Lifting and Fixing Tools
The forces acting on a lifting insert should be calculated taking into account:
- The precast unit weight.
- The adhesion to the mould.
- The lifting machinery (dynamic coefficient).
- The sling angle.
- The number and position of inserts (number of efficient points).
Precast unit weight
The weight to lift has to be calculated taking into consideration the weight of the precast element, but also all the other parts lifted with the precast unit (formworks, preassembled parts).
Actions from adhesion to the mould
The adhesion effort qadh is present at time of the first lifting, it is function of formwork type and specified in the following table:
| Formwork and condition | qadh |
|---|---|
| Oiled steel mould, oiled plastic coated plywood | 1 kN/m² |
| Varnished wooden mould | 2 kN/m² |
| Rough wooden mould | 3 kN/m² |
The area to be used in calculations is the total contact area between the concrete and the form. For some types of uneven concrete surfaces, forces may be much larger than given in the table, and should be considered separately. Force may be zero in some special cases: post-tensioned elements, elements concreted in lost formwork ...
Dynamic actions
During lifting and handling, lifting devices are subjected to dynamic actions. The magnitude of the dynamic actions depends on the type of lifting machinery. Dynamic effects should be taken into account by the dynamic coefficient Ψdyn given in the table here after.
| lifting machinery | dynamic coefficient (ψdyn) |
|---|---|
| Stationary crane, rail-mounted crane, speed > 1m/s | 1.15 |
| Stationary crane, rail-mounted crane, speed > 1m/s | 1.30 |
| Bridge crane, speed | 1.15 |
| Bridge crane, speed > 1m/s | 1.60 |
| Lifting and moving on flat terrain | 2 |
| Lifting and moving on rough terrain | 4 |
Other dynamic influences than covered by this table should be based on special provisions or engineering judgement.
Sling angle
If the ropes, chains or slings are not perfectly vertical when lifting, this will create a sling coefficient given in the table here after, depending on the angle α which is the angle on the top of the slings.
| Angle | Sling length L | Sling coefficient (φsling) |
|---|---|---|
| 0° | - | 1 |
| 30° | 2 D | 1.04 |
| 45° | 1.3 D | 1.08 |
| 60° | D | 1.16 |
| 90° | 0.7 D | 1.42 |
| 120° | 0.6 D | 2 |
With D = distance between 2 opposite lifting inserts
Number of efficient lifting points
In a statically indeterminate system, the load distribution on inserts depends in most cases on the unknown stiffness of the ropes and the inserts itself. Therefore in most cases, only maximum 2 efficient lifting points should be used in the calculation of the actions on inserts. Only unless it is ensure by suitable means (speader, beam) that the load is distribute equally on all the lifting inserts, then the number of efficient point could be equal to the total number of lifting points.
Calculation of actions on lifting inserts
The actions, Ed, should be determined from Equation:
with:
- G = weight of the precast concrete element (kN)
- qadh = mould adhesion (kN/m²)
- Af = form area in contact with concrete (m²)
- φdyn = dynamic coefficient
- φsling = sling coefficient
- Neff = number of efficient lifting points
Conclusion and other considerations
The Safe Working Load of the lifting insert must be higher than Ed.
In some cases, it can be necessary to calculate the action on inserts at different stages of the life of a precast unit (at the precast factory and on site for example).
The concrete strength needs to be clearly defined when lifting at the precast factory and on site, to select and design the type and the size of the inserts.
The load capacity of the anchorage (concrete failure and steel failure) has to be checked by qualified people.