the web and the top slab of the bridge box girder. As shown in Figs. 7 and 8, the base slab formwork consists of metal sheathing plates, channel sections, and I-beams to support the bottom slab of the bridge box girder. The base slab formwork has two identical parts connected with pins during the concreting phase. The base slab formwork is supported on the L-frames through a number of I-beams. Sliding plates with Teflon of 5 mm thickness are attached between the L-frames and the I-beams to accommodate the transverse movement of the base slab formwork during the advancing phase. As shown in Fig. 7, the outer formwork consists of a skin for the top slab, a skin for the web, and diagonal struts. Each skin consists of sheathing, channels, and I-beams to support the fresh concrete. The diagonal struts transmit the loads carried by the two skins into the L-frames through the base slab formwork. As shown in Figs. 9 and 10, the inner formwork is separate from the system and used to support the web and the top slab of the bridge box girder. The shores of the inner formwork are equipped with wheeled dollies to facilitate the erection process and the pushing into a new position during the advancing phase. The inner and outer formworks are connected together by anchor tie-rods during the concreting of the web and the top slab of the bridge box girder. The loads carried by the inner formwork are transmitted directly into the base slab formwork. Hydraulic Equipment and Accessories The stepping shuttering is provided with four groups of hydraulic jacks to control the movement of the system during both concreting and advancing phases. Three hydraulic jacks; namely, main, temporary, and pushing jacks, are mounted on the horizontal beam of the support frame, as shown in Fig. 11. A drawing for these jacks is shown in Fig. 12. The capacity of the main jack is 350 tons with a stroke of 250 mm, and its function is to raise the main truss by cm over the seesaw rollers during concreting to prevent any longitudinal movement of the system. The temporary jack has a capacity of 20 tons with a stroke of 2,000 mm, and its function is to FIG. 5. General View of Stepping Shuttering and Its Components 208 / JOURNAL OF CONSTRUCTION ENGINEERING AND MANAGEMENT / MAY/JUNE 2001

4 FIG. 6. View of Main Girder, Support Frame, and Hanger Frame control the transverse movement of the main truss to achieve the required horizontal curves for the bridge. The capacity of the pushing jack is 20 tons with a stroke of 2,000 mm, and its function is to push the shuttering during the advancing phase until reaching its new position over the next pier. The fourth group of the hydraulic jacks is used to control the transverse movement of the base slab formwork. The capacity of this jack is 5 tons with a stroke of 1,800 mm and is used to pull the two base slab formwork parts in a transverse direction across piers. The pulling process of the two base slab formwork parts will continue until the clearance between the pier and the base slab formwork becomes at least 5 cm. This distance is required to avoid obstruction with the piers during the advancing process. An important device provided with the system is the Dywidag bars (D W bars) which are used for adjustment of elevation. Each bar is made of very high steel strength with a diameter of 32 mm, and its function is to support the intermediate part of the base slab formwork during the concreting phase. As shown in Fig. 6, the D W bar consists of two identical parts connected together. The lower part passes through the bridge deck and is connected with the base slab formwork, while the upper part is connected with the hanger frame. The D W bar is raised or lowered by hydraulic jack mounted on the hanger frame to adjust the distance between the main truss and the base slab formwork. ADVANCING OF STEPPING SHUTTERING After performing the concreting phase for each bridge span, the system is advanced to cast the next span. To achieve the advancing phase, the following steps are performed as illustrated in Fig. 7: Step 1. Take off the anchor tie-rods that connect the outer and inner formwork FIG. 7. View of System during Both Concreting and Advancing Phases JOURNAL OF CONSTRUCTION ENGINEERING AND MANAGEMENT / MAY/JUNE 2001 / 209

