collapsing of l’ambiance plaza

COLLAPSING OF L’AMBIANCE PLAZABRIDGEPORT, CONNECTICUT, U.S.A.

SYED IDRIS - FARID ISKANDAR - LAILY FARHANAN - NAJLA SALLEH - AISYAH AZLIN

MEASUREMENT & APPLICATION 152

INTRODUCTION OF THE L’AMBIANCE PLAZA

• L’Ambiance Plaza collapsed during the construction on April 23rd 1987

• 28 workers were found dead• L’Ambiance Plaza is actually a 16-storey building (13

apartment levels, with 3 parking levels)• Method used for the construction on L’Ambiance Plaza is

“lift-slab”(casting post-tensioned floor slabs, one on-top of another, at ground level and then hydraulically jacking each level into place)

• Cause of collapsing are still unknown

DESCRIPTION OF L’AMBIANCE PLAZA

• Planned to have two virtually identical towers, with floor plans measuring approximately 63 ft by 112ft, and a neighbouring parking garage.

• Lift-slab construction method, which was employed on L'Ambiance Plaza, is used in the design of two to five story buildings.

• The Lift-Slab process used consisted of casting floor slabs at ground level, raising those floor slabs to the desired elevation with hydraulic jacks, and fixing the slabs in position mechanically. By casting the floor slabs on grade, the need for shoring or formwork underneath each slab was eliminated, and only side forms were required.

BEFORE COLLAPSE HYDRAULIC JACK

AFTER COLLAPSE

CAUSES OF COLLAPSEStill unknown, but there are 5 theories of why

L’Ambiance Plaza collapsed by various experts of building failures

THEORY #1INSTABILITY OF THE WEDGES SUPPORTING THE

12TH FLOOR & ROOF PACKAGEby- Thornton Tomasetti Engineers

• They concluded that the epicenter of the collapse was a core column, 3E, of the west building

• A wedge supporting the 12/R package rolled out leaving the shearhead at this level supported by a single wedge

• The horizontal load from the jack used to plumb the building caused the remaining wedge to roll slightly as evidenced by rounding and bending in the west weld block of the shearhead

• The slabs and shearheads then began to slide down the column, impacting the floor slabs below, ripping them from the column, and progressively collapsing the west tower

• The east building then collapsed from horizontal forces transmitted through the pour strips or horizontal jack, or impact of the west tower debris

THEORY #2JACK ROD & LIFTING NUT SLIPPED OUT DUE TO A DEFORMATION OF AN OVERLOADED STEEL ANGLE

WELDED TO SHEAR HEAD ARM CHANNELby- National Bureau of Standards (NBS)

• concluded in their investigation that the failure began at the building's most heavily loaded column, E4.8 or the adjacent column, E3.8, as a result of a lifting assembly failure

• each column the shearhead reinforces the concrete slab, transfers vertical load from the slab into the column, and provides a place of attachment for the lifting assembly

• Steel channels are cast in the slab, allowing room for the lifting angle. Lifting rods, raised by hydraulic jacks above them, are passed through holes in the lifting angle and fastened with lifting nuts

• The angles deformed under the excess force of the three 320 ton slabs, causing the jack rod and lifting nut to slip out of the angle and hit the column

• The building then collapsed in the same method as described by T-T after the lifting nuts impacted the column.

• NBS testing determined that when the shearhead and lifting angles were loaded with forces nearing 80 tons, they had a tendency to twist

• During the lifting process the shearheads and lifting angles were loaded close to their maximum capacity

THEORY #3IMPROPER DESIGN OF POST-TENSIONING

TENDONSBy- Shupack Suarez Engineers, Inc.

• They examined the unusual layout of the post-tensioning tendons in the west building

• The arrangements of the tendons at the west building are not arranged & placed properly

• Therefore the load transferred to the column is not uniform

• Thus, it causes instability to the column and then collapse

THEORY #4SUBSTANDARD WELDS AND QUESTIONABLE

WELD DETAILSBy- Occupational Safety and Health Administration (OSHA)

• OSHA stated that two weld design details were questionable

• The welding method for the connection does not follow the industry standards

• The welds were of unknown and unspecified penetration depth and therefore had an unpredictable strength

• Upon further investigation by OSHA hired consultant firm, Neal S. Moreton and Associates, it was determined that of 30 welds investigated at Column E3.8, levels 7,8, and 10; 17 welds did not meet industry standards.(McGuire, 1992)

THEORY #5GLOBAL INSTABILITY CAUSED BY LATERAL

DISPLACEMENTBy- Occupational Safety and Health Administration (OSHA)

• OSHA focused on the response of lateral loading and overall torsional instability

• The shearhead connection is rotationally stiff when the concrete slab is temporarily resting on the wedges and when it is fully welded in its final position it becomes a rigid connection

• However, when the slab is lifted off the wedge it can rotate freely. In the presence of lateral loading or displacement, such as that from the horizontal jacking just after lunch, the slab could be lifted off a wedge and the building would become laterally flexible

CONCLUSION

• Instability of a structure leads to failure. Thus, it must be well & properly planned & designed

• Method of construction must follow the industrial standards

• The method of jointing the slabs, columns, beams etc must be very clear between engineers & contractors

• The lift-slab method is not suitable for building a high rise building

• Safety measure must be taken a count during the construction