The Wretched Life of a Trench Box Strut

14 Jun

Following on from the last blog article on trench box panel design, I thought I’d better complete the task as promised and continue the discussion on the components that hold the panels apart; that is the struts.

Struts or spindles as they are sometimes known perform the understated but essential task of transmitting the forces exerted by the ground between opposing box plates. During their life cycle they get subjected to all sorts of use and abuse as they tend to be in the firing line of a swinging excavator bucket; but without them being able to perform their function under harsh use, catastrophic collapse of a box can easily occur. To set the scene I enclose a “rogues gallery” of damaged box struts that we commonly see in our depots.

Ignoring slide rail systems for now, there are three basic types of box strut namely:

 

  • Fully adjustable
  • Incrementally adjustable
  • Fixed length

The fully adjustable strut is traditionally known as a spindle due to its relative slender profile. It comprises an adjustable left and right handed screw threaded section a series of fixed length (“plug in” or bolt together) extension pieces and a pair of end mounting units or spring spindle holders located at each end of the strut assembly to form the connection to the box panels. The latter allow a degree of strut articulation to facilitate a typical dig and push installation sequence. The spindle assembly, although robust, does need to be specified with care at longer spans as the “plug in” joints between the extension bars will promote a degree of sag in the struts reducing its load carrying capacity due to potential buckling effects. 

Due to its greater versatility and more robust design, the trend nowadays is to use an incrementally adjustment. This family of struts comprises inner and outer telescopic steel box sections with a series of alternative pin location holes along the length through which a shear pin passes. Again an end rockers allow a degree of strut articulation as with the spindles to facilitate a typical dig and push installation sequence.

Some boxes utilise fixed length struts in a variety of profiles however these tend to be not that popular due to their limited versatility on site.

Design considerations

Due to the fact that most box struts are adjustable in some manner, the structural analysis can be complex due to the mechanisms and required and component clearances involved.

For example long struts that use a plug-in extension system can exhibit alarming buckling inducing sag when several joints are present. We actually limit the length and number of joints to help alleviate this effect. Add this to the difficulty in assessing the local structural strength of the adjustment screws, means that the best way of assessing structural capacity of these struts is from test data.

Incrementally adjustable struts offer a much simpler calculation model. The sliding sections are generally quite substantial, typically RHS members that are not susceptible to lateral buckling under normal loading over the strut lengths specified (4m to 5m max). It is the more local effects such as shear in the adjustment pins and bearing in the sliding members which dictate overall structural capacity.

One final point concerns the age old chestnut of using struts to close off with sheeting the ends of the box. This is definitely a no-no with the spindle type of struts that are not designed for any sort of side loading. The more substantial typical RHS section incremental struts do indeed have spare capacity to withstand some side loading but this would need first principal verification for combined loading effects. Far better to use end closure panels such as Endsafe that have been developed specifically for this purpose.

That’s just about concludes the more technical aspects of trench boxes. I can now go off on holiday for a couple of weeks and will return in the summer with another technical blog offering from the world of shoring. For those of you that read New Civil Engineer, we have a few Viewpoint articles planned on topical subjects over the next few months. I wish I had gone on that creative writing course now! 

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