Structural Calculations

[Handy1971]

If I know the ratio and safe working load of a lintel from the manufacturer how can I work out the proposed lintel is up to the job.
E.G.
A window span of 1.8m with 2 courses of blocks over the top and a pitched 20 degree roof.
700mm returns each side.
So a 2.1 lintel having a SWL of 23kn

Dead Loads
Tiles 0.48kn/m2
battens and felt .04
rafters .015
insulation 0.06
plasterboard 0.15
Brick and render 2.3
3.17?

Live/imposed Load
Roof Load .75kn/m2

That example then could have a first floor joist above creating another load
I just cant seem to corrolate the 23kn linear load of the lintel in to a formula that makes the lintel adequate. I know Catnic will do it for free,but its annoying I cant grasp it.
Sorry for the thickness, and thanks for any help

 

[tony1851]

Aside from the load of the blockwork, the other determinant of load will be the span of your roof;eg a roof covering a space of 6m will impose twice the load on the beam as a roof spanning only 3m.

Assuming it's a simple lean-to roof, what you need to do first of all is determine the total distance from the edge of the eaves, up to where the roof meets the house wall - then halve that; let's say for the sake of argument it comes to 1.9m. Then you simple tot up the figures you have for the build-up of the roof to give a total load per sq meter. I reckon yours will be around 1.5kN/m² and multiply that by 1.9, giving a total roof (dead + live load) of 2.85 kN per meter run.
So a 1.8m span gap means a total of 2.85 x 1.8 = 5.13 kN on the lintel.

To this, you have to add the weight of the blockwork on each skin.
The toal load will come well under 23kN.

 

[Handy1971]

I was hoping you would reply Tony, thanks for that.
I'll sit down over the weekend and have another go.

 

[jeds]

For a standard tiled roof I always allow 0.85kN/m² for dead load. That’s high enough to cover most situations and pitches. If your roof is lean-to spanning 4m you should allow 2m on the lintel. So you’ve now got 0.85x2 = 1.7kN. Multiply that by the span of the lintel (2100) gives you a roof dead load of 3.6kN across the lintel. You could work out the same for 0.75 live load – or you could just double it to 7.2kN. A couple of courses of masonry will be no more than 2kN. So total is less than 10kN.

 

[Handy1971]

Aside from the load of the blockwork, the other determinant of load will be the span of your roof;eg a roof covering a space of 6m will impose twice the load on the beam as a roof spanning only 3m.

Assuming it's a simple lean-to roof, what you need to do first of all is determine the total distance from the edge of the eaves, up to where the roof meets the house wall - then halve that; let's say for the sake of argument it comes to 1.9m. Then you simple tot up the figures you have for the build-up of the roof to give a total load per sq meter. I reckon yours will be around 1.5kN/m² and multiply that by 1.9, giving a total roof (dead + live load) of 2.85 kN per meter run.
So a 1.8m span gap means a total of 2.85 x 1.8 = 5.13 kN on the lintel.

To this, you have to add the weight of the blockwork on each skin.
The toal load will come well under 23kN.

Thanks John,

0.85kN/m2 for most domestic roofs,flat and pitched?
When working out the masonry load above the lintel,is it the span plus the height of the masonry,straight line vertically, to how ever high? (this example 2 courses above the lintel)

Lean to roof extension
Concrete lintel at 2.5m span 100x140 with a UDL of 4.4KN/m from lintel table, total length 2.7
Roof span from house wall to edge of eaves 3.8m


Dead Load
Roof 0.85kN


Live Load 0.75kN
Total 1.6kN

1.6kN x 1/2 span of roof x 1.9 3.04kN
Lintel span 2.5m x 3.04 7.6kN

Blockwork 2.5mx.450mm 1.125m2 x 2.3kN/m2= 2.58kN (2.3kN per m2 is this high for concrete block?)
7.6kN+2.58kN= 10.18kN

Concrete lintel fails

Thanks again both, done another example to make sure ive got it right,no motive other than learning.

 

[jeds]

Masonry above a lintel is taken as a 45° panel, unless it would cut an opening - which it wouldn't here. A panel above a 2.5m span is 1.56m². Medium density blocks are 1.9kN/m² laid. 1.56x1.9 totals 3.0kN. Divide this by the span gives you 1.2kN/linear metre.

 

[Handy1971]

Cheers