Indeed, there are many advantages to prefabricating components in a controlled environment prior to transporting to the jobsite for final installation. But there is an additional challenge involved in this construction method, offloading and handling of the prefabricated component.
We discussed the process of shipping precast concrete in a previous article, Best Practices for Shipping Precast Products, and below we will discuss best practices for offloading and handling precast concrete structures.
CRANE LIFT PREPARATION & PLANNING
When you are dealing with precast concrete, you are working with a strong and heavy product that requires proper planning and coordination before the product gets to the jobsite. It is important to understand the safety risks, have the right equipment on hand, and communicate this information to everyone involved.
Part of the planning process typically includes an initial jobsite visit by the crane or equipment operators who will be lifting the product. During this visit, they can review the location for placing the precast structure, determine where the crane will be setup, determine the path of access for the heavy haul delivery truck, and assess the area for any overhead obstructions.
Larger structures requiring more complex rigging formations should be reviewed by a certified crane operator and possibly a professional engineer. A lift plan should be created in advance showing the distance the crane will need to reach, the capacity of the crane relative to the angle and height position of the boom, the weight of the structure along with the rigging gear, and the capacity of each component of the rigging gear, which may include spreader bars. These lift plans will help the personnel on the jobsite understand the limitations of the excavation, identify potential overhead obstructions, identify the correct rigging equipment needed, and help the determine the appropriate crane capacity needed.
EXCAVATION, SHORING & SUBGRADE PLAN
Some of the more common steps that get overlooked during the prior to shipment are the timing of when excavation takes place, how the excavation is shored, and the preparation of the subgrade. As we have discussed in a separate article, Tips for Preparing Subgrade & Installation of Precast Concrete Products, the method of shoring can have a big impact on the type of crane and the necessary capacity to reach the final placement position. Additionally, it does not matter how well the structure is designed if the subgrade foundation is not prepared correctly. The most efficient jobsites will mark the final position of the structure on the subgrade to give clear guidance to the crane operators and riggers.
We recommend those involved, including site personnel, crane operators, riggers, and heavy haul truck drivers, to have one final pre-lift meeting to discuss the swing path of the lift, confirm the weight of the product, confirm the capacity of each of the rigging components, verify there are no overhead obstructions, and point out any potential hazards around the jobsite. Workers should maintain a safe distance of 10 to 15 feet from the structure when it is being offloaded, never walk underneath a suspended load, and they should avoid putting themselves in between the lifted product and a danger zone (“between a rock and hard place”).
After all the preparations and planning steps have been satisfied, the job site is ready for the structure’s arrival.
Once the structure arrives and the delivery ticket has been verified by the field representative, actions can begin to start offloading the structure. The equipment typically used to lift precast structures are cranes, forklifts, and excavators. Chains or slings are attached to the lifting devices with one or a combination of shackles, hooks, or specialize lifting clutches. There are various types of lifting embeds used in precast structures so it is critical to have detailed information on the type of lifting device in advance.
The structure should be lifted slowly at a consistent rate along the planned path and should never be suspended over a person.
To help guide the structure, crane spotters or riggers on the ground will communicate with the crane operator and may even utilize ropes as tag lines to help guide and steady the structure as it is set into place. Any joint sealants or other pieces that are required for the installation can be installed once the structure has been offloaded.
With lifting plans, safety discussions, the right equipment, and clear communication, offloading and handling precast structures becomes a seamless process.
Stay tuned for our next article where we discuss preparing the subgrade!
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