Figure 1: Side Member Failures Located at and near the ‘Make Shift’ Joint.
 

Figure 2: Corresponding Failure Sites on Remaining Length of Ladder.

Objectives

An investigation was required to determine the root cause for failure of the ladders.

Investigation

The following information was gathered through a visual inspection and from the incident report:

• The ladder was composed of two straight aluminium ladders attached by means of 4 pieces of duct tape and two polypropylene lashings
• The ladder was pitched at an angle far less than the recommended 60 -75 degrees.
  
• Failure occurred without warning, and a distinctive cracking sound was heard immediately prior to failure.
• The adjoined ladder consisted of a 2500 mm and a 3500 mm ladder.
• The length of overlap of the two attached ladders was ~ 875 mm.
• Failure of the ladder occurred in both side members (Figures 1 and 2).

Figure 3 shows the failure site of one side member. The plastic deformation and final failure regions of the material are indicated. It is clear from this feature that the side member material failed by plastic yielding, as deformation of the material is clear.
 

Figure 3: Yield Failure of Side Member

Figure 4 shows the failure site of the remaining side member. The initiation site and fracture surface are highlighted. The features of the failure site are indicative of material fast fracture.

Figure 4: Fast Fracture Failure of Side Member

Spectrographic chemical analysis of the ladders was carried out and the Brinell hardness of the material was measured. The results were within specification.

Results

Due to the nature of operation of the ladder, it can be concluded that the failure resulted from the application of an abnormal bending moment. The possible orientation of the adjoined ladders and their likelihood of producing a similar failure as that observed are shown in Table 1.

Table 1: Assessment of Possibly Ladder Orientationss

Orientation No.	Potential Ladder Orientation	Likely/Unlikely	Explanation
1		Likely	Results in a bending moment capable of producing a similar appearance of the yield failure site as observed.
2		Unlikely	Results in a bending moment incapable of producing the yield failure site appearance observed.
3		Unlikely	Results in a bending moment incapable of producing the yield failure site appearance observed.
4		Likely	Results in a bending moment capable of producing a similar appearance of the yield failure site as observed

Conclusions

The possibility of the ladder overlap, ladder over length and horizon service angle inducing the abnormal bending stress was examined and concluded to be sufficient to result in yielding failure observed in one side member, followed by fast fracture failure of the remaining side member. The root cause for the failure therefore is the minimal horizon service angle, ladder overlap of the adjoined ladders as well as the resulting over lengthening.