Lift Slab Ppt

Presented byShweta Choudhary Roll No.Advanced Building Construction YCMOU M.Arch. Sem2 Aayojan School of Architecture.

Faculty InchargeProf.Archana Rathore

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Lift-slab construction was a revolutionary idea invented and developed in the early 1950s by a collaboration of Philip N. Youtz and Thomas B. Slick, resulting in what came to be known as the Youtz-Slick Lift-Slab Method of Construction.

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Since then it has become a basic method of economical concrete construction, especially for office buildings, apartments, parking garages, hotels and other structures characterized by repetitive framing from floor to floor.

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Basically, the method entails casting floor and roof slabs on or at ground level and jacking them up into position.

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Flat plate floors are commonly used because they are so well suited to stack-casting, requiring for work at only the edges of the slab and at floor openings.

The most famous IIR (Institute for Inventive Research) invention was the Youtz-Slick lift-slab method of building construction, developed concurrently by Mr. Slick and Philip Youtz, a New York architect, in 1948.

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Originally , lift-slabs were reinforced with mild steel reinforcing, which limited the column spacing or required very thick slabs. With the advent of post-tensioning , however the column spacing was increased and the thickness of the slabs were reduced. Contemporarily, all lift-slabs are post-tensioned. 

Photographs of what is believed to be one of the earliest, if not the first, lift slab structure constructed in the US.

Developments in the construction field have changed lift slab techniques over its 33 year history, increase use of pumping and prestressing has made cast-inplace flat plate work more efficient.

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The traditional lift slab construction sequence as illustrated in the figure. Special lifting collars or share heads are provided in the slabs at the columns. Bond breaking compounds are applied between slabs to separate them. After the slabs have cured long enough to reach a prescribed strength powerful hydraulic jacks mounted on top of the columns lift the slabs into their respective positions. A console connected to each hydraulic jack synchronizes the number of turns of the check nuts to assure that the concrete slabs is being raised the same amount at all points.

Further developments…. 

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In the conventional system hydraulic jacks were mounted on top of the columns, limiting the height of the columns and making it necessary to remove the jacks before splicing on the next upper column tier. The new approach allows columns to be erected as tall as 6 stories without filed splices. The jack is supported off the column by a welded plate that is later used to support the slab shear head.

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Recent changes in lift slab construction include supporting the hydraulic jack off the column by a welded plate. The old approach used jacks mounted on top of the columns. Columns

Another refinement controls the amount of lift at each column. A steel tape runs from each column to a central sensing device in a console which monitors the relative movements and automatically operates the pumps switching them off and on as necessary to keep the floor perfectly level as it moves upward at approximately one inch per minute. Safety is provided by electrically driven nuts which follow the movement of the hydraulic cylinders. If a gap develops above a nut, an alarm first goes off. When the gap grows to more than 3/8 inch, the associated pump stops in turn stopping the entire lifting operation. This assures fall back protection in case of hydraulic failure.

Advantages 

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The big advantage of erecting concrete buildings using lift slab construction is elimination of most form work; only the sides need to be formed , an important factor in areas where labor cost are high. Lift slab can be used for heights upto about 16 stories. Economical column spacing ranges from 22 to 32 feet. Columns may be pipe, tubes or wide flange sections; concrete building columns may be used in 3 to 4 story buildings not requiring splices. Another advantage is reduced handling and hoisting of materials and supplies that can simply be placed on top of the slabs and lifted with them. There is little need for finishing the bottom of the slabs, since they will be as smooth as the floor finish of the slab below and thus the bottom of the slab can be used directly as a ceiling. The technique offers good fire resistance and good acoustic ratings. Mass designed into walls, floors and roofs helps to reduce the effects of daily temperature changes.

Limitations •The method has limitations too, the principal one being that buildings must be specifically planned for the same , or it will not have any economic advantages over conventional construction.

