Yet another Padstone question.

[Firsttimebuilder]

Having read a lot of the threads on padstones, there seems to be very little concensus or certainty.
I am working my way through my first extension, a single storey of about 100m2 that includes an integral double garage. The door opening is 4.5 metres wide and I have had a special lintel made by Keystone. I have obatined the (worst load case) loadings from the roof trusses from the truss company (Donaldsons) and spent about 40 minutes on the phone to the techncial department as Kestone running through the loadings from the limited amount of masonary and the trusses (one particular point load from a girder truss 1.5m in from the end meant that none of the standard lintels were acceptable in shear).
The construction is fairly typical, 100mm thermalite blocks on either side of a a 100mm cavity with render on the ourter skin. On either side of the garage door opening I have used 7N/mm2 Thermalite blocks to give extra compressive strength to allow for the imposed loads from the roof trusses on the lintel (I have alos used a steel re-inforcing mesh in every other course to improve stability, but cannot find much info on what effect this has - though there is also a non-buttressed run of just over 9m and this should help prevent cracking - or so I am told).
My question is, do I need to use padstones for the bearing of the garage door lintel? It has a bearing of 200mm at each end, and supported by the two skins of blockwork and the return that closed the cavity I get an area of bearing of about 50000mm2, which should support 350kN. The total load on the lintel is 46.9kN, but with an uneven load distrubution, so the maximum load at one end is about 30Kn. So, from this I cannot see why I would need to use padstones rather than just have the lintel bear directly on the blocks.
But, from what I have read, there seems to be a strong prejudice against this - have I missed somehing (or done my maths wrong)?

Also, I have been advised in another forum that the max bearing stress of 7N/mm2 blocks is about 1.8N/mm2. CAn anyone tell me why it is not 7N/mm2?

This is my first post, so hope it make sense.

 

[noseall]

Generally speaking proprietary pressed steel lintels, i.e. Catnic, I.G. Birtley etc, do not require padstones and sit directly onto block.

I personally have only had to fit padstones beneath u' beams (rsj, girder type).

 

[^woody^]

That's the odd thing

Swap out your Catnic, and put a RSJ in it's place, and the BCO will require a padsone - even though loads have not changed

 

[Firsttimebuilder]

The beam that I am installing is an I beam (two C sections welded toegether, plus a plate on the bottom to extend it to the outer skin of blocks). Whilst the beam itself is heavy, about 370kg, the loads are dominated by the roof truss loadings.

I cannot see why I need a padstone, and my building inspector is too busy to come out (he came out for foundations and when I had some drains work done before the block and beam floor went in, he now says he doesn't want to come out until the roof trusses are in - I have requested visits, but he has declined as he is too busy!).

My current thinking is to just use some plate lintels (80mm thick) of 440mm length; these do not spread the load much, but will reduce the the stress on the blocks a little).

Thanks for the replies so far

 

[^woody^]

The principle is to spread the load sideways and down, and prevent diagonal shear stress right at the bearing ... which will crumble the corner of the block/brick and reduce the bearing and lead to further failure

A long steel plate or padstone or bricks/blocks (and mortar) of sufficient strength all do the same thing

The most important thing is to spread the load across any joints in the wall immediately are below the padstone, to avoid vertical travel of the load

 

[dave54321]

Hi,

I know this is an old post but I have found a free padstone calculator, which can be downloaded from;

http://steelbeamcalculator.co.uk/freepadstonecalculator.aspx

Hope this helps others with the same problem.