SE detail translation

[LSB]

Hello

Can someone out there translate the below to 'english' for me, as in explain what the acronyms.

 

The roof is split so I assume that this is discussing the first roof which is 4m.

 

Sorry about the quality, but it was sent to me like this, drawn on graph paper by hand

 

I have a ground worker coming next week to discuss my foundations and I just want to understand what this means.  I've tried speaking to the SE, but he's not particularly helpful.

I realise this page is about the roof, but I like to know a bit of what I'm talking about when discussing the build with experts.

 

Thanks

 

 

 

[Temp]

Others can probably do a better job but..  He's been working out the size of rafters you need.

 

He assumes they are about 4m long in the title.

 

The next bit is the load they need to carry allowing for things like: Weight of materials,  normal loads, an allowance for someone up there doing maintenance and for solar panels.

 

In the table he assumes the rafters will be 0.4m centres. 

 

He concludes by checking that rafters 47mm * 150mm C24 will be ok. The C24 bit is the timber grade.

[Temp]

You mention there are two bits of roof. Would the rafters in the other bit be shorter than 4m?  If so you can use same size for both pitches. If the other bit needs longer rafters there might be other calculations for that. 

[Gus Potter]

On 01/01/2022 at 16:33, LSB said:

Hello

Can someone out there translate the below to 'english' for me, as in explain what the acronyms.

 

The roof is split so I assume that this is discussing the first roof which is 4m.

 

Sorry about the quality, but it was sent to me like this, drawn on graph paper by hand

 

I have a ground worker coming next week to discuss my foundations and I just want to understand what this means.  I've tried speaking to the SE, but he's not particularly helpful.

I realise this page is about the roof, but I like to know a bit of what I'm talking about when discussing the build with experts.

 

Thanks

 

On 01/01/2022 at 18:23, Temp said:

Others can probably do a better job but..  He's been working out the size of rafters you need.

 

He assumes they are about 4m long in the title.

 

The next bit is the load they need to carry allowing for things like: Weight of materials,  normal loads, an allowance for someone up there doing maintenance and for solar panels.

 

In the table he assumes the rafters will be 0.4m centres. 

 

He concludes by checking that rafters 47mm * 150mm C24 will be ok. The C24 bit is the timber grade.

 

Temp..Yes good /great summary.

 

@LSB Is there another page or two of calcs relating to the roof? I have deduced that the calcs are based on Eurocode 3 rather than BS 5268. I did this by comparing the SLS (serviceability limit state) to the ULS (ultimate limit state) ratio. Your calcs give a ratio of 1.35 and 1.5 for SLS and ULS respectively and that correlates with the Euro Code safety factors. The BS code is different.. I'll leave the explanation out for brevity.

 

Before making any further comment  it would be good to see all the calcs relating to the rafters and the cladding on top. A 6" x 2" rafter spanning 4.0m invites interest even at 400mm centres then you have the insulation.. closer centre rafters the more bridging effect.. sometimes the lightest structural members can be counter productive?

 

I can see an alpha factor... 16.0mm Assuming this is some kind of deflection limit then  BS 5268 recommends a limit of span *0.003 = 12.0mm but the Eurocodes allow for the limits to be agreed to some extent between the Client and Engineer... they cut you a bit more slack.. allow you to innovate.

 

I'm interested to see the if the rest of the calcs take into account the deflection limits set by the metal cladding provider and the internal lining provider.. say Gyproc? While the roof may not collapse it may bend so much that it tears the cladding fixings and causes cracking in the ceiling. If you are going to stretch the deflection limits as an SE you should be pointing this out to your Client and let them decide what level of risk they want to take on.

 

Have you got a good builder who has said.. mmm, these rafters look a bit small?

 

 

 

[LSB]

do you have any idea what

 

EW = 

 

ZYYREG =

 

stand for

 

The 2nd roof is quite a lot bigger than the first at 5.5m long, for that one he is recommending larger joists.

 

 

[LSB]

22 hours ago, Gus Potter said:

 

Temp..Yes good /great summary.

 

@LSB Is there another page or two of calcs relating to the roof? I have deduced that the calcs are based on Eurocode 3 rather than BS 5268. I did this by comparing the SLS (serviceability limit state) to the ULS (ultimate limit state) ratio. Your calcs give a ratio of 1.35 and 1.5 for SLS and ULS respectively and that correlates with the Euro Code safety factors. The BS code is different.. I'll leave the explanation out for brevity.

 

Before making any further comment  it would be good to see all the calcs relating to the rafters and the cladding on top. A 6" x 2" rafter spanning 4.0m invites interest even at 400mm centres then you have the insulation.. closer centre rafters the more bridging effect.. sometimes the lightest structural members can be counter productive?

 

I can see an alpha factor... 16.0mm Assuming this is some kind of deflection limit then  BS 5268 recommends a limit of span *0.003 = 12.0mm but the Eurocodes allow for the limits to be agreed to some extent between the Client and Engineer... they cut you a bit more slack.. allow you to innovate.

