Construction formwork and scaffolding design

Formwork and support work should be designed to ensure it will be strong enough to withstand and carry the weight of wet concrete, reinforcing, cast-in items, and the people and equipment used to place the concrete. The design normally assumes a rate of pour, and if this rate is exceeded it could lead to overloading of the formwork. Also materials should not be heaped on formwork, or scaffolding, since this could result in the equipment carrying a load it was not designed for.
 
Case study:
I was the Project Director on a project that involved constructing a reinforced concrete slab, two metres thick, and eight metres above an operational railway line which the supporting scaffold had to span. The support-work was designed by our Formwork Design Engineer, who also produced detailed drawings of how the support-work should be constructed. The quantity of equipment was ordered from these drawings.
There was a full-time Project Manager allocated to the project, and at the time, I was responsible for five other projects, situated hundreds of kilometres apart, so I visited the project approximately every two weeks. The day before the concrete slab was due to be poured I visited another project, and at the last minute decided to make a detour on my homeward journey to check the slab was ready for the concrete.
On arriving on site I was informed the slab was ready to receive concrete, however, I noticed a number of steel beams that had been ordered were lying unused on the ground. I queried this, and was informed the Supervisor had decided the beams were superfluous and omitted them. I contacted the Formwork Design Engineer, and explained my concerns. He immediately checked his calculations, and called me back to say he had major concerns, and was on his way to the site – some 200 kilometres from his office.
When our Engineer arrived and inspected the support-work, he found the omission of the support beams meant the load of the wet concrete on the slab would not have been distributed evenly, and would have resulted in some of the scaffold support legs carrying double their load capacities. To compound the error, the Engineer found a mistake with his original calculations, which would have anyway resulted in the legs carrying an extra 20% over their capacity. This additional 20%, together with the doubling of the load, would have resulted in some scaffold legs carrying nearly two and a half times the load they were capable of carrying.
If we had gone ahead with placing the concrete on the slab there is no doubt the support-work would have failed with catastrophic results, people would probably have been killed and others seriously injured, the railway would have been closed for weeks, the project would have been delayed by months, and there would have been costly property damage.
As a result we postponed the concrete pour until we were able to install additional support-work to carry the load, ensuring none of the scaffold was over-loaded.
 
There are several lessons from the above:

1. the formwork design must be double-checked to ensure there are no errors or erroneous assumptions
2. the design drawings should be followed and Supervisors should not omit items, no matter how unnecessary they may seem, unless they first check with the Designer
3. it’s important to check that the support-work and formwork has been erected according to the design drawings. These checks should also include:
   1. ensuring all bracing is installed
   2. all support-work is erected straight, vertical and level
   3. the equipment is in good condition
   4. the supporting ground is firm and capable of carrying the load
   5. the support-work and formwork is all tightened and secured properly
   6. remembering that support-work erected on top of existing elevated slabs transfer the load on to these slabs, so they must be designed, and supported in such a way that they can carry this load

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