Formwork Blog

Posts Tagged ‘Back Propping’

Location: A Construction Site In Sydney Australia on the 26th of September, 2008

Brand new (nine day old) 250 tonne Leibher Crane crashes through concrete floor on a Sydney Construction Site. I have not been told of any injuries or fatalities. If you know anything about this incident, your comments are welcome.

Photo 01 – Crane Crash 26-September-2008 Sydney

It would appear that the concrete floor did not have enough Back Propping underneath, and I cannot make out if large enough Sole Boards were used (if any) under the outrigger pads.

This concrete floor would also have a safe working load limit. If the crane exceeded the safe working load limit, the entire path where the crane traveled should be back propped underneath in order to increase the safe working load of the floor.

Photo 02 – Crane Crash

In most cases, a crane coordinator from the crane hire company usually sees things like this in advance and makes the necessary arrangements. At other times, experienced crane operators, riggers and doggers are competent enough to anticipate back-propping requirements.

Photo 03 – Crane Crash

The next photo reveals an outrigger pad with no sole-boards underneath it!

Photo 04 – Crane Crash

To me, Sole Boards are standard and should be placed underneath outrigger pads whenever the crane is setup. I was always told that outrigger pads can penetrate concrete floors and to always use sole boards underneath outrigger pads when setting up a crane.

Photo 05 – Crane Crash

Tip: Always position plenty of back-propping under a concrete floor directly underneath the outriggers, and always use big solid sole-boards underneath the outrigger pads in order to distribute the weight over a larger area.

Even when sufficient and correct back-propping is in place, outrigger pads have been known to penetrate concrete floors where sole boards have not been used.

Extra Sole Boards In order to distribute the weight over a larger area, it can be a good idea to stack two rows (instead of just one row) of hardwood sleepers under the specific outriggers which will be under the most load.

If you are ever unsure, ring the crane coordinator and demand an official decision.

Thanks for the photos Joe

Location: A Surfers Paradise High-Rise Construction Site, Gold Coast Australia

After reading about a killed worker as a result of being buried under wet concrete after a suspended slab collapsed, I wandered if some of us too relaxed with our attitude towards Back-Propping.

The Do It Later Attitude

The photo below shows a small part of a formwork deck which in my opinion was done in a hurry due to the large number of incomplete Back-Propping assignments. This can only be the result of a flawed Work Method Statement.

Unsafe Back-Propping

You’ll notice that the Props are sitting on a flat timber which also needs to be propped before the concrete pour. The attitude here is “we’ll do it later”, or worse, “it’s okay” as it is. The problem with this attitude is that inside this maze of formwork frames, it can be very difficult to remember or spot all these important do-it-later jobs.

Often this do-it-later job is allocated to a Formworker experienced with spotting danger areas of a deck and back-propping accordingly. Too often this job is allocated too late only allowing time to insert the props, but not to tie them off with Scaffold Tube and Clamps (Fittings).

Top is Back Propped, bottom is Not

The other reason that this finish-it-just-before-the-concrete-pour attitude is flawed is because the deck will continue to be loaded with additional bundles of plywood, timber and steel well before the concrete pour day. Heavy compressors could also be landed on the deck along with big bundles of pipes for the concrete pump.

What this means is that it is possible to overload and collapse parts of the deck even before the concrete pour day if the Back-Propping is incomplete.

Do It Now

Never leave these Back-Propping jobs incomplete. Do them immediately and completely as you go. This procedure should be placed in the companies Work Method Statement in order to ensure that your supervisor or leading hand can not take you to another job before the Back-Propping is 100% complete.

Communicate

You should also communicate with the Deck Head informing them of the areas where construction materials should not be placed. Deck heads should also inform Doggers the correct way to lay the Dunnage/Gluts.

For example, the Dunnage should always go across several Joist timbers instead of longways in between the Joists where the loaded Dunnage could easily penetrate through the plywood. This mistake is made very often so keep an eye on beginner doggies with no formwork experience.

Secure Those Props

Too often props are not nailed at the top because there is simply no way to reach up there safely. A single prop on it’s own falling over can threaten life or limb as well as be responsible for the collapse of a formwork deck.

Even if the props are nailed to the Header, there is no guarantee that the Header is nailed properly to the Joists. For this reason, props should be secured with scaffold tube and clamps back to the frames which will ensure the props remain in the desired position.

Plan To Minimize Back-Propping

A lot of Back-Propping in the above pictured deck could have been eliminated altogether by planning the layout of the frames better. If frames are erected at the minimum required span or less, the need for Back-Propping will be reduced to a minimum.

Of course this method assumes that the formwork company does not take short-cuts by deliberately using lots of unnecessary props because there is a shortage of Frames. The general rule for this should be if a Frame can fit, put it in there.

Cause And Effect

Because