This is tricky without a drawing...however the implied gallerdurksplan is definitely a mezzanine, gallerdurk is absolutely grating. If it's steel though is not known. Could be aluminium, fibreglass or a special type of plastic. (But let's assume steel as its most common in ordinary use.) Pelarupplag is a generic word for column support, which can differ rather a lot in how it is done. Is your gallerdurksplan lowered onto it's supporting columns?
Hope this is of some use.