Is a padstone always required?

[OrangeJuice09]

Hi,
I'm having built single storey extension with a bifold leading to the patio. It will have a flat roof, so not much weight.
The opening is 3.6m brick to brick, thermalite inside and concrete block outside, 100mm cavity. I assume this is too wide for a pre-stressed lintel so considering a steel (178 x 102) with a 300mm plate welded to it -Steel end to sit on the inside wall, or alternatively, an expensive catnic.

The builder said a padstone is required eitherside that spans both leafs, but I was thinking that this would cause a thermal bridge?

I thought :
1. If the steel / catnic overhangs by 200mm a padstone is not needed at all.
2. If a padstone is to be fitted, there should be a seperate padstone for the inner and outer wall.

Are the above two points correct, if not what are the options?
Thanks

 

[RonnyRaygun]

Correct and correct if it’s only carrying a couple of metres of flat roof.

You don’t usually have a pad stone on the outer leaf anyway (especially if facing brick you don’t want to interrupt the brick face) as it’s assumed the load bears onto the inner leaf with the plate transferring the load. For this reason (the eccentric load on the plate) the engineer should also check for torsion and sometimes UB sections fail with RHS sections having much better resistance to torsion.

The max length for a standard Catnic is 3.6m so you’d need an extra heavy duty - is that definitely more expensive than a steel with a plate? Don’t forget the steel should be galvanised or protected in another way.

Also, the plate is usually 25mm short of the front face of the supported masonry so that it isn’t as easily visible.

 

[Notch7]

I would’ve thought an off the shelf heavy duty catnip would be fine.

you don’t need a pad stone outside if it’s concrete block

 

[Notch7]

Stupid autocorrect

catnic not catnip

 

[^woody^]

You or your builder should not be guessing, and clearly need the advice of a more competent person to work this out for both the steel section size and the need for and type of padstone.

Lengths of bearings don't determine the need for padstones.

 

[Dereekoo]

Lengths of bearings don't determine the need for padstones.

Thats an interesting statement Woody, so if a 600 deep padstone was required below a beam with a 150 bearing to take the load down and out 600 to a safe bearing capacity is it that extending the beam to have a bearing of 600 there would still be a need for a 600 padstone. I know its not likely to happen but would be interested on your view as its Christmas

 

[^woody^]

Padstones are used primarily to prevent bearing shear, rather than spread loads, so no matter how long a bearing, there is still potentially the same load on the face of the bearing.

A secondary function of preventing local crushing may come in to play, but that should be dealt with by the compressive strength of the wall generally - so the padstone is more of a substitute for weak wall not a means to spread load - although admittedly, a way to deal with a weak compressive strength of a wall can be to spread the load over it!

If you consider the sizes of typical padstones in the shops , they are no bigger than a brick or block so in that context they are not spreading loads over any greater distance than a brick or block so their primary purpose can't be to spread loads.

Your 600 padstone scenario would only seem to be related to the bearing capacity of the wall and not specifically the loads from the beam - ie if the wall was made of stronger bricks or blocks, there would be no need for such a deep padstone

 

[RonnyRaygun]

Padstones are used primarily to prevent bearing shear, rather than spread loads, so no matter how long a bearing, there is still potentially the same load on the face of the bearing.

Hmm, are you sure? There's more risk of shear through the masonry for an in-line bearing, but a longer bearing will reduce that risk.
I think the other issue with excessively long beam bearings (and no pad stone) is that beams deflect, and any deflection will reduce load at the end of the beam and concentrate it at the edge of the support. Is that what you mean Woody? A pad stone of the correct length will not deflect and will help to spread the load with a triangular stress distribution.

If you consider the sizes of typical padstones in the shops , they are no bigger than a brick or block so in that context they are not spreading loads over any greater distance than a brick or block so their primary purpose can't be to spread loads.

The pad stone is, where possible, positioned concentrically under the beam bearing, whereas the blocks in a wall probably won't be. It would be possible to position a 440mm 3.6N block concentrically under a beam bearing, but would it be strong enough under crushing or shear loads to distribute the loads effectively? Would require more calculations to be sure and even then I'm not sure I'd want to rely on it.

Your 600 padstone scenario would only seem to be related to the bearing capacity of the wall and not specifically the loads from the beam - ie if the wall was made of stronger bricks or blocks, there would be no need for such a deep padstone

He's talking about a 600mm long bearing isn't he, not deep?

I've just been playing around with TEDDS bearing design this lunchtime and it does some very strange things. Either an anomaly in the equations or yet another glitch in the TEDDS software

 

[tony1851]

TEDDS bearing design..... does some very strange things. Either an anomaly in the equations or yet another glitch in the TEDDS software.

Get back to hand calcs!!

 

[^woody^]

He's talking about a 600mm long bearing

He said deep. I can only work with the tools given to me

 

[Dereekoo]

He said deep. I can only work with the tools given to me

To be fair I perceived one 600 long x 600 deep on the premise that the load imparted by the beam ( bearing 100mm onto the block) would then splay out in a 45 degree downward manner and reduce the bearing to something of about a sixth of what it was at the top? The beam to be in line with the wall not perpendicular to it. Perhaps a bit simplistic?

 

[tony1851]

600 long x 600 deep?!! That's a flagstone!

 

[Dereekoo]

600 long x 600 deep?!! That's a flagstone!

Yep, just using it to try and simply illustrate my point (thick flagstone though, more a gravestone)

 

[RonnyRaygun]

To be fair I perceived one 600 long x 600 deep on the premise that the load imparted by the beam ( bearing 100mm onto the block) would then splay out in a 45 degree downward manner and reduce the bearing to something of about a sixth of what it was at the top? The beam to be in line with the wall not perpendicular to it. Perhaps a bit simplistic?

The other thing is that to (arguably) keep the centroid of the load within the middle third you need the beam bearing to be 2/3 the length of the padstone. Arguably, because that assumes a linear load distribution. Once you start looking at triangular load distribution the centroid moves closer to the edge - This might be where TEDDS gets a bit confused. I would check but it's twenty past five on a Friday and I'm outta here in 10 mins

 

[^woody^]

more a gravestone

With the name of whoever is expected to fit it etched on the front