Lift Slab Construction - Research Paper Example

Lift slab construction: Introduction: The lift slab construction is a time saving and systematic construction process in which concrete slabs are cast in-situ, one above the other, separated by membranes, and are threaded through the previously erected columns in order to be lifted to their respective elevations with the help of a hydraulic jack or other mechanical methods. The hydraulic jacks are lifted on the top of each column, and are secured in position with the help of a pinning mechanism. They pick the slabs up using the lifting rods which are connected with the lifting collars, that are intentionally cast in the concrete slab to serve the purpose of lifting. Once the slab has been raised to its desired level initially without proper leveling, it is joined with the columns with the help of the wedges which are “tack welded”. The area all around the slab is connected with the column through complete welding of the interface as soon as the slab has been leveled to attain its permanent position. The slab is held in position by steel wedges or other holding structures designed with the columns. The sequentially raised slabs become floors to higher slabs. Initial casting of the concrete slabs takes place at the ground level. However, with subsequent rising, the successive slabs are cast above the previously raised floors, which are in turn raised to their respective heights. The columns are also extended, in multiples of about a maximum of 5 storeys side by side as more and more slabs are raised to their heights. (webs.demasiado.com, n.d.). As the building reaches its anticipated height and all of the slabs are raised to their respective positions, the hydraulic jacks are removed. The significance of the employment of this technique in the construction of high rise structures is its time-saving and cost- reducing nature and because it offers a more organized, simple and systematic approach towards the construction of high rise buildings. (puka.cs.waikato.ac.nz, 2006). Advantages of the lift slab construction system: Lift slab construction eliminates the provision of beams and provides a smooth and finished surface on both sides. The technique serves to reduce the construction time and material cost each up to about 30 % as compared to the costs incurred on conventional construction techniques. (webs.demasiado.com, n.d.). In addition to that, the casting of concrete slabs right on the site results in cutting the traveling and miscellaneous charges from the concrete plant to the site. History of failures in the lift slab construction process: Lift slab construction has long been a much practiced technique for the quick construction of multi-storey buildings. The use of this technique can be traced back to 1948, when it was first introduced as a method of construction by Youtz and Slick. (911research.wtc7.net, 2010, para. 2). Since the development of this technique, a large number of accidents have been observed on the projects where this technique has been adopted for construction during the phase of execution. Although in a vast majority of cases, the cause of failure has little to do with the methodology of construction and is more about human negligence, yet the lift slab construction technique has been much criticized irrespective of the origin of error leading to failure, given some of the most disastrous accidents that have happened in the slab lift construction projects in the past. Chances of the occurrence of a failure are much pronounced whenever the pulling stress exerted by the lifting equipment in the course of erection exceeds the ultimate capacity of the slab. Also, any flaw in the performance of the lifting equipment brings along with it many chances of failures. Failures can also result if the formwork for higher levels of concrete slab is not adequately braced or supported, since subsequent floors are cast over the underlying floors and then raised to their respective heights. Maintenance of equilibrium is fundamental to an accident-free lift slab construction process. Immense care must be taken to ensure this because very little horizontal forces are required to disturb the equilibrium and hence put the integrity of the structure in danger. The following text discusses some of the examples of failure from the history and analyzes the possible and potential causes of failure as figured out in the subsequent researches conducted on those accidents. 1. The lift slab construction failures near Miami, Florida (1952): In the course of construction of a 79000 kg (about 87.5 tonnes) heavy section of the roof slab in a building near Miami, Florida in 1952, three different types of failures, one after the other were experienced. (Carper and Feld, 1997, p. 310). In the first attempt to lift the slab, the failure was caused by the twisting of a 125 mm (5 inches) diameter pipe which was there to support a jack mounted on the column-top. Dislocation of the holding pipe caused the jack to fall. In the second attempt to lift the slab, weak strands of the flange placed to lift the slab began to distort, resulting in the failure of operation. In the third attempt, failure occurred due to a shear failure in the concrete cap of the pier. Commentary on the causes of failure: Although three different types of failure were experienced in the three attempts to lift the roof slab in this case caused by three different causes, yet the causes can be linked one way or the other to one basic source, i.e. human negligence. In the first failure, probably the pipe was not adequately located or balanced, so it got buckled. The second failure was clearly because of the use of low strength material in the flange meant to support the slab. The error could be eliminated by careful checking of the material strengths before their use in the equipment. Therefore, the error can be considered as ignorance of the concerned engineer. In the third failure, noncompliance with the design and specifications stipulated in the drawings and the contract documents resulted in the development of inadequate shear strength of the concrete cap which caused the collapse. The error can again be traced back to the ignorance of the staff in general and the quality control department in particular. The responsibility of ensuring that the concrete is tested for the cube strength and various other