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Bus Stop bench (Advanced)
Suggested viewing: Bus stop bench (Intermediate)
DESCRIPTION AND LOCATION
This is a really well designed and detailed stainless steel bench, located outside the fire station on Eyre Street in the centre of Sheffield (i.e. around the back of the new Market at the bottom of the Moor, and not far from Decathlon), the postcode for which is S1 3FG.
APPLIED LOADS
The first thing to do with any structure is to think about the loads which are likely to be applied to it. These are usually fairly obvious, but not always. Indeed, structures are sometimes subject to abnormal loads, which the designer needs to try and anticipate, and design for as appropriate.
In this case, obvious loads include the self-weight of the bench itself, and the weight of people sitting (or even standing) on the bench, as well as any bags and packages they may have. There will be some wind load on the bench (although the holes in the seat will reduced vertical wind loads), although this is likely to be nominal, and can therefore be ignored.
Not so obvious loads might include somebody trying to kick the bench over (in other words, a horizontal load applied to the seat or at the top of the post, which is likely to be far more onerous than any wind horizontal loading). Not that this would be an accidental load case, so EC1 allows use to apply a reduced load factor of 1.05 to it, as opposed to the standard load factor of 1.5 for an imposed load.
CONCEPTUAL/QUALITATIVE BEHAVIOUR
Supports:
Here, there is only one support (i.e. at the base of the vertical post) so all loads applied to the bench (including its self-weight) must be transferred through the horizontal seat, to the vertical post, and down to this support.
Unbalanced Imposed Load Scenario 2
Whilst it is likely that the two imposed load scenarios considered in the intermediate tutorial will give critical member forces, support reactions and deflections, for completeness we will consider another unbalanced load scenario, based on an adult on the left hand seat and a child on the right hand seat, as shown below (note that there could potentially be an adult sat in the middle as well, which would increase the axial force in the post and the support reaction, but we have not considered that here).
This loading arrangement will give the bending moment diagram shown below. Here, the out of balance moment at the head of the post = 0.9L (from the left hand seat) minus 0.3L (from the right hand seat) = 0.6L.
Similarly, the shear force diagram will be as shown below.
Finally, the deflected form diagram will be as shown below, which is similar to the case in the intermediate example. However, the deflected form of the right hand cantilever will be curved here, due to the presence of a load on this member. Further, whilst the head of the post will still rotate and deflect, the magnitude of the movement will be less than that in the previous unbalanced load case due to the partial balancing effect of the load on the right hand seat/cantilever.
Accidental Loads:
Last but not least, it is always important to consider accidental loads. This could potentially include vandalism, in the form of somebody kicking the seat. This could be considered as a horizontal load applied to the end of the seat, either along the length of the seat or at right angles to it, as shown on the right. This could result in axial force, shear and horizontal bending to the seat itself, as well as shear and bending to the vertical post.
OTHER THINGS TO THINK ABOUT
How does the seat itself work structurally?
It’s essentially a channel section bending about its minor axis. If you had the dimensions of the section, its section properties could be calculated using the parallel axis theorem. Under normal loading conditions (i.e. when the seat is subject to dead and non-accidental imposed loads) the section is bending about its minor axis, so lateral torsional buckling effects would not need to be considered.
Why use a rectangular hollow section (RHS) for the post?
Whilst box sections are very good for members subject to axial loads and bending (due to them being less susceptible to buckling effects than open sections) the relatively low forces and low effective lengths involved in this structure suggest that a box section was probably chosen for aesthetics and reduced maintenance requirements (an open section would attract dirt and rubbish more easily).
Can you explain the orientation of the post?
Well it certainly wasn’t for structural strength or stiffness reasons, since the bending moment and associated deflection in the post due to normal loading conditions will be about its minor axis. The most likely explanation is that the orientation chosen makes for a stronger connection between the seat and the post (i.e. the width of the post is not much less than the width of the seat).
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