Constructability in Bridge Design #7: Initial Sizing and Planning of Cranes

When planning a bridge erection methodology using cranes, selection of appropriate crane model(s) is crucial. This not only ensures safety but also helps in efficient project and cost management. Here's a rule-of-thumb method for initial sizing and planning.

Step 1: Estimate Weight

The first step in determining crane capacity is to estimate the weight of the segment(s) to be lifted. This can be estimated using the member dimensions and the material's density. Include allowances for the weight of rigging and splice plates, cross frames, bolt’s and studs (if any). Estimate the load in metric tonnes.

Step 2: Crane Location and Lift Radius

Next, conceive potential crane locations, particularly meant for critical girder segment lifts. Use the ballpark footprints to position your cranes and verify whether there's sufficient room:

• Up to 200-tonne Crane – 7m wide x 12m long

• 300-tonne Crane – 8m wide x 14m long

• 500-tonne Crane – 10m wide x 18m long

Also, estimate the lifting radius (R) at this stage after positioning the crane roughly from where it will sit to lift the load and set it down. Be mindful of any possible interferences by adjacent physical structures, right-of-way limits, by the structure being constructed or overhead lines etc.

A planning exercise can be done using a plan and elevation view of the bridge site and structure using the rough footprints of cranes provided above. The excerpt below is a sample planning exercise from the Context Sensitive Conceptual Bridge Design course offered by the Spannovation Bridge and Seismic School using footprints for 300 tonne cranes.

Crane planning exercise to lift concrete girders from an existing parallel bridge

Step 3: Rough Crane Size

Ballpark size of the crane in terms of maximum lift capacity can be applied using the following formula:

Size of Crane (tonnes) = Load (L) x Radius (R) / 2.5

Round up to the nearest 50 tonnes to determine the crane size in terms of maximum lift capacity.

Example

To calculate the size of the crane required to lift 60 tonnes at a radius of 10m.

• Size of Crane (tonnes) = Load (L) x Radius (R) / 2.5

• Substituting the given values:

• Size of Crane (tonnes) = 60 tonnes x 10m / 2.5 = 240 tonnes

• Rounding off to the next 50, we get 250 tonnes.

So, a crane with a capacity of 250 tonnes is required to lift 60 tonnes at a radius of 10m.