What UB or lintel over bifolds?

[Cluffers]

I've found so much varying information and nothing for my specific scenario. I'm having a single storey rear extension with 3m bifolds out the back. Warm flat roof.

100mm block on each skin and 150mm cavity w/ full fill ins
4m (3.65 internal) out and 5.7m across (5m internal) - the ceiling joists at 9x2 running the width of the extension, so will not be bearing either on the rear of the existing house or over where the bifolds will be.

In this case, I don't know whether to put a 152x89 steel in above bifolds with an 8mm welded plate to catch the outer blockwork, or whether this is just overkill for something which will essentially just be taking 2-3 courses of blocks above the bifolds? I'm just concerned about any deflection w/ a standard duty cavity lintel pinching the doors.

 

[Dereekoo]

Well a standard lintel will be designed with a 1/300 defection limit at design load which in your case means 10mm, usually a limit of 5mm for bifolds but depends on manufacturer. But as your lintel will not be loaded to its limit the deflection will be less, determine the load and ask the manufacturer what the deflection will be or if your actual load is 50% of the design load assume the deflection will be 50% of the 10mm limit. You could always test the lintel before installation by loading it with blocks (human bodies) and actually measure the deflection

 

[^woody^]

Bear in mind that a "standard" lintel relies on a section of masonry being built on it as part of a composite design.

The biggest problem with his type of scenario, is not the heavy loading, but the light loading and the relative flimsiness of the wall above the opening - which becomes susceptible to vibration and movement and then cracking of finishes. A substantial beam or intermediate restraints would deal with this.

 

[Cluffers]

Bear in mind that a "standard" lintel relies on a section of masonry being built on it as part of a composite design.

The biggest problem with his type of scenario, is not the heavy loading, but the light loading and the relative flimsiness of the wall above the opening - which becomes susceptible to vibration and movement and then cracking of finishes. A substantial beam or intermediate restraints would deal with this.

Hi woody, when you say substantial beam, what would that look like? 152x89 or something heavier

 

[tony1851]

A good beam would be a 152 x 152 x 23, with 6mm plate welded underneath. If supporting 3 courses of dense block cavity wall (and no load from the roof), the deflection would be around 3mm.

If there are 2 or 3 block courses above, put a couple of 30 x 5 steel straps built into the inner skin and screwed to the underside of the first 3 joists parallel to the wall. That will help restrain the wall.

Why are the joists running the length of the extension? Its usual to run them front-to-back, in which case the joists and roof boarding give some lateral restraint to the wall above the beam.