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An Insight into the Roots of Formwork Failure

Abstract: Almost every construction operation in every part of the world utilizes formwork and falsework systems in or-der to support structural elements, materials and workmen. The collapse of forms or scaffolds not only leads to work delays and property loss, but has also been responsible for numerous worker injuries and deaths. The collapse of con-crete formwork used to support concrete slabs, decks and roofs is a big problem. Accidents not only affect workers but also their families. ‘Safety problems are just getting out of control’. The demand for construction speed is having a di-sastrous effect on worker’s health and safety. Advancement in formwork technology is limited as compared to concreting & reinforcement. So, focus has to be shifted to formwork & falsework. The construction industry is a multifaceted indus-try, characterised by a broad range of high risk activities and complex work arrangements ,

Construction industry has the largest number of work relat-ed deaths than any other industry

ACI 347 states “Formwork failures can be attributed to human error, substandard materials and equipment, omission, and inadequacy in design. Careful supervision and continu-ous inspection of formwork during erection, concrete place-ment, and removal can prevent many accidents”.

Roots of Failure

1. Uncontrolled acceleration of formwork removal or im-proper stripping

2. Improper or inadequate shoring or re-shoring3. Unstable shoring foundations4. Alignment of shoring, non-verticality of supports5. Inadequate bracings, diagonals or X bracings6. Lack of control of rate of Concrete placement7. Vibration/shocks8. Lack of attention to formwork details9. Improper , or lack of design of formwork10. Adequate slab thickness, Improper placement of rein-

forcement 11. Onsite modifications to formwork due to site conditions

not addressed in the design12. Inadequate strength of formwork materials13. Inadequate inspection, no architectural or engineering

inspection done

14. Handling, Erection and Dismantling procedures15. Overloading of the formwork during concrete pouring

operationsMainly failures are due to faulty designs, faulty materi-

als, and not following scheme drawings etc. Each year hun-dreds of men are seriously injured or killed.

Case study

Tropicana Casino Parking Garage in Atlantic City, New Jersey (2003)

- The parking garage was a part of the Tropicana Casino and Resort expansion project

- collapse resulted in the failure of five levels of an exterior bay

- The ten-story parking garage was designed as a reinforced concrete structure with shear walls and intermediate moment resisting frames to resist lateral loads

- the 3,000 car parking garage was a part of an ongoing project

- The framing generally consisted of a one way slab span-ning in the east-west direction, supported by continuous cast in place beams in the north-south direction

Major Causes

1. Improper placement of reinforcement Omission of the reinforcing steel compromised the in-

tegrity of the structure. Misplace the required embed-

Sameer S MalvankarDeputy Manager-Engineering, Gammon India Ltd.,

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ment length for the welded wire mesh at the intersection of the exterior columns and the slab/beam as per struc-tural details. Failed to detail, fabricate and place bottom reinforcing steel, as required by the structural contract drawings.

2. Inadequate inspection, No architectural or engineering inspection done

The collapse could have been averted if immediate atten-tion was sought. Failed to seek the immediate attention of the contractor or construction manager or structural engineer in spite of workers brought in notice the cracks around exterior column.

3. Improper or inadequate shoring or re-shoring Only one level was re-shored instead of the required

three. Given the fact that cracks were earlier noticed at and around the exterior columns, the fewer levels of re-shores were highly detrimental to the integrity of the structure. This contributed to the collapse. Failed to re-shore an adequate number of floors, as required by the project specification, at the time concrete was being cast.

4. Lack of attention in detailing of formwork Even when the basic formwork design is soundly conceived,

small differences in assembly details may cause local weakness or overstress loading to form failure. Engineers not prepare shoring drawings for the area of the collapse.

main consideration is the gain strength of concrete to ensure that the member when released can support its own weight together with any imposed construction operation loads. Additional considerations are the elements of mechanical damage, reduction in thermal shocks and limiting excess deflections. The minimum concrete strength at the time of striking the formwork should be specified in the drawings or technical specifications. Premature removal of shores and careless practices in re-shoring caused numerous failures or deficiencies in the complete structure.

