Second opinion on structural calcs...

[Chaser]

I'm in the process of sorting out the plans for an extension. Semi detached 2 storey house. House is 6m wide, but i'm going out at the side and out at the back. Original construction is brick, cavity, block. Extension at the rear is blockwork only.

I've had calcs for a span across the back of the house to open it up and i'm advised 2 356x127x33 bolted together with a 665mm padstone on both sides.

The depth of the beam means I wont be able to achieve a flush finish (the extension is vaulted sloping upwards towards the house) and 790mm piers to sit the padstone can reduce if I replace the inner leaf with brickwork.

Im no SE myself, but one of the builders quoting has questioned the size of the beam as being excessive but I'd be more inclined to go with the SE over the builder! Whats the thoughts on whats been provided?

 

[oldbutnotdead]

Does the SE know you want a flush finish? Those beams and pads sound a tad chunky but it is the SE's neck on the line if he underspecifies so it wouldn't be a surprise if they've gone for a conservative solution. The calcs they will have provided with the drawings should give you a clue- if there's a massive safety margin then there'll be scope for redesign (but not by the builder!)

 

[^woody^]

Ask the SE. If you live on Station Road or something, he may have got you mixed up with a railway bridge.

 

[tony1851]

If the house is 6m wide and you are going out at the side (a further 2.5m??), the clear span will be around 8m. The optimum beam for that span would be one of the 254 x 254 'H' sections. Theese are heavy but you can have them in two lengths bolted together.

The piers do seem overly-large and intrusive for your kitchen units - with care in design they can usually be considerably reduced.

On that span, you will never get a flush ceiling because the depth of the beam will be greater than the thickness of the floor.

 

[Chaser]

Im out at the side but it's not pulling the room across. Thats a separate utility and garage. The opening is a fraction under 6m

 

[tony1851]

In that case, 356-deep beams are definitely too big.

 

[Chaser]

The floor joists are 9 inches (whatever that is metric) so I'll probably struggle to get flush anyway but I wasn't hoping for such a big bullnose before it vaults. If the piers are built from Brick I can go down to 440mm

 

[tony1851]

Do the floor joists run onto the wall, or parallel to it?
Is the wall a cavity wall or solid wall?

 

[Chaser]

Its a cavity and the joists run in to the wall.

 

[tony1851]

In that case, a 254 x 254 beam could just about give you a flush ceiling. The ends of the joists can be notched on the underside to sit flush on the bottom flange and, taking into account the thickness of the floorboards, you should be OK for minimal intrusion upstairs.
The beam would be heavy, but there are ways of having it in two or three sections bolted together once in position.
I'd have a word with your SE regarding your point about a flush ceiling. If you use 356 deep beams, the best you would have is a 100 downstand without allowing it to protrude upstairs.