Hi thanks for your question. My name is ***** ***** I can answer this for you.
The starting point is to understand what your obligations were under your agreement with your client. Presumably, it was to design a beam suitable for the specifics of the property concerned. The question, then, is did you comply with that contractual obligation and deliver services to a satisfactory standard?
If th beam design was suitable to that extent, then it seems that you would have complied with your obligations vis-a-vis your client. As such, there would be no claim against you.
The cost of alternative suppliers is presumably not relevant, unless you were asked to design a beam that would cost a certain amoun or under that amount. If this is the case, then you have to determine whether you complied with that obligation. And yes, the range of suppliers them comes into the equation and the likely cost of the beam.
If a reasonably competent builder could fit the beam you designed, and it was otherwise in accordance with the design paramaters, then I expect there is no claim against you at all.
Hope this answers your question. If you wish to clarify anything, do let me know, otherwise please remember to rate the answer for me.