Extension structural beam installation advice

[dirtbiker3]

I’m interested to see what the more experienced members on here would do in my situation. Hopefully someone has done a very similar job in the past and can share their experience with me.

I have a 2004 built persimmon house which is a conventional built brick n block house with a 50mm cavity. I am in the process of adding a rear extension to the property, full width (6.25m) double storey. It’s fairly simple as the ground floor is an extension to our open plan kitchen/ diner and the first floor will extend the bedrooms. The roof will have a hip at the back and stepped into the existing roof. The existing roof will be supported by a newly formed girder truss when the new roof is installed. This negates the need for a structural beam on the first floor.

My next step in the build is to install a structural beam on the ground floor. This is where I need some input from experienced builders with regards to fitting supports in the correct places to ensure the building is safe. We plan to open up the downstairs completely, meaning two channel beams bolted back to back (180x90 PFC) will span the full width of the ground floor and sit on padstones either end. A structural engineer was consulted in the design. We plan to fit the beams under the floor joists. He has indicated he is happy for us to take the block work completely out meaning there will only be 100mm bearing on each end as we will simply be continuing the blockwork through. One problem here, as the existing cavity is 50mm and the extension is 100mm, we have a 50mm step. To get over this, we are planning to use a padstone 440*215*140. Does this sound acceptable? I had expected the SE to specify the need for a ‘nib’ to act as extra support for the beam but he is more than happy that this isn’t needed. Mainly due to the fact the existing walls above it will be knocked through and replaced with a timber stud wall to divide the bedrooms and the existing roof will be supported via a girder truss on the new roof.

Hopefully I haven’t confused you all yet

Like I said, we have now bricked half height so ready to fit the structural beam. I plan to do it in the following way;

*Fit acrow props (max 900 centres) inside under the existing floor joists, 5.7m span so say 7 acrow props with scaffold boards above and below

Q. Should I remove the existing plasterboard to expose the joists and prevent it crushing?

Q. Initially there will be a lot of weight above these joists as the inner block work and roof is being supported. Will this method still be acceptable


*For the outer leaf brick work, we plan to use strong boys / acrow props at max 900 centres

Q.Is this a sufficient number of acrows, given the fact that when using strong boys the load capacity is only 340kg?


*The beams will be cited at the base of the opening before supports are positioned. They weigh around 160kg each so I may use a genie lift to make life easier. Once in position, they can be bolted together.

Q. Should the padstones be laid first to the correct height, then when the beams go in, make any adjustments using metal shims/packers?


Once the steels are in, I will use acrows as extra temporary supports until the roof is finished. This was advice for the SE, to be on the safe side. The acrows and strong boys will also have to remain in place to support the outer brickwork as the beams are not wide enough to span both leafs of brick/blockwork.


Hopefully, the above description explains things well enough to get some helpful advice and guidance. Thanks in advance for any constructive comments you can offer.

 

[tony1851]

Has your SE said anything about lateral stability of the house? Because if he hasn't, Building Control certainly will.

 

[dirtbiker3]

Tony, that was my initial worry about the lack of lateral restraint. I queried this with both the SE and BO and they are happy with the workings the SE has provided.

What are your thoughts on this?

 

[tony1851]

Your plan doesn't show the whole ground floor of the house, so it's not possible to know what other structural walls are running across-ways and providing lateral stiffness.
What doesn't help is that it seems to be a detached house, so it can't rely on any stiffness from an adjoining property.
I have to say I'm surprised that the bco has taken the SE's word without question. Not saying there is going to be a problem, but in my own experience, when the back wall of a house is being removed completely at ground floor level, b/c have either asked for a frame, or alternatively, figures to prove that the house has enough residual stiffness without resorting to a frame.

 

[^woody^]

Why does a wall need lateral restraint from a beam at a point along the wall's length?

 

[tony1851]

It doesn't need 'lateral restraint from the beam', it needs lateral restraint for wind load on the side wall.
The two-storey extension has increased the area of the side wall of the house (and therefore the wind load on the house) while pushing the buttressing walls further out.
This isn't to say that the house is unstable, just that it's an issue that in this case should not be dismissed so easily by the SE and bco, particularly with it being a detached. I would suggest it's a case where particular attention should be given to fixing the new floor joists and roof structure to the new walls, rather than just paying lip-service to these items as most builders tend to do.

 

[^woody^]

A two storey house with of the sizes described in this thread does not need an special additional wind loading precautions to what is already there. Taking out that rear wall may well need a wind frame if the extension was not there, but it will be, so it wont.