5 FIG. 8. Base Slab Formwork during Concreting Phase using the temporary jack, until the end can pass the pier by at least 5 cm. Step 9. Advance the shuttering into the next pier using the pushing jack. The advancing process is achieved through several increments where the length of each increment is 180 cm. After achieving each increment, the pushing jack is hydraulically loosened and the stroke returns back to zero to start up a new increment, and so on. DIFFICULTIES ENCOUNTERED DURING CONSTRUCTION 1. During the assembling process some pins did not fit the holes. This was solved by increasing the diameter of these holes during field operations. 2. After connecting the two parts of the D W bar, it was found that the length of the bar was not long enough to be held by the hydraulic jack. This problem was solved by connecting three parts of the D W bar instead of two parts. 3. The inner formwork was hard to withdraw after the concreting phase, and several wheeled dollies were broken. 4. After pulling away the two parts of the base slab formwork, it was found that the clearance between the base slab formwork and the piers could create obstruction with the piers during the advancing phase. This problem was solved by rotating the L-frames to make an obtuse angle with the cross beams of the hanger frames. EVALUATION OF SYSTEM The stepping shuttering system is most suitable for moderate spans ranging from 40 to 70 m. The most common systems in Egypt for this range of spans are the traditional formwork and the incremental launching truss systems. In the following paragraphs, the merits and drawbacks of the stepping shuttering system, compared with other systems, are discussed based on the construction cost, schedule, and flexibility. FIG. 9. Inner Formwork during Concreting and Advancing Phases Step 2. Dismantle the fixing struts that connect the outer and base slab formwork Step 3. Lower the main girder by cm, using the main jack, to rest on the seesaw rollers Step 4. Put off the pins that connect the two parts of the base slab formwork Step 5. Dismantle the two parts of the inside D W bar; the upper part is lifted and the lower part is fixed on the base slab formwork Step 6. Move the outer and base slab formwork by 35 cm, and then fix the outer formwork on the L-frames. Step 7. Dismantle the two parts of the outside D W bar. Step 8. Pull away the two parts of the base slab formwork, Cost The unit cost for the stepping shuttering used in this project is E313/m 2 of the bridge deck. The cost for the traditional formwork and incremental launching truss systems are analyzed for several bridge projects with similar conditions. The results indicate that the unit cost for both traditional formwork and incremental launching truss is E265/m 2 and E169/m 2, respectively, of the bridge deck. For sake of comparison, the unit cost for all construction systems is plotted versus the number of bridge spans, as shown in Fig. 13. As can be seen, the unit cost is decreased as the number of spans increased for both stepping shuttering and incremental launching truss systems. The traditional formwork is depreciated on five spans, therefore, the unit cost-span relationship is constant for each successive five spans. Compared with the traditional formwork system, the incremental launching truss and the stepping shuttering systems prove more economic for long bridges of 1,000 and 1,700 m or more, respectively. Harris (1994) reported that the launching truss and the stepping shuttering systems are most appropriate for long bridges in the range of m. As can be seen in Fig. 13, the incremental launching truss system proves the most economic system for long multispan bridges. Unfortunately, this very economical system can only be applied when both horizontal and vertical alignments of the deck are perfectly straight, or alternatively of constant radius. Schedule Harris (1994) reported that the production rate for the stepping shuttering is about 300 m 2 of deck surface area per week 210 / JOURNAL OF CONSTRUCTION ENGINEERING AND MANAGEMENT / MAY/JUNE 2001

