Shipping containers are designed to store heavy items and transport heavy equipment around the world. Therefore, both 20ft and 40ft containers can store extreme weights, especially if they are being used for static storage and do not need to be transported with the weight inside.
20ft containers can store up to 28 tonnes and 40ft containers can store up to 26 tonnes. The floor of shipping and storage containers have steel supports underneath which are overlaid with heavy duty marine ply flooring. As long as any cargo loaded inside the container is evenly distributed across the floor, you will be fine storing heavy equipment.
If you have a single heavy item or several small heavy items which require storage, the floor of the container can be strengthened in the area that will take the stress of the load, this solution is ideal for static storage, but, if you are shipping the container further modifications may be required on the container.
When you are storing heavier items in your shipping containers, it is certainly worth considering upgrading to a tunnel container, these unit types have cargo doors to both 20ft or 40ft end making it easier to load and unload heavier items without needing to empty the whole container.