The following scenario:

I am tasked to design a 6.6m high reinforced concrete wall in an industrial building. We have a 10 ton Telehandler (telescopic loader) with 7m maximum reach, loading 1500kg loose material in a scoop.

I need to cater for a "runaway vehicle collision" where:

the vehicle reaches a maximum velocity of 2m/s before it collides with the wall.

I need to find out the worst case collision force on my wall.

Usually I would employ the design methodology for vehicle barriers and parapets in car parks in Eurocode 1 (EN 1991-1-1). But in this case the point of contact is significantly higher than the centroid of the colliding vehicle.

I would assume that the front wheels of the loader leave the ground during the collision, thus there is some sort of energy dissipation there too.

I am pretty sure that the long boom just frightens me and is enough of a distraction to stop me from thinking straight here.

Any help would be greatly appreciated.

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In a situation like this you will need to resort to energy methods. Depending on how complicated you wanted to get you could do a dynamic calculation also.

I would recommend looking at the guidance in Eurocode 1991-1-7 (Accidental Actions). In particular look at Annex C which describes dynamic design for impact. This essentially involves equating the kinetic energy of the object impacting the wall to the energy absorbed by deformation of the wall. There are a few simple formulas which can be used depending on how you classify the impact.

You will need to consider a variety of loading cases for example:

  • Load at various heights - separate (or possibly simultaneous) impacts of the loader and load.
  • Load and loader all impacting as one 'lump'
  • etc

From my experience with impact calculations it is unusual for a structure to be able to withstand any significant impact through elastic deformations alone. In which case the structure will obviously be damaged. In this case you have a few options:

  • Protect the structure from being hit in the first place
  • Perform a plastic analysis and provide appropriate detailing so that the resulting plastic configuration can be achieved
  • Allow the structure to be damaged in some controlled way (imagine a fuse or some kind of energy dissipating system)
  • Accept that whatever is being hit will not exist after the impact and deal with the consequences (structural integrity)

In my view it is prudent to consider the consequences of your constraints being exceeded:

  • What if someone is driving too fast?
  • What if someone is carrying a load which is too heavy?

To address your comment below. Clause 4.4 in EN 1991-1-7 states for accidental actions caused by forklift trucks:

The National Annex may give the value of the equivalent static design force F. It is recommended that the value of F is determined according to advanced impact design for soft impact in accordance with C.2.2. Alternatively, it is recommended that F may be taken as 5 W, where W is the sum of the net weight and hoisting load of a loaded truck (see EN 1991-1,1 , Table 6.5), applied at a height of 0,75 m above floor level. However, higher or lower values may be more appropriate in some cases.

So one way of approaching the problem would be to apply an equivalent single degree-of-freedom analysis: First obtain the maximum dynamic interaction force from Equation C.1 (substituting the stiffness of your wall for k as specified in clause C.2.2(1) ): $$ F = v_r \sqrt{k m} $$ Where $v_r$ is the velocity at impact, k is the wall stiffness, and m is the mass of the impacting object.

The time duration of loading can be obtained from equation C.2: $$ \Delta t = \sqrt{\frac{m}{k}} $$

The static deflection can then be calculated by: $$ \frac{F}{k} $$ A dynamic load factor assuming a rectangular pulse load (see Fig C.1) can be calculated according to [1]: $$ DLF = 1 - \cos{2\pi\frac{\Delta t}{T}} $$ Where T is the natural frequency of vibration of the wall. The dynamic displacement is then: $$ \delta = DLF \frac{F}{k} $$ You will immediately notice that: $$ 0 \leq DLF \leq 2$$

You can then obtain the desired section forces/moments by using the dynamic displacement in the standard deflection formulas.

The value of the wall stiffness k will depend on the impact location, construction of the wall, support conditions, if the wall remains elastic (if it is not elastic than the above analysis is no longer valid), etc.

The book cited below is quite good for explaining how to perform an equivalent single-degree-of-freedom analysis.

[1]: Biggs, John M. "Introduction to structural dynamics." (1964). (One of the best introductory books on structural dynamics in my opinion - despite its age)

  • $\begingroup$ Thank you for the detailed reply. I will consider the various recommendations. I was particularly looking for tips for analysis of the problem. i.e. soft impact / hard impact scenario. $\endgroup$
    – SlydeRule
    Sep 22 '15 at 15:18

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