Bison Beams

[johno12345]

Hi,

Does anyone know what sort of wall a bison beam should be on. I have a 6m span bison beam roof supported on Thermalite blocks which are showing signs of cracks.

Thanks

 

[Static]

You talking 6m spanning hollow core concrete slab roof? Well assuming minimal screed and no roof plant loading on the flat roof (also that its a cavity wall its bearing onto), 3.5N blocks should be adequate..

Where are the cracks occuring? How old is the wall? etcetc more info please

 

[johno12345]

Hi,

There is some mineral insulation then a bitchumen roof. The 6m slabs are supported by the internal skin only on each side.

The blocks are 6" and it was built in 1984. the cracks are appearing everywhere and are about 3-4mm wide. They are worse around the windows and doors. The outer brick skin has no signs of cracking at all.

The whole building was designed for a first floor to be added .

Thanks

 

[Shytalkz]

Vertical, horizontal, diagonal cracks? Top, middle, bottom? Combination of the foregoing?

What's a **** roof? Has the censoring thing gone mad again?

 

[johno12345]

Hi,

Vertical cracks down the wall in intervals and they are diagonal at the window lintols above and diagonal beneath the lintols pointing downwards to the window reveal. The lintols are 325mm concrete ones.

Many of the cracks run from the top to the floor apart from the ones arounf the windows. The floor slab is sound and shows no cracking.

The roof is made of that sticky black stuff applied while hot and steamy yes the autocensor didnt like it

Thanks for your help

 

[Shytalkz]

Diagonals at the lintel bearings sounds like the blockwork is possibly overstressed at those points, or the lintel is deflecting, which the diagonals above (if running upwards and towards the centre of the opening) also suggest. What lintels are they? Are there any?!

Verticals at intervals suggests drying shrinkage - did it **** down with rain when they were being laid? Long walls, far in excess of 6m length, without vertical movement joints? Crack widths constant? If varying, wider at top or bottom?

 

[Static]

Hmm, was about to ask where movement joints and and length of wall but someone beat me to it..

Are the foundations adequate?

 

[johno12345]

I tried taking photos but u cant see. If you imagine the windows as a human body, (you with me here ) doing starjumps, your arms and legs are the cracks.

the lintols are 325mm steel reinforced concrete with a steel on thee cavity side to support the external brickwork.

I cant comment on the weather in 1984 the vertical cracks, which are almost plumb are every 1.5-2m along the walls, even the non-load bearing walls. the crack widths are contstant.

I think we are going to have some steelwork installed to take the load of the bison beams.

 

[Shytalkz]

You mean the cracks move up and down??

Well, that amount of floor span shouldn't be overstressing the blocks, especially as they are 150 thick! Is this a solid wall, or cavity?

If you've got concrete lintels, the diagonals at the bearings are most likely due to dissimilar thermal movement, rather than overstressing. Max opening width?

All in all, can't see that there's any specific problem with them, think they must have been either poorly-cured blocks and/or the weather was cack at the time they were laid, allowing them to get soaked.

If you're concerned about the lintels (even though they should have been designed to take two storey loading), build a lintel into the walls above the Bison floor and directly over the openings extending min 150 either side to take the additional loading. Why weren't ones like Catnics or IG used, ooi?

 

[johno12345]

nah, the cracks stay the same ) it is a cavity wall but the beams are only supported by the lightweight blockwork on the inner leaf. the windows are 6' wide and the lintols extend 300mm beyond the opening.

I dont know why catnic lintols or IG werent used. Thee whole site used RC lintols. maybe it is a custom of our old builders.

Perhaps the blocks were wet through when fitted then.

A structural engineer has been and he is drawing up a belt and braces steel system for the roof.

Thanks!

Johno