Help,

We have a bungalow built on a sloping site with a square bay window on the back which is made up from UPVC windows on a studd wall made up from 4" x 2" studds with plasterboard internally and T&G boarding on the outside.

Because of the sloping site the floor of the window is actually some 1.5 metres above the ground and is supported on two cantilevered timber beams each 8" x 2" and which extend approximately 400 mm out from the wall. (The window is only 400 mm deep).

We want to replace the bay with some sliding patio doors with a tall narrow window on either side to retain the existing profile and give access to a proposed balcony at a later date.

I have had two quotes from local window companies but the third was concerned the cantilevered beams wouldn't be strong enough to take the weight of the patio doors and side windows. These will weigh approximately 150 Kg.

I would have thought it would be OK because each cantilevered beam would only be taking around 75 Kg plus some load from the floor of the window and the flat (felted) roof above and at the worst this is only 400 mm from the wall.

I could possibly work out the loads in the beam (I was a mechanical engineer) but it's a long time since I did any thing like this and I've no real idea what is an acceptable level of stress and deflection in the beams.

Could anybody point me in the direction of a suitable formula and / or acceptable stress figures.

John.

Not an engineering that type of stuff but wouldn't think it would be a big problem as you would be removing a smillar amount of weight before fitting the doors.

I think I can visualise from your description, but a sketch or two from you help get more replies.

Not sure if these help:


[img]
Rear Bay Window [/img]


Cantilever Beams

John

I'm a Civil Engineer, it's been a while since I have done any structural engineering and tbh my speciality is motorway bridges, but, I can't see a patio door weighing much more than the arrangement you have at the moment so in my opinion you should be fine. The loads are pretty small and the capacity of an 8"x2" beam is fairly substantial.

how will you be supporting the balcony? that may be where the confusion is setting in...

If you want to check the loads it's easier to consider it as a simple analysis. treat it as a 400mm long cantilever with a point load on the end to calculate the bending moment (W.L) the point load would be a combination of the dead loads (the self weight of the structure) and the live load (people in the building)

for your dead loads you need to consider the weight (in kN) of the windows, the roof, the flooring and the beams themselves.

the live load from Eurocodes is 2kN/m2 for floor area supported by the cantilever (the area inside the wall footprint x 2 will give you the total load in kN)

add the dead and live loads together then divide by 2 for the point load at the end of each cantilever, this is a worst case scenario for the bending moment calculation.

there is a worked example for timber design to Eurocodes in the link below, you would only need to consider Bending, Shear and Deflection for your application though.

http://www.roymech.co.uk/Related/Construction/Timber_Ex.html

I hope this helps, It's fairly well referenced but any questions give me a shout and I will try to help if I can.

[Edited on 4/10/16 by Attilauk]

[Edited on 4/10/16 by Attilauk]