Slab on grade? Crawler crane or wheeled crane? I suppose that this is too easy but what about crane mats? Wheeled crane may take a little more cribbing due to the small out-rigger area.
It's a wheeled crane with outriggers, anf for which, I'll probably have to have several supports beneath the slab directly under the outriggers. This is what I have to calculate (my first time!)
The first thing you need to do is to contact the company who is hiring you their crane to ask if they have crane mats to place beneath the outriggers. These are generally timber mats of differing sizes which will re-distribute the outrigger load across their area. You also need to be sure that you have their published maximum outrigger loads for all orientations of the jib.
You then need to check the bearing stress between the crane mat and the concrete slab. If you do not have mats then take the load over the area of the outrigger foot and compare this with the bearing capacity of your concrete. If the concrete fais you need a larger mat. Following this check, you then distribute the load from the edge of the mat/ outrigger pad at a gradient of 1:1 through the depth of the concrete slab (in both directions when viewed on plan) and check the bearing capacity of the subgrade over this larger footprint at the slab/subgrade interface. If you have not exceeded the safe bearing capacity of your subgrade then all is well and you can proceed with the lift.
Please check that there are no known voids below the slab in the area. If there are then you will have to check the bending/shear capacity of the slab to span these troublesome areas. If you are working in the UK you should have a delegated named person who is the "Lift/crane coordinator" who will need to have all of this info in his posession and check your specified requirements prior to the lift taking place.
Civeng,
Do not forget settlement calculations. U do not want to end up in a scenario where one of the pad is settling. Remember instances where u will have more forces on one pad compared to the other.
Riz