parameters rests fundamentally with the quality control department of a firm. 2. The San Mateo, California accident (1954): In the event of demonstration of the techniques employed in the lift slab construction, a 3 inch deflection caused in the columns because of a load imbalance resulted in a 5 meter sway in the 19.8m * 21.3 m slab followed by a collapse of the whole structure. (Carper and Feld, 1997, p. 310). About 10 people were injured as a result of the accident. Investigation of the causes of failure: At that time, the architect and the contractor were suspected for inappropriate designing and construction of the slab respectively. However, when investigated later, the major cause of failure was found to be the load imbalance. During the time of demonstration, more workers had gathered on one portion of the slab as it was being raised by the jack giving rise to eccentric loading which caused both the columns and the slab to sway towards the mass and led to the ultimate collapse of the whole structure. Thus, eccentric loading on the lift slab construction can be considered as one of the causes of failure. It should be noted here that the error is again human negligence and can easily be prevented by careful location of loads on the slabs so that the location of the resultant load matches with the centre of gravity of the slab. No issue as such was noticed in the mechanism of the lift slab construction process other than the load imbalance caused by the ignorant workers’ crew. 3. The L’ Ambiance Plaza accident in Bridgeport, Connecticut (1987): After the San Mateo, California accident of 1954, very little or no further accidents are recorded in the history with reference to the lift slab construction till the huge catastrophe experienced in the failure of the L’ Ambiance Plaza in Bridgeport, Connecticut in 1987. (Carper and Feld, 1997, p. 310). Prior to analyzing the potential causes of failure of the structure, it will be customary to briefly describe its size and various components it was made up of. As per the plans, the L’ Ambiance Plaza was supposed to be a 16 storey tower with three parking floors. It was essentially designed as a unit of two individual plazas joined together by an escalator. According to (Martin, 1999), the size of either of the two plazas in the plan was 63 ft * 112 ft. Frame of the structure comprised concrete slabs having a depth of 7 inches and columns. (Cuoco, 1992 cited in Martin, 1999). The columns were made of steel and the slabs were post-tensioned. These 16 slabs for the 16 floors were all cast together at the ground around the columns, one above the other with separating membranes between the adjacent slabs. Lateral stability of the unit of the two towers was intended to be achieved with the help of 2 shear walls built in either of the two buildings. The shear walls covered all except for the top-most storey and the storey beneath that. “State of the buildings construction at the time of the collapse”. (Martin cited in 911research.wtc7.net, 2010). Failure: Half of the unit was already constructed at the time of failure. The Western building was the first to collapse followed by the Eastern wing. Failure occurred by the higher slabs banging into lower ones and the shift of energy continued until whole of the structure was converted into a mound of trash in a period as little as 5 seconds. It was a matter of moments. It is considered as the “worst lift-slab construction accident” that caused 28 fatalities in total. (Martin, 1999). An analysis of the causes of failure: Because of this being the worst accident in the history of lift slab construction, the matter was thoroughly investigated and various theories were put forward to explain the reasons of failure, each having its own significance and approach towards the analysis of failure. The causes of failure as assessed by different scholars are summarized below: One of the biggest mistakes made by the managers in the case of the L’ Ambiance Plaza was the distribution of the responsibility of different structural components including various parts of the slab to different subcontractors. “The result was inadequate designs for the slab in the bay with the elevator openings (and) for the slab at shear wall slots in close proximity to exterior columns”. (www.engineering.com, n.d.). The prevalent American building design trends have adversely affected the L’ Ambiance Plaza on the basis of four major areas of deficiency which can be categorized as: 1. Allocation of the design responsibility to more than one contractor. 2. Inadequate specifications for complicated designs of structures. 3. Unestablished safety provisions in various building codes. 4. Tests in the field are generally carried out by technicians instead of experienced structural engineers. (www.engineering.com, n.d.). Miscellaneous causes of lift slab construction failures: 1. Inadequate design and supervision of the construction of different structural members. 2. Multiple contracting system. 3. Undeveloped quality assurance / quality control (QA/QC) system. 4. Improper load distribution on the slabs as they are being raised. 5. Unnecessary gathering of workers under the slab and in their vicinity. 6. Use of low strength materials in construction. 7. Improper adjustment of various components of the lifting equipment. Measures to avoid failures in the lift slab construction: Owing to the large amount of failures in the history of the lift slab construction, ensuring safety should be taken as a big consideration in the various activities involved in the process of lifting slabs. Some of the measures that can be taken in this regard are mentioned below: Design responsibility: The responsibility to design the structural members should preferably be rested with one entity, which would be fully responsible for the adequacy of the design. If it is not achievable, the design must be cross-checked by a capable engineer before it is practically adopted. Lift plan: The slab lifting crew should regularly prepare a lifting plan in order to accommodate any variation in conditions. One such variation is the change in the number of slabs that the jack is supposed to raise to the required heights. Initially, the lifter only picks up the top 2 slabs cast at ground around the columns to a height slightly above the second storey. These slabs are located temporarily at this