John Rumm wrote:

Coming back to the joists, I had a gut feeling that deflection would
be more than desired and your results back this up although you have
quite rightly taken the worst case with the load in the centre of the
span. In our case most of the load will be near to a sleeper wall for
each leg. But I would rather be conservative and use worst case!

That also assumed the leg directly over one joist - if it were to the
side a little the the load would be more spread between two (I did leave
the load sharing set to 2 however)

No problem, you have confirmed what I thought and unless one is a
structural engineer it is better to err on the side of caution.

One idea was to double the joists up i.e. putting extra joists in
between but in the areas of high load to have double joists joined,
but I had thought of glueing them together and bolting through the
neutral access. Any problems with glueing and bolting?

I would be a little wary of bolting a joist that is only 100mm high.
Your standard 12mm bolts would hence be taking out over 10% of the joist
depth at the point.

Yes, I see that but wouldn't the fact that the beam was drilled only in
the neutral axis i.e. along the centre where I believe that the beam was
neither under compression or expansion make this OK? I would also clamp
the edges tightly whilst the glue sets.

You could glue, or use the spiked timber connectors on the bolts if
going that route. I would have though that just nailing the joists side
by side would be enough in this circumstance.

You are right. I just have an inherent distrust of nails, even big ones!

I also had an extension to this idea...I thought of cutting 8" strips
of 18mm WBP, glueing two together and then sandwiching that between
two joists, all glued and bolted. In effect a 'T' beam. Obviously the
ply can't extend the total length because of the sleeper walls but
that would probably not matter as shear is unlikely to be a problem, I
think. Any comments on this idea?

Doable, but possibly overkill. If you wanted a beam stronger than a pair
of timbers side by side, then the next option would usually be a flitch
beam[1] (i.e. a pair of beams with a steel plate sandwiched between
them, and bolted together). Not sure how realistic that is on only 4" of
depth though.

I did not know about flitch beams. Maybe my thoughts about deep depths
of plywood sandwiched between the joists are my primitive way of
re-inventing the wheel :-) However that has got me thinking that this
might be the neatest, most elegant and best solution. Presumably a steel
stockholder would cut and 'passivate' them for me.



[1] Example 8mm steel plate flitch (with red passivated coating):

http://www.internode.co.uk/loft/images/flitch.jpg

Yes, I see it there - what does it have to support?

Thanks again John.

Steve