The process ….. Stage 1 Preparation The "Lift-Slab" system of construction is a system of mechanically elevating horizontally inclined partitioning slabs on pre-positioned columns. Diagram #1 shows a pair of columns with five slabs prepared at ground level Diagram #2 shows two lifting boxes mounted, one on each of the two columns, and a single slab elevated to the first floor level

Stage 2 Simple and Time Saving The slabs are easily fabricated on site at ground level and systematically elevated to their required positions. This graphic process description shows two columns at each progressive stage. The system is capable of accommodating sequential single slab lifts on up to 40 columns, a lifting box mounted on to each of the 40 columns

Stage 4 Simple Pinning System

Stage 3 Central electronic Control 

The slabs are sequentially lifted and easily secured in position, creating flooring at the required levels. Central electrical control console regulates the lifting process and maintains horizontal alignment and flatness for slabs each weighing up to one thousand five hundred tonnes.

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The Initial maximum height of the columns is five storeys The lifting boxes are sequentially raised to higher positions to enable the partitioning slabs to be lifted to the required levels The lifting boxes and the partitioning slabs are held in their relative positions by a simple pinning system

Stage 5

Stage 6

Further expansion

Flexibility

•The system is easily expanded to create higher rises

•The vertical columns are extended in stages, increasing the height in multiples of up to five storeys •The first five slabs are elevated further to make room for the subsequent five slabs •Buildings of up to thirty storeys have been successfully constructed using the "Lift-Slab" method of construction

When all slabs have been fabricated and lifted to pre-determined positions, the final positioning takes place starting from the bottom and continues upwards 

Diagram #12 shows three partitioning slabs in their final position, higher slabs ready for further elevation 

In the case of higher rise buildings prefabrication of slabs is carried out at a higher level making the first floors free for subsequent construction works 

Stage 7 Final Positioning 

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At the completion of the Lift-Slab erection process, the LiftBoxes are simply lifted from the tops of the columns Special design parameters allow for generous spacing between columns After completion of the Lift-Slab erection process columns may be eliminated in special cases to provide larger unhindered floor spacing

High Rise Benefits •No horizontal beamsthe slabs are self supporting and create rigidity to the final structure. •Material saving-upto 30% savings in cement and reenforcing bars. •Time Savingtypically 30%saving in construction time compared with traditional construction methods.

General Considerations…… The lift slab method of construction presents certain unique engineering considerations, during both the design phase and the construction phase of a project. These considerations must be recognized and adequately addressed during the structural design, during the planning of the lift-slab operation, during the preparation of the shop drawings, and during the construction. Structural engineering is required in all of these phases by various engineers employed by different organizations and with different responsibilities.

THE PRINCIPLE OF LIFTING THE LOAD… 

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Main components of machine are the cylinders and two threaded winches between two steel beams. Winches are connected with screw bundle concreted into the ceilings, lower bridge of the machines is underpinned by steel pipes. Cylinders lift up the upper bridge together with winches, so the ceiling's bundle is lifted up. Cylinder's length of stroke is 10 cm(clearence). Lifting winches lean against lower and upper bridges with nuts (female screws). When piston reaches the clearence, weight of lifted bundle is loaded over from upper bridge to lower bridge and upper bridge is let back to zero setting. (Nuts on winches are driven by cog-wheels, so that the winch gets into lifting position again.) Lifting is carried out in 10 cm steps so, that this way is also done in controlled parts.

Lifting jack

Side elevation of a building being constructed in accordance with with this technology

Side elevation showing an additional slab being constructed in preparation for raising.

The lifting process…..

video

Since the development of the Youtz-slick system, other lift-slab system have been developed. One of these is the Lift-plate system, developed by Peter Vanderklaauw, which has many similarities to the Youtz-Slick system. Other successful systems have been developed as well such as the Multileveling Component system, developed by Kolbjorn Saether, and the Cortina system, developed by Pablo Cortina Ortega. Various new methods with little variations in concrete casting methods and lifting processes have also developed. We can here discuss the Youtz-Slick and the Lift-plate methods since they are the two systems most widely used.