6 FIG. 10. View of Inner Formwork during Concreting Phase construction site as is often the case with the incremental launching truss system. 2. The stepping shuttering system can be adapted for future projects with different spans without adding significant cost. 3. The stepping shuttering system is used many times over, thereby avoiding the need for almost continuous relearning as is often the case with the traditional formwork system. 4. The system is easy to erect, operate, and dismantle. The interviewers concluded that the construction operation using the stepping shuttering system is the most comfortable system compared with other available systems. FIG. 11. Jack Seesaw Rollers, Main Jack, Temporary Jack, and Pushing for spans up to 40 m. In this project the average time for the construction of each bridge span is 21 days which is similar to that reported by Harris (1994). For similar projects, the average time for the construction of each bridge span is 24 and 31 days using the incremental launching truss and the traditional formwork systems, respectively. Based on these figures, the construction progress rate for the stepping shuttering is 14% and 48% higher than that for the incremental launching truss and the traditional formwork systems, respectively. It might be noted here that the indirect cost for the stepping system is expected to be lower than that for other available systems. Flexibility Interviewed project team members agreed that the major advantage of the stepping shuttering system, compared with the other available systems, is its flexibility to accommodate different local conditions. The stepping shuttering system is most appropriate where a bridge is constructed over poor soil, water, or is high above ground level. In cases where bridge geometry is complicated, the most feasible construction solution generally involves falsework to support either an in situ concrete or a precast concrete deck, or a combination of both. In this project the stepping shuttering system is used to construct superelevation up to 5% and horizontal curves with radius down to about 300 m. Other Issues 1. The construction operation for the stepping shuttering system does not infringe on traffic or property below. In addition, it does not require a manufacturing area in the The principal advantage of the stepping shuttering system is the saving in falsework, especially for high decks. Constructability improvements are achieved in this method by eliminating delays in erecting and dismantling formwork for each bridge span, and it allows for higher construction progress rates compared with other available systems, and in performing horizontal and vertical alignments of the bridge deck. LESSONS LEARNED It may be concluded that this project was successful. The success is shown in the ability of the constructor to perform the project within the budget and the schedule. Keys of the project s success can be attributed to the planning and design phases. Designers paid careful attention to the method of construction, e.g., designers selected the shape of piers to facilitate both concreting and advancing phases. Thus, attention to constructability issues in the design phase is essentially required in avoidance of potential problems during construction. One of the major contributions to the project s success was the constructor s 6 months spent planning the project prior to the start of construction. During this period, the constructor was able to carefully plan the construction process. Much attention was paid to the design and manufacture of the stepping shuttering. The constructor hired a consultant to analyze the stresses on the shuttering induced during construction. Most of the shuttering elements were manufactured in the contractor s workshop. Some of the shuttering items, such as the hydraulic jacks, the Dywidag bars, the seesaw rollers, and the sliding plates, were imported from Germany. This required the involvement of a German supplier in the fabrication, erection, and advancing of the stepping shuttering. Meetings were frequently held between the project team and the German supplier to discuss the concerned operations and the construction method. The project team agreed that the communication with the German supplier, who had previous experience on similar works, had an advantage in expediting the gain of experience. JOURNAL OF CONSTRUCTION ENGINEERING AND MANAGEMENT / MAY/JUNE 2001 / 211

7 FIG. 12. View of Hydraulic Jacks FIG. 13. Relationship of Formwork Unit Cost versus Number of Bridge Spans Also, the teamwork improved the working environment, and hence, resulted in a more successful project. At the end of the project, the constructor has the experience and the equipment that enable him to compete with other systems on similar projects. It is worth mentioning here that after the success of performing this project, the constructor was awarded two similar projects to be constructed with the same technique. Hence, the implementing of new technology not only improved the constructability, but also increased the opportunity of gaining new work. CONCLUSION This paper describes an innovative method of construction of concrete bridges that offers substantial opportunity for enhancing constructability using the stepping shuttering system. The 6th October bridge extension built in Egypt demonstrates the effectiveness of the stepping shuttering system for moderate bridge span in an urban environment. Significant data related to the construction system was documented and the difficulties encountered during construction were investigated. In addition, evaluation of the system and the lessons learned were studied. Industry professionals involved in bridge construction may find the description of the construction methods and techniques of practical value. ACKNOWLEDGMENTS The writers would like to express their gratitude to the staff of Arab Contractors Co. (Osman, Ahmed, and Osman, Co.), under the supervision of Arab Consultants (Moharam/Bakhoum). Special thanks are due to project manager Sanaa Mahmoud and site manager Salah for their valuable helps. REFERENCES Constructability: A primer. (1986). Construction Industry Institute, University of Texas at Austin, Austin, Tex. Gab-Allah, A. A. (1989). Special building construction systems Bridge construction. MS thesis, Zagazig University, Zagazig, Egypt. Hampson, K., and Fischer, M. (1997). Structural designs and construc- 212 / JOURNAL OF CONSTRUCTION ENGINEERING AND MANAGEMENT / MAY/JUNE 2001

8 tion technologies for California highway bridges. J. Constr. Engrg. and Mgmt., ASCE, 123(3), Harris, F. (1994). Modern construction and ground engineering equipment and methods, Longman s, London. Mondorf, P. E., Kuprenas, J. A., and Kordahi, E. N. (1997). Segmental cantilever bridge construction case study. J. Constr. Engrg. and Mgmt., ASCE, 123(1), O Connor, J. T., and Davis, V. S. (1988). Constructability improvement during field operations. J. Constr. Engrg. and Mgmt., ASCE, 114(4), JOURNAL OF CONSTRUCTION ENGINEERING AND MANAGEMENT / MAY/JUNE 2001 / 213