Hi all,
I'm just trying to get your advice as to whether a concrete lintel is sufficient for my needs as my number crunching is at odds with a steel RSJ supplier who is recommending one of their own products.
It's a bit long-winded but I wanted to show you my full workings.
I would like to cut a 1000mm doorway through the ground floor of a two-storey gable wall constructed with a double skin of Victorian red brick. For clarity, the wall is no longer external since an extension was built onto it many years ago. The wall has no openings above where I would like to cut through and rises to a pitched roof of clay tiles. The first floor joists which sit immediately above the proposed lintel position (ignore the rubbish scale to my sketch below!) support a 3.6m x 3m bedroom with a ceiling height of 2.3m. Since the bedroom ceiling extends above the wallplates the loft space is only 1.5m. The loft is not boarded and remains empty.
I have a drawing to illustrate:
So... I calculated the load as follows:
Height of ground floor above lintel = 0
Height of first floor above lintel = 2.3m
Height of loft = 1.5m
Total wall height = 3.8m
Total area of column above 1m opening = 3.8m2
60 bricks per m2 @ 2kg per brick = 120kg per m2
Total weight of column above opening = 456kg
456kg x 10 = 4560 Newtons or 4.56kN
I then read about multiplying by 1.6 to account for live load on the first floor; not sure where I read it but it stuck in my head and may explain the discrepancy if that's totally wrong!
Load adjusted to account for first floor joists = 7.29kN
So, I went looking at specs for concrete lintels and found that I could buy a 100mm x 65mm x 1500mm lintel (giving me a 250mm end bearing each side) with a load bearing capacity of 9.84kN/m, a fair amount over what I calculated my requirements to be. Obviously being double skin I was going to install two lintels, one on each skin at the same height.
However, when I gave the same sketch to Catnic for their input they came back and suggested I use one of their steel products across both skins with a SWL of 49kN but they didn't supply their full calculations.
Clearly there is a huge difference in me thinking a 9.84kN/m lintel is sufficient when someone else is recommending a 49kN steel!
Where have I gone wrong in my fag-packet calcs?
*Bbefore anyone says, "Just ask an SE", I prefer to try and get a good grasp of a problem before proceeding hence the question here. Full calcs from a pro will be obtained to satisfy BC when the work is started.
I'm just trying to get your advice as to whether a concrete lintel is sufficient for my needs as my number crunching is at odds with a steel RSJ supplier who is recommending one of their own products.
It's a bit long-winded but I wanted to show you my full workings.
I would like to cut a 1000mm doorway through the ground floor of a two-storey gable wall constructed with a double skin of Victorian red brick. For clarity, the wall is no longer external since an extension was built onto it many years ago. The wall has no openings above where I would like to cut through and rises to a pitched roof of clay tiles. The first floor joists which sit immediately above the proposed lintel position (ignore the rubbish scale to my sketch below!) support a 3.6m x 3m bedroom with a ceiling height of 2.3m. Since the bedroom ceiling extends above the wallplates the loft space is only 1.5m. The loft is not boarded and remains empty.
I have a drawing to illustrate:
So... I calculated the load as follows:
Height of ground floor above lintel = 0
Height of first floor above lintel = 2.3m
Height of loft = 1.5m
Total wall height = 3.8m
Total area of column above 1m opening = 3.8m2
60 bricks per m2 @ 2kg per brick = 120kg per m2
Total weight of column above opening = 456kg
456kg x 10 = 4560 Newtons or 4.56kN
I then read about multiplying by 1.6 to account for live load on the first floor; not sure where I read it but it stuck in my head and may explain the discrepancy if that's totally wrong!
Load adjusted to account for first floor joists = 7.29kN
So, I went looking at specs for concrete lintels and found that I could buy a 100mm x 65mm x 1500mm lintel (giving me a 250mm end bearing each side) with a load bearing capacity of 9.84kN/m, a fair amount over what I calculated my requirements to be. Obviously being double skin I was going to install two lintels, one on each skin at the same height.
However, when I gave the same sketch to Catnic for their input they came back and suggested I use one of their steel products across both skins with a SWL of 49kN but they didn't supply their full calculations.
Clearly there is a huge difference in me thinking a 9.84kN/m lintel is sufficient when someone else is recommending a 49kN steel!
Where have I gone wrong in my fag-packet calcs?
*Bbefore anyone says, "Just ask an SE", I prefer to try and get a good grasp of a problem before proceeding hence the question here. Full calcs from a pro will be obtained to satisfy BC when the work is started.