•The foundations are constructed and backfilled and then the slab on grade is constructed. • Openings are left in the slab on grade to permit the erection of steel columns which are then erected and plumbed. • In the Youtz-slick system a lifting collar is cast into each slab at each column. This collar provides a method to hook up lifting rods at each collar, so that the slab can be lifted, and a method to secure the lifting collar to the supporting column, either permanently or temporarily. •When the first tier of columns is erected, all the lifting collars for all the slabs to be lifted are installed over the columns of this tier and are temporarily suspended above the ground .

•The lowest slab to be lifted is constructed first by lowering its lifting collars into place on the slab on grade, erecting the side forms (which may be high enough to provide edge forms for all the slabs to be lifted), installing post-tensioning strands and reinforcing steel, and placing, finishing, and curing the concrete. •A bond breaker is sprayed onto the finished floor slab, and each of the other floor slabs are constructed on top of the slab below in a similar manner. When the slabs reach the proper strength, the slabs are post-tensioned. •The building may be divided into sections for lifting as shown in Figure 1-4, depending on available equipment and the size of the floors. •The sections are joined by pour strips after lifting has been completed.

•In the Youtz-slick method, the jacks are first mounted on top of the columns of the first tier, Two threaded lifting rods are attached to each jack, one on each side of the column. •These rods are then attached to the lifting collars of the slabs to be lifted; if more than one slab is lifted at a time, the rods are attached to the lowest of these slabs. •The operation of the jacks is normally coordinated by a central console on the roof slab, in order to keep the slabs level within a specified tolerance (typically a fraction of an inch).

•If necessary, the jacks can be manually operated to adjust the elevation of the slab at individual locations.

•The Lift-plate system differs from the Youtz-slick system in that a pair of jacks are mounted on each column, one on each side. •This feature makes it possible to have high tiers of many stories, up to about six stories. •The length of column above the jacks is unloaded and does not affect the stability of the column. Whereas a Youtz-slick lifting collar is one piece and needs to be threaded over the columns of the first tier, a Lift-plate lifting collars comes in two pieces which are bolted together after being put in place. • The lift-plate system is the same as the Youtz-slick system in regard to the construction of the foundations and slab on grade, and the casting and posttendering of thee slabs to be lifted.

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A new lift slab system has evolved in which concrete bearing walls are lifted simultaneously with the slabs. Concrete bearing walls are cast flat in the same stack with the slabs and attached to the slab with loops of plastic rope, forming hinges. As the slab is raised, each wall panel automatically unfolds into position. Since the walls are load bearing, there is no need for expensive steel columns or lifting collars as used in conventional lift-slab work. The steel columns used for erection are removed and reused elsewhere.

•Walls of only 4-inch thickness are adequate because they have a long bearing length. •The openings in the walls will be filled with masonry walls, precast panels or other curtain wall materials. •The columns and bridges, reusable up to several hundred times, can be taken apart for easy transport to the next job. •The absence of decking formwork shores, scaffolding, hoists and cranes further illustrates the simplicity of the lift slab operation.

Buildings constructed using various Lift Slab Systems

Fargo high rise building

The Foss firm pioneered the use of "lift-slab" construction in the region. The method was used in constructing the Fargo high rise building east of Island Park.

•During the 1960s, the firm pioneered the use of "lift-slab" construction in the area, using the method in construction of the high rise apartment buildings in Fargo and Moorhead - Foss Engelstad and Foss was the consulting engineer firm on the Fargo high rise for a St. Paul architectural firm - Nelson Hall and Neumaier Hall dormitories on the Moorhead State University campus, and Park Towers apartments Bethany Towers in Fargo. •At one time, the Fargo high rise was the tallest lift-slab building in the world.

The Johnstone Hall complex

•Erected in 1954, the Johnstone Hall complex design became a model for college dormitories, implementing a new raise-slab construction method, a practice which was featured in many architectural magazines at that time. \ •This method - the Youtz-Slick "lift-slab" method lifted reinforced concrete slabs onto columns with hydraulic jacks. • These slabs weighed 224 tons and were nine inches thick, 122 feet long and 43 feet wide. •Johnstone Hall was the largest building complex erected using this method. •Campus legend had it that two other similar structures built elsewhere collapsed before completion. •Today, only one of the original Johnstone buildings is still standing on the campus.