Clause 11.3 of IS 456:2000 states that Formwork shall not be released until the concrete has achieved a strength at least twice the stress to which concrete may be subjected, at the time of removal of the formwork. The strength referred to shall be that of concrete using the same cement and aggre-gates and admixture, if any, with the same proportions and cured under conditions of temperature and moisture similar to those existing on the work.

2. Punching shear strength of flat slab

A typical flat plate punching shear failure is character-ized by the slab failing at the intersection point of the column. This results in the column breaking through the portion of the surrounding slab. This type of failure is one of the most critical problems to consider when determining the thickness of flat plates at the column-slab intersection. Accurate pre-diction of punching shear strength is a major concern and absolutely necessary for engineers so they can design a safe

Bailey’s Crossroads in Virginia (1972)

- 26-stories + apartment building- Forms were supported by floors 7-days old or older- Failure occurred on the 24th floor, where it was shored

to the 5day old 23rd floor- The overloaded 23rd floor failed in shear around one or

more columns- Triggering a collapse that carried through the entire height

of the building

Major Causes

1. Uncontrolled acceleration of formwork removal or im-proper stripping

The time and procedure by which soffit formwork of beams and slabs can be struck should be carefully controlled. The

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structure. Supports of slabs, beams or girders when it’s supported over successive RC members shall be checked for shear capacity of slabs.

New York Coliseum (1955)

- a flat plate waffle slab with solid slabs at the column caps - The floor that collapsed was the first floor above grade

supported on two tiers of shores- Used power buggies

placement of supporting members and it turn failure of formwork. Inadequate cross bracings, horizontal bracings of supports are one of the factor most frequently involved in formwork accidents. High shoring with heavy load at the top is vulnerable to eccentric or lateral loading. High shoring must tie to permanent structures is possible to avoid over-turning failure.

Major Causes

1. Lack of control on concrete placement

Failure to regulate properly the rate and order of placing concrete on horizontal form produce unbalanced loadings and consequent failures of formwork.

2. Inadequate bracings

Lateral force on supporting members will cause dis-

Other Causes

1. Alignment of shoring, non-verticality of supports

All vertical supports shall be checked for verticality. In case of out of plumb supports the loads may eccentrically act on the member and bend in member may occur. Eccentri-cally loaded support may lead to failure of one support and it will goes on successive failure of entire system. Height of

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adjustable jacks at base or top of falsework legs should be maintained as per design.

2. Vibration/shocks

Forms sometimes collapse when their supporting shores or jacks are displaced by vibration caused by passing traffic, movement of workers and equipment on the formwork or the effect of vibrating concrete to consolidate it. Shores pro-vide with diagonal bracing can help prevent failure due to vibration.

3. Unstable soil under mudsills

Formwork should be safe if it is adequately braced and constructed so all loads are carried to solid ground through vertical members. Shores must be set plumb and the ground must be able to carry the load without settling. Shores and mudsills must not rest on frozen ground; moisture and heat from the concreting operations, or changing air temperatures, may thaw the soil and allow settlement that overloads or shifts the formwork. Site drainage must be adequate to pre-vent a washout of soil supporting the mudsills.

Conclusion

Everyone in a project must be alert to unsafe conditions, and all work must be performed in accordance with safety regulations and the requirements specified in the form de-sign.

- Educated and trained crew- Strictly follow the construction methodology- Follow the drawing-If not available, insist for the same- Prepare a hazard analysis for a job at site- Take necessary precaution-Think in advance- Close supervision- Before, During and After- Prepare Check list for work- Do not repeat the mistakes

“Preventable Accidents, if not prevented due to negli-gence, its nothing short of a murder”. Dr. S. Radhakrishnan

References

- Hurd M. K. “Formwork for concrete”, American concrete institute, 1915, 6th edition

- Wikispaces- OSHA accident investigation reports w

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