IStructE exam tips: What are ‘two distinct and viable solutions’?

One phrase that occurs in Section 1a of every question in the Chartered Member exam is:

“Prepare a design appraisal with appropriate sketches indicating two distinct and viable solutions for the proposed structure”

But what exactly is meant by “two distinct and viable solutions”?

It's a common question from candidates, and that's partly because this issue is not black and white, it's open to interpretation. A paper can be marked down if the two schemes are deemed ‘not distinct enough' – take this quote from the 2014 examiner's report on question 3, for example:

“Many candidates proposed two composite solutions with concrete slabs supported by steel plate girders or pre-tensioned precast Y or U girders keeping the rest or the structure identical. These may have been viable but were not sufficiently distinct to score high marks.”

So let's break this down, and give some examples:

Viable
This is the easy part. Both of the solutions you come up with need to be viable, which means they need meet the following criteria:

1. Strength: The structure must be strong enough to support the applied loads. There must be a clear load path for loads to take from the point of application down to the foundations – You will demonstrate these load paths when you draw the load path diagrams in section 1a.
2. Stiffness: The structure must be sufficiently stiff to support the loads without excessive deflection. Particularly if you have suspended structure, you should demonstrate that the deflection will be within acceptable limits.
3. Stability: The structure must have a stability system to resist lateral loads in BOTH orthogonal directions. For example a steel frame can be braced in both directions, or it can be braced in one direction and portalised in the other.
4. Buildability: The scheme should be buildable. Although this criteria is not as critical as the others, you could be marked down if you develop a scheme which is totally beyond all realms of reasonable possibility. If there is a particularly difficult to build aspect of your scheme then you should acknowledge this and suggest how this could be solved.

Distinct
Your two schemes should be sufficiently different to each other that there is as little overlap between the two designs as possible. You should try to avoid having identical elements in both schemes. The reason for this criteria is to allow you to demonstrate your skills to the examiner. Being able to come up with alternative options for a client is an important skill, and it also shows the examiner that your design ability is not just limited to churning out exactly the same type of structure time after time, you need to demonstrate flexibility.

There are a number of different attributes of a scheme which can be varied in order to generate two distinct solutions. Below is a list of attributes you could try to vary in your schemes, with some different examples for each one:

1. Material
   1. Steel
   2. Concrete
   3. Timber
   4. Masonry
2. Frame type
   1. Braced frame
   2. Loadbearing walls
   3. Suspended slabs
   4. Cantilever slabs
3. Grid size
   1. Square / rectangular
   2. Distance between columns
   3. Constant or varied grid
4. Primary / secondary beam layout
   1. Only primary beams
   2. Long secondary beams and short primary beams or vice versa
5. Slab span direction
   1. One way or two way span
   2. Single span or multiple span
6. Stability system
   1. Braced frame
   2. Moment frame
   3. Portal frame
   4. Shear walls
7. Foundations
   1. Shallow pads / strips
   2. Piles and ground beams (different pile types)
   3. Raft foundation
   4. Piled raft
8. Construction method
   1. Insitu / precast
   2. On site erection / prefabrication

The more attributes you vary between your two schemes, the more distinct they will be. I would suggest that only one variation (such as a simple material change from steel to concrete) would be considered inadequate. Changing two attributes might be sufficient to pass, but you may be penalised for not making them distinct enough. Changing three or more attributes should be a reasonable goal to aim for when developing your schemes.

Example
Scheme 1: Braced steel frame on a square grid, with composite deck slabs spanning one way between primary beams. Possibly pad and strip foundations, and suspended insitu slab, or driven piles and ground beams.

Scheme 2: Insitu concrete moment frame with a two way spanning flat slab, on the same square grid (grid size could be set by the question). With an insitu concrete raft foundation or CFA piles. (Could also have shear walls for stability in one or both directions)

Discuss
That's just a simple example, so if you have anything to add to this or other IStructE exam tips then discuss it in our forums here.

We'd love to hear what you think!