OTP bank office complex Veszprém LIVING COMPLEX

Plovdiv

Center of banks office complex Budapest

The L’Ambiance Plaza •It was a 16-story residential project under construction in Bridgeport, Connecticut. Its partially erect frame completely collapsed on April 23, 1987, killing 28 construction workers. • Failure was possibly due to high stresses placed on the floor slabs by the lift slab technique. •There was a school of thought that this accident highlighted the deficiencies of lift slab construction. • This accident prompted a major nationwide federal investigation into this construction technique, as well as a temporary moratorium of its use in Connecticut.

•L’Ambiance Plaza was planned to be a sixteen-story building with thirteen apartment levels topping three parking levels. It consisted of two offset rectangular towers, 63 ft by 112 ft each, connected by an elevator. •The entire structure collapsed, first the west tower and then the east tower, in 5 seconds, only 2.5 seconds longer than it would have taken an object to free fall from that height. Two days of frantic rescue operations revealed that 28 construction workers died in the collapse, making it the worst lift-slab construction accident.

•At the time of collapse, the building was a little more than halfway completed. •In the west tower, the ninth, tenth, and eleventh floor slab package was parked in stage IV directly under the twelfth floor and roof package. •The shear walls were about five levels below the lifted slabs. •The workmen were tack welding wedges under the ninth, tenth, and eleventh floor package to temporarily hold them into position when they heard a loud metallic sound followed by rumbling. An ironworker who was installing wedges at the time, looked up to see the slab over him ―cracking like ice breaking.‖ •Suddenly, the slab fell on to the slab below it, which was unable to support this added weight and in turn fell.

Description of Building •L'Ambiance Plaza was planned to have two virtually identical towers, with floor plans measuring approximately 63 ft by 112ft, and a neighboring parking garage. •The towers were separated by 4 ft and would have been joined by castin-place concrete during the final stage of construction. •The structural system consisted of steel columns and 7" thick two-way unbonded post-tensioned concrete flat plates with shearwalls at four perimeter and four interior locations. •The Youtz-Slick method of lift-slab construction, was employed.

Causes of Collapse Theory 1: Instability of the wedges supporting the 12th floor and roof package Theory 2: Jack rod and lifting nut slipped out due to a deformation of an overloaded steel angle welded to a shear head arm channel Theory 3: Improper design of posttensioning tendons

Structure Geometry Prior to Collapse - Drawn by Liam McNamara based on description from Culver, 1987

Theory 4: Substandard welds and questionable weld details

Theory 5: Global instability caused by lateral displacement Shearhead and Hydraulic Jack

Shearhead picture

Cleveland Lift-Slab Parking Garage Cleveland parking garage under construction

The building consisted of eight-story twin towers, 28 by 6.4 m each (91 by 21 ft) in plan. The slabs were 200 mm (8 in) thick and weighed 800 kN (180,000 lb). The slabs had been jacked into place on 18.5 m (61 ft) tall columns spaced 6.7 m (22 ft) on center. All of the slabs of the west tower were in place, secured by temporary steel wedges On the evening of April 6, winds gusted up to 56 – 80 kph (35 – 50 mph). The structure shifted 2.1 m (7 ft) out of plumb in the long direction, with the fifth floor very close to an adjacent building. The east tower, with only the second and third floors in place, was not affected by the wind. The building was secured with guys and temporary shoring on orders of the Cleveland building commissioner, who remarked ―if they had welded at each floor as they went along the building would have been braced and this would not have happened.‖ The west tower was eventually jacked back into place .

Sidesway displacement of Cleveland parking garage

Restoring the Cleveland parking garage to plumb

Pipers Row Car Park, Wolverhampton •Pipers Row Multi-Storey Car Park was built in 1965 by British Lift Slab Ltd (BLS) using the Lift Slab system of construction. •In January 1996 the local NCP management reported to the NCP Regional Building Surveyor, that damage had developed after a period of frost and that ponding was leading to leakage and lime damage to cars. The lime damage risk lead to the closure of some parking spaces below the top slab. An area of 350m2 was recommended for resurfacing. •A 120 tonne, 15m by 15m section of the top floor collapsed at about 3am on 20th March 1997 when punching shear failure at one column led to a progressive collapse as similar failures followed at eight adjacent columns.

