Help please identifing concrete blocks

[justinpollard]

Hi Friends,
I am building a rear kitchen extension which involves the insertion of pair of steel beams and removal of cavity wall. I want to keep the opening to a maximum thus the brick & block returns/piers either side of the opening to a minimum. The maximum size of the opening specified by the structural engineer depended on the newton strength of the existing internal skin block walls. My question is - how do I identify if I have 3.6n or 7n blocks? I've removed some of the spotted plasterboard to expose some of the block work as per the picture. The blocks are much darker than breezeblocks and they are what I would describe as course/granular/aerated and you could NOT cut them with a handsaw. Your thoughts would be most appreciated. [net]208032/66128_94355839.jpg[/net]

 

[tony1851]

Can't get your pic, but from your description it sounds like the blocks are probably cinder blocks.

If so, their compressive strength will be hard to quantify, though probably they would be stronger than the Celcon-type block, but possibly not up to 7N strength. Perhaps best to play safe and go with the lower strength.

But it seems odd that the SE has stipulated that the width of the opening should be based on the strength of the blockwork. Correctly designed padstones would allow for a weaker block and still maintain a reasonable opening width.

 

[justinpollard]

Thanks Tony, I'm not sure why my photo won't load. Essentially the SE specified 750mm returns each side with concrete pad stones assuming it's built with 3.6n blocks, but I could cut the walls back to 450mm each side if I rebuild the bearing ends with 7n blocks down to floor level, again pad stones at the top. Thus why I'm keen to id the blocks, if already 7n blocks all I have to do is chop in the specified concrete pad stones (n/b I'm going for a clear opening of 3800mm wide, worktops running down both sides and I'm hoping to keep the returns from breaking through the line of the worktops). If I'm unable to easily id the blocks, I may just have to bite the bullet and rebuild the returns with 7n blocks. I've tried adding the photo again.

Blockwork

• justinpollard
• 8 Sep 2013
• 1

The images is of the block work above an existing Catnic type lintel

Untitled

• justinpollard
• 9 Sep 2013
• 1

Untitled

• justinpollard
• 9 Sep 2013
• 1

 

[tony1851]

Comparing the sizes (3800 opening and two 750 returns?) it looks as though your SE has just used the 'rule-of-thumb' guidance in the Building Regs, which states that normally the opening should not be more than 2/3 the total length of the wall.

If I may say so, that seems a bit lazy of him. With fairly simple calcs, it's possible to show that smaller returns than that can be achieved in lightweight block.

Before you start hacking away, I'd suggest asking him on what basis he specified such large returns. The design load each end of a 3800 span will not be that much (assuming a two-storey house?).

 

[justinpollard]

Wow, still up Tony, thanks again for your thoughts. The SE did appear to do a thorough job, he certainly produced detailed calculations and took the time to try and explain it all to me. As you rightly say, I think it may be wise to speak to him again and perhaps get him to pop by and take a look now it's exposed. It would be nice to save unnecessary building work, I have enough of that to do as it is. p.s. Yep it's a 20yr old two story brick build house, conc roof tiles etc.
Again thanks for your time and advice.