height. After that, the pull rods of the lifter are lowered down and connected to the underlying slabs, which are raised to their respective permanent positions i.e. the first and the second floor. This is followed by the permanent location of previously raised slabs which is achieved by attaching the pulling rods to a cap fixed on top of the columns which are temporarily extended to allow higher slabs to be raised to their locations. The process continues till all the slabs have been raised to their respective heights. Once this is achieved, the temporary column extension above the top-most slab is removed. (Zallen and Peraza, 2004, p.16). It is quite evident from the lifting operation described above that the lifting crew experiences a variation on daily basis in the number of slabs to lift and the heights to which they are to be raised. Therefore, a lift plan needs to be prepared and modified on daily basis owing to the rapid progress in work. It should be ensured that the lift plan is carefully prepared and strictly followed. Development of a lift plan is a regulatory requirement in most of the projects which involve lifting the concrete in the form of panels, slabs or pre-cast members. Besides, the development and implementation of a lift plan is widely acknowledged as a good and safe construction practice for lift slab construction. Inspection of the lifting equipment: Whenever components of the lifting equipment are adjusted to suit the changed elevations and the scope of load, it is imperative that the equipment is thoroughly inspected and the adjustments are approved by a capable engineer before the access to its use is granted. Tool-box talks: This rapidly changing and risky nature of lifting operation calls for a need to demonstrate the risks associated with the change in the key variables of the lift slab construction process regularly. The lifting crew needs to be educated on the foreseeable risks and the methodologies that can be adopted to mitigate those risks. The engineer-in-charge should conduct tool-box talks and meetings prior to the start of work every day so that these issues can be addressed to the crew as a whole. OSHA regulations: The importance of safety in the lift slab construction process can be estimated from the fact that the Occupational Health and Safety Administration (OSHA) has addressed safety concerns about the lift slab construction in the 29 CFR 1926.705. The general requirements of a slab-lift plan have been addressed in subsection (a) up to subsection (p). (Eckhardt, 2001, para. 3). They can be briefly summarized as follows: 1. The lift plan should be inclusive of drawings and provisions to specify the lifting operation in detail and to ensure the achievement of the building’s stability in the phase of construction respectively. The person who prepares the lift plan should be a registered professional engineer and the owner should ensure strict compliance to the plan. 2. OSHA specifies that the maximum load capacity of the lifter should be displayed on it by the manufacturer, and in no case should the lifter be overloaded. According to the “OSHA 3106, 1998 Revised” mentioned in (www.masonryworktools.com, 2008), “Jacking equipment must be marked with the manufacturers rated capacity and must be capable of supporting at least two and one-half times the load being lifted during jacking operations”. 3. The lifting equipment should be equipped with a safety device. In case of inappropriate functioning of the lifting equipment, the load will be lifted with the help of that safety device. 4. While the slab is being lifted, it should be ensured that no workers, other than those actually involved in the operation shall be allowed to enter the structure unless the structure is adequately reinforced and a registered professional engineer guarantees its stability. Also, the workers should be strictly forbidden to stand under a slab being raised, especially when they are not playing an integral part in the process of lifting. It can be observed that OSHA has majorly covered all necessary areas of precaution. It is essential that the owners take particular interest in the safety of their staff and ensure that the regulations are complied with. Conclusion: Lift slab construction is a quick and cost effective means of multi-storey building construction and has been in use in the construction industry for ages. Although it provides an economic and time saving solution for the rapid development in the construction sector, yet the method needs to be practiced with immense care. This is, probably one of the most sensitive methods of high rise construction. As analyzed in this paper, causes of failure in a vast majority of cases in the history of lift slab construction can be rightly related to human negligence. The parties involved in the construction process should ensure the allocation of the right duties to the right persons, and comply with the rules laid by OSHA for different health and safety concerns of the practice. Also, the QA / QC department needs to be exceptionally strong and dominating in lift slab construction projects. The construction operations need to be carefully supervised and adjustments in various components of the lifting equipment must be inspected and approved prior to the notification to proceed. With the implementation of proper quality control and quality assurance principles, as well as a sense of responsibility in the attitude of the personnel involved in the construction process, circumstances can be improved and the number of injuries and fatalities can be dramatically minimized. Works cited: “Campus (lift slab construction, 1952), Trinity University”. Council of Independent Colleges Washington, DC. 2006. 4 July 2010. . Carper, Kenneth L. and Feld, Jacob. “Construction failure”. New York: John Wiley and Sons, Inc. 1997. 4 July 2010. . “Concrete and Masonry Construction”. OSHA 3106 1998 (Revised). 2008. 4 July 2010. . Eckhardt, Bob. “Developing a lift plan”. 1 Feb. 2001. 4 July 2010. . “LAmbiance Plaza”. 9 – 11 Research. 23 June 2010. 4 July 2010. . “LAmbiance Plazza”. n.d. 5 July 2010. . Martin, R. “L’Ambiance Plaza Collapse”.1999. 4 July 2010. . “Stage 5 - Further expansion”. n.d. 4 July 2010. . Zallen, R. M. and Peraza, D. B., “Engineering considerations for lift-slab construction”. USA: American Society of Civil Engineers. 2004. 4 July 2010. . Read More