Collapsed 4th floor slab

Plan of 4th floor slab at Pipers Row showing repaired areas

cutting out for repairs at I2-3, 20th March 1996.

•The initial punching shear failure in the concrete around the shear head could have occurred at H2 or I2,as at both the deterioration and/or repair substantially reduced the anchorage to the top steel essential for shear strength. •Because of sensitivity to support wedge construction tolerances, H2 and I2 had similar indeterminate wide ranges of possible effective shear force from self weight at the time of collapse. •The Lift Slab shear head and column connections remained intact. G1 column, with bottom steel welded to shear head, did not fail in punching shear

Photo from above the collapsed slab

general view

Circular hotel guest room tower, in Portland, Maine. The investigation revealed that the lateral load resisting system for the structure had substantial deficiencies. •The first deficiency was that the planes of all the brace frames were radial and interested at a single vertical axis (at the center of the circle.). •There was no other significant lateral load resistance; thus the structure was torsionally unstable.

The lateral load resisting system of lift slab buildings usually consists of reinforced concrete shear walls, or braced frames with steel diagonal members connected to the steel columns.

•A second deficiency was that one end of the diagonal cross-bracing members did not terminate at a column, but terminated within the span of the slabs. • Under lateral load, the forces from the bracing would impose large vertical concentrated load on the slabs which in turn would impose large bending effects (moments) on the slabs, which the slabs were not designed to resist.

•A third deficiency was that the diagonal bracing was designed for strength but not stiffness.

Rules after collapse…… The Occupational Safety and Health Administration’s standard for concrete and masonry construction— Subpart Q,Concrete and Masonry Construction, Title 29 of the Code of Federal Regulations (CFR), Part 1926.700 through 706— Sets forth requirements with which construction employers must comply to protect construction workers from accidents and injuries resulting from the premature removal of formwork, the failure to brace masonry walls, the failure to support precast panels, the inadvertent operation of equipment, and the failure to guard reinforcing steel.

• Lift-slab operations must be designed and planned by a registered professional engineer who has experience in lift-slab construction. Such plans and designs must be implemented by the employer and must include detailed instructions and sketches indicating the prescribed method of erection. The plans and designs must also include provisions for ensuring lateral stability of the building/structure during construction.

•Jacking equipment must be marked with the manufacturer’s rated capacity and must be capable of supporting at least two and one-half times the load being lifted during jacking operations and the equipment must not be overloaded. Such equipment includes, but is not limited to, the following: threaded rods, lifting attachments, lifting nuts, hookup collars, T-caps, shearheads, columns, and footings.

•Jacks/lifting units must be designed and installed so that they will neither lift nor continue to lift when loaded in excess of their rated capacity; and jacks/lifting units must have a safety device which will cause the jacks/lifting units to support the load at any position in the event of their malfunction or loss of ability to continue to lift. •No employee, except those essential to the jacking operation, shall be permitted in the building/structure while any jacking operation is taking place unless the building/structure has been reinforced sufficiently to ensure its integrity during erection. The phrase ―reinforced sufficiently to ensure its integrity‖ as used in this paragraph means that a registered professional engineer, independent of the engineer who designed and planned the lifting operation, has determined from the plans that if there is a loss of support at any jack location, that loss will be confined to that location and the structure as a whole will remain stable.

• Under no circumstances shall any employee who is not essential to the jacking operation be permitted immediately beneath a slab while it is being lifted.

Refrences…. -OREGON OCCUPATIONAL SAFETY AND HEALTH DIVISION DEPARTMENT OF CONSUMER AND BUSINESS SERVICES -Zallenengineering .com -www.concreteconstruction.net -en.wikipedia.org -Lift-slab Mirim Bau Consortium -United States Patent Applcation Publication -Engineering Considerations for Lift Slab Construction, Published by ASCE -www.concreteconstruction.net -Webs.demasiado.com

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