A water load bag is a flexible, heavy-duty bag filled with water on-site to apply a controlled proof load during testing. It is used to test cranes, davits, winches, structures, and other lifting equipment in their working environment.
Find answers to common questions about Cookes × JWA water load testing. This page covers how water load bags work, when proof load testing is legally required in New Zealand, what documentation and certificates are issued, and the practical considerations involved in planning, measuring, and delivering a proof load test. Whether you're managing a crane fleet, planning a five-year marine survey, or assessing a civil or industrial structure, use this page to understand the method, the compliance context, and what to expect from a Cookes-delivered job.
What is a water load bag?
A water load bag is a flexible, heavy-duty bag filled with water on-site to apply a controlled proof load during testing. It is used to test cranes, davits, winches, structures, and other lifting equipment in their working environment.
What tonnages are available?
JWA water load test bags are available from 250 kg to 110 tonnes in the standard range, with low-headroom bag options from 250 kg to 15 tonnes for more constrained testing environments. Note: individual bag capacity differs from total system capacity. Confirm the right configuration for your required proof load with our team at quotation stage.
Can bags be combined for larger test loads?
Yes. Bags can be used individually or combined to build up the required test load. For large or unusual test loads — beyond what local stock can handle - cross-rent capability through JWA's global network means there is no fixed ceiling on the test tonnage that can be supported. Confirm availability and lead time with our team.
How is the load measured accurately?
Load is measured using a calibrated load measuring device — typically a load cell, crane scale, or dynamometer. Fill volume alone is not sufficient for precise, auditable verification. A calibrated load cell is required for every proof load test using water bags, and the calibration certificate must be current on the day of the test.
How is the bag filled on-site?
The bag is transported to site empty, visually inspected, and the serial number is matched to the inspection certificate before use. The bag is then rigged into position, hoisted slowly to allow it to unroll clear of snag points, and filled with water on-site - progressively building the required test load while the calibrated load cell is monitored. This avoids the need to transport the full test weight as a solid mass.
What happens if a bag leaks?
Water load testing is carried out using inspected equipment, a pre-use condition check, and controlled procedures. Bags are inspected before each use and any bag that does not pass the condition check is quarantined before deployment. As with any proof load test, a lift plan, exclusion zones, competent supervision, and a risk assessment are required. A leaking bag causes the load to reduce gradually - unlike a solid weight, the load does not fall. The procedure for stopping or abandoning a test safely is part of the lift plan.
Are there situations where water bags are not the right choice?
Yes. Water bags may not be suitable where there is no viable water supply on-site, where freeze risk is present (alpine or extreme southern NZ locations), where an OEM test bed or factory acceptance setup is required, or where high-impact drop or shock-load testing is involved. Additional care is required in high-wind conditions or where headroom and geometry do not suit a hanging bag arrangement. If you are unsure whether the method suits your site, speak to our team before committing the scope.