The creepy secrets of metals and plastic
By Jim Anderton, technical editor
In southern Pennsylvania, there's a famous house designed in the '30s by the American architect Frank Lloyd Wright, called Fallingwater. The home features balcony-like floors that cantilever out from ...
In southern Pennsylvania, there’s a famous house designed in the ’30s by the American architect Frank Lloyd Wright, called Fallingwater. The home features balcony-like floors that cantilever out from a rock face without supporting columns, apparently defying both gravity and good engineering sense. I say “apparently” because on closer inspection, the cantilevered floors are indeed sagging. Fallingwater is falling down, slowly. The reinforced concrete floors are experiencing a phenomenon known as “creep”, the slow deformation of a bulk material under an applied load that’s less than the amount normally needed to permanently deform the material.
The strange and potentially dangerous aspect of creep is that it happens under loads that are well below anything normally considered serious, and well within the margin of safety for most shapes and structures. Apply the load for a little while, and the material simply springs back, whether it’s polyurethane or granite. Apply the same forces over a long period of time, and the material eventually flows in the direction of the applied force. Metals creep (which in jet engines is a serious problem) and so do polymers.
Creep is a factor in the plastics business in two ways: one is the way the polymers react, and the other relates to the metal equipment that we use to mold them. Ever move an old machine to a new location and find new problems that never existed before? That press may be a collection of stout castings and steel weldments, but on a microscopic scale it’s as flexible as rubber, and if it’s left in one place for a long enough time, will “take a set”. Leveling the machine periodically goes a long way to reducing the effects of the phenomenon, as does making sure that the unit rests on the mounting points the manufacturer intended.
Surprisingly, the effect seems to be worse for lighter weight equipment than for the heavy cast base machines. I’ve seen smaller presses operating on wooden blocks and shims — a sure way to introduce stresses where the frame designer never intended. Is vibration a problem? If you’re considering anti-vibration mounts, talk to the machine manufacturer first. Modern equipment is designed to take it’s own weight into account, weight that’s fed to the ground through mounting points that are designed to maintain machine alignment and perhaps minimize transmitted vibration and noise. Use four mounts instead of six, for example, and you’re setting yourself up for mysterious issues later. Even adding support can be counterproductive where machinery is aligned to take it’s own weight into account. Heavy frames sag, and that’s not a bad thing as long as it’s compensated for, so reconsider before “improving” the machine designer’s ideas.
The same logic goes for bracing and triangulation. There are few, if any, unnecessary components in a good machine design, and sometimes it would be easier to service or maintain a machine if that damn cross-brace wasn’t in the way. The fact that the equipment seems to work fine without it (for now) isn’t a reason to leave it off.
And what about polymer products? Plastic lumber made from recycled materials is a good example of the potential for creep. In un-reinforced form, recycled thermoplastics need to be used carefully in safety-critical designs to avoid creep. A sagging picnic table is one thing, but a balcony is another. Even Fallingwater is now supported by steel columns while the reinforced concrete is post-stressed as part of the building’s restoration. For my humble deck, however, I’m looking forward to using thermoplastic lumber for structural applications, such as the cantilevered beams, instead of just for the planks.
