Preventing Bin Failure...Warning Signs to Look for on Steel and Concrete Tanks

This article is based on a presentation by Bill Socha, assistant vice president and engineering manager for Borton LC, Hutchinson, KS (800-835-9310/www.bortonlc.com).

He spoke in August at a Safety and Regulatory Conference sponsored by the Kansas Grain and Feed Association in Salina, KS.

Storage tank failures at grain handling facilities can be extremely devastating, threatening the lives of employees and causing extensive property damage and grain losses. They are also normally preventable.

The key is to look for tell-tale warning signs.

Steel Tanks

Corrugated steel tanks have been popular over the years because of their relatively low cost per bushel to build. However, without proper maintenance, they can deteriorate over time and fail.

Here are some warning signs:

� As the concrete foundation expands and contracts with temperature changes over time, radial cracks can form in the foundation. These are normal by themselves, but watch for openings appearing between the foundation and the bottom of the tank where the foundation is cracked. Also, watch out for unusually large cracks.

� As bolts expand and contract, bolt holes can take on an elongated, slotted shape. Over time, this transfers radial load forces to the concrete floor, which is not designed to handle such loads.

� Watch for bolt heads that have popped off. That will transfer added loading onto the other bolts, greatly accelerating tank wear. Replaced these bolts as soon as possible.

� Another key area to watch is where a loadout spout is attached to the tank walls. Things to watch out for are any ratholing that may appear above the spout mounting, any ratholing that may appear above the spout, and any type of distortion in the tank wall that may be related to the spout.

� A general rule of thumb � the smaller the steel tank, the stronger it is. As tank size increases, the force of the grain against the tank structure when the tank is full increases exponentially.

Concrete Tanks

Slipform or jumpform concrete storage tanks present a different set of issues related to maintaining the integrity of the structure.

� A certain amount of cracking and spalling inevitably takes place on concrete walls, and much of the time, isn�t a cause for concern.

What is more critical is spalling to the extent that rebar becomes exposed. Moisture in the air can cause this rebar to rust away, meaning that the rest of the rebar in the tank walls must take up the load. Eventually, failure occurs.

Fixing exposed rebar is relatively simple. It involves cleaning out any loose material, sealing it up with a bonding material, and patching up the wall with grout or expandable epoxy.

� Water ponding on the roof of the tanks is a sign that the roof may be starting to sag. In the worst cases, roof beans can �walk� out of the pockets holding them in place and fall into the tank.

The only way to know for sure if this is happening is to get down into the tank and inspect the roof from below. (Be sure to follow all confined space entry regulations.) If roof beams are starting to move from their pockets, installing new beams above the roof is at least a temporary fix.

� Moisture is the long-term enemy of concrete structures. Maintaining roof coatings provides a relatively inexpensive form of insurance against tank wear.

� Another thing that can happen with the roof is that the roof cap can shift due to natural expansion and contraction, as well as differential settlement. As the slab shifts, large chunks of concrete can break off and fall, and the roof can crack enough to let moisture in.

� Cracking on the outside walls isn�t always a concern except where the cracks run deep enough to allow moisture to reach the rebar. In some cases, moisture also can leach into tunnels, damaging tunnel walls or even reclaim conveyors.

� The unfortunate truth is that some concrete elevators, especially older ones, were built with inferior concrete. Such concrete generally is too soft for extended use. It is damaged by normal expansion and contraction over time, and near rail lines, the vibration from passing trains also causes damage.

Two signs that call for immediate action � any sudden appearance of cracks deep enough to cause chunks of concrete to fall out (which is a safety hazard) or any bulges or misshapen sections of the walls.

Written by Ed Zdrojewski, editor