Aluminum Extension Ladders: Engineering & Construction

Introduction

Significant improvements in the ANSI and UL ladder codes have eliminated several formerly common types of accidents. I will review the problems I observe on ladders produced to the current codes and will list the failures which were formerly common.

Aluminum Alloys

Alloys for siderails and rungs have carefully controlled compositions such that following extrusion the alloys become stronger over a period of time. The process is called precipitation hardening, or aging. At room temperature the hardening is well along in a few months and it is substantially complete in about a year. Reduced temperatures delay hardening while sustained high temperatures reduce the strength as the alloy overages.

In the past, most ladder makers extruded aluminum billets which came directly from the big-3 suppliers: Alcoa, Reynolds and Kaiser. The compositions were consistently well controlled and I never observed an alloy which failed to harden at least adequately. The ladder makers now often buy aluminum ingots on world markets and melt these with appropriate alloying element additions to make their own billets for extrusion into side rails and rungs. Because their quality control is somewhat less stringent than that of the "big-3", there is a potential for inconsistencies in the rate and effectiveness of the aging reactions.

When a ladder accident may have been the result of inadequate metal strength the ladder must be promptly appropriate. Since aging may still be occurring, the hardness must be measured as soon as possible after an accident.

The rest of the metallurgical testing can be conducted at a later time. The issues which we consider include:

  • Chemical composition
  • Deoxidation practice
  • Metal cleanliness
  • Grain structure
  • Response to precipitation hardening
  • Change of hardness with time
  • Metal strength
  • Compliance with standards: ANSI A14.2, OSHA, UL 184

Feet and Caps

Feet and caps are involved in many of the accidents I analyze. Some of the shortcomings stem from the testing for code compliance. The codes call for removing the feet and caps during most tests. As a direct consequence I observe feet which have inadequate mechanical stability and both feet and caps which are less impact resistant than the rest of the ladder. Completely bypassed by the codes are the environmentally induced aging and degradation of plastics leading to increased brittleness, decreased friction, and stress relaxation which loosen fittings over time.

Foot and cap cases we have studied include:

  • A brittle, plastic cap fractured while in use, causing the ladder to shift and throwing the user off balance so that he fell.
  • The friction pad on a foot was secured by a steel staple. The staple rusted through and the foot slipped off the pad, causing a slipout accident.
  • Pivoting feet were made of folded sheet metal. Poor design allowed a foot to rotate while the ladder was in normal service, which caused a slipout accident.
  • Elastomeric feet which originally complied with the code's friction tests aged rapidly and lost much of their friction properties, causing a slipout accident after only two years.

Problems with Ladders Conforming to Older Codes

Rung to Siderail Joints

When the relatively limber Type III ladders are used with more weight on one siderail than on the other, the differential deflection puts torsion force on the rungs. When the rung to siderail joints start to slip accident victims report a "creaking sound" and are then thrown off to one side.

This was a fairly common accident which seems to occur in ladders complying with the new code only when there are manufacturing defects in the rung to siderail joints.

Fire Fighter Ladders

Fire fighters now carry much more equipment and larger hoses so that we are running into tragic failures involving ladders which once had adequate capacity for fire fighters and their equipment but are now overloaded by air packs, radios, and the weight of larger hoses. Loss control specialists should check on the adequacy of older ladders used by fire companies.

Hardware

The codes now require cycle testing of the sliding and latching hardware. This has largely eliminated accidents due to hardware malfunctions and failure. Because the codes do not effectively address the environmental durability of ladders, adjusters need to be alert to selective corrosion which leads to hardware failure. Most often seen are steel pivots which are too small to accommodate foreseeable corrosion loss. Laboratory work is needed to identify the mode of the corrosion, confirm the alloys, and determine that the particular type of corrosion and rate of corrosion fall into the foreseeable category. Through the discovery process we work with counsel to check that hardware in an accident ladder matches what was in the ladders used for certification.

Suggestions for Adjusters

Sight down the ladder and record your observations. Rung to side rail joint slippage may leave one siderail bowed more than the other or it may show an overall end to end twist. With handling, the distortion may snap out so make observations and measurements before the ladder is moved. In witness interviews probe for information concerning the presence and extent of such distortion immediately following the accident. Check to see if the victim recalls the creaking sound just preceding his being thrown off.

Have a hardness test performed ASAP if there has been a structural failure and if the ladder is less than about two years old. Call us for instructions.

Find and retain missing friction pads, caps and metal parts. A metal detector can help you find missing metal but not plastic. Metal detectors are stocked by equipment rental firms.

Take close-up photographs of distorted feet and hardware to forestall claims of alteration following the accident.

Keep the ladder clean and dry so that corrosion is arrested and the laboratory does not have to deal with the issue of which corrosion preceded the accident and which was subsequent.


March 1, 1999 by Fred Hochgraf

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