 

I'm interested to see the if the rest of the calcs take into account the deflection limits set by the metal cladding provider and the internal lining provider.. say Gyproc? While the roof may not collapse it may bend so much that it tears the cladding fixings and causes cracking in the ceiling. If you are going to stretch the deflection limits as an SE you should be pointing this out to your Client and let them decide what level of risk they want to take on.

 

Have you got a good builder who has said.. mmm, these rafters look a bit small?

 

 

 

 

the whole report is 16 pages long, I just wanted to know what the different symbols meant so I could look at the whole report.

The build has 3 different roofs, all technically flat, this one 4m the next one 5+ and the final one 6.3, although that may be tiles as it was previously different from the rest of the barn.

 

The SE has barely spoken to me and when I ask him questions he doesn't answer them, he doesn't have a good client facing attitude, newly qualified.

 

One of the possible ground workers came yesterday, although this isn't for him, but I discounted him when he said not to worry about the SE calcs and just suck it an see !!!!

 

I do have another 3 coming to quote.

 

 

We haven't finalised on builders or roofers yet, none of the ones we have spoken to have seen the drawings yet, they only arrived just before Xmas and everyone seems to have been closed for 10 days.

 

 

 

Edited January 3, 2022 by LSB

[Gus Potter]

2 hours ago, LSB said:

do you have any idea what

 

EW = 

 

ZYYREG =

 

stand for

 

The 2nd roof is quite a lot bigger than the first at 5.5m long, for that one he is recommending larger joists.

 

 

EW is ΣW (Sigma W) which is the sum of the all the factored load (W is the loads from the ULS column) on the beam. Thus ΣW = sum of total ULS load =0.46 + 0.45 = 0.91 kN/m (kilo Newtons per metre run) * 4.0m (beam length) = 3.64 kN (kilo Newtons) = the factored design load applied to each beam.

 

Zyy Req...

 

Z is called the section modulus and the yy indicates about which axis of the beam the value of Z applies to. The British Standards and Eurocodes use a different axis of notation. Your SE is using the Eurocode here.

 

The letters "req" are short for required. The SE is working out what value of Z (a geometric property) the beam needs to have as a minimum to carry the load.

 

To calculate the Zyy required we use the formula  Z req = maximum bending moment / maximum design stress allowed in the timber.

 

The bending moment is shown as m = 1.8 kNm and the design stress is 14.8 N/mm^ (Newtons per mm squared).

 

We need to make the units compatible so to convert kNm (Kilo Newton metres) to Nmm (Newton millimtres) we multiply by 10^6 as below.

 

Thus Zyy req = 1.8 * 10^6 / 14.8 = 121621 mm^3 = 121.6 x 10^3 mm^3 this is the minimum value of Zyy required to carry the load.

 

Next the SE is checking to see if a 150 x 47 timber has a Zyy value greater than the required minimum.

 

 

Zyy for the proposed timber is calculated by the formula b*d^2 / 6.

 

Beam width = b = 47mm,

Beam depth = d =150mm

 

thus Zyy = 47 *150 *150/6 = 176250 mm^3 = ~ 176.3 x 10^3 mm^3 (millimetres cubed) > 121.6 x 10^3 required thus OK.

 

The Iyy is called the second moment of area (units mm^4 ) and you'll probably see this on the next page when the SE checks the deflection.

 

 

 

[saveasteading]

Or in other words, complicated Engineering stuff. Much more encouraging to see this than the standard computer outputs where you put in some numbers and an answer comes out. 

What is behind your concern?

[LSB]

21 hours ago, saveasteading said:

Or in other words, complicated Engineering stuff. Much more encouraging to see this than the standard computer outputs where you put in some numbers and an answer comes out. 

What is behind your concern?

no particular concerns, I'm just a bit OCD about understanding things, just a bit

 

Although Gus wasn't convinced that these beams were big enough, or I think that's what he said.

 

I will post the other calcs as well when I can, but that's more complicated as the roof is split in different ways.

 

 

 

 

[Gus Potter]

@LSB It's good to ask questions, that is what BH is about.. sharing views and knowledge. I learn loads here and enjoy reading the posts on ASHP etc and stuff that is just interesting.

 

From time to time I may be wrong..but better to ask questions now rather than after something is built.

 

LSB. An experienced SE, Building Contractor, serial BH self builder will often just look at beam/ rafter sizes and form of structure and go..that looks a "bit tight" lets have a closer look at that.

 

I can see that your 150 x 47 rafters are tightly sized.. based on the one page of calcs you have posted.  Getting "it tight is ok".. that is economic design.

 

I use my own name here on BH. I'm not criticizing your SE but I am highlighting that the rafter choice should be justified in your calcs, particularly in terms of your deflection limits and whether the rest of the roof covering and internal linings are compatible with those limits.

 

It's important to recognise that if you have say a steel cladding over a timber joisted roof then while the timbers may be strong enough to carry the loads they may bend (deflect) so much that it over stresses the fixings on the cladding. The cladding supplier will specify what the maximum deflection of the underlying roof is to be. Exceed this and you void your cladding warranty.

 

Sometimes on an lean to extension at a 4.0 m span rafter it's easier to specify a 195 x 45 rafter.. this gives you plenty depth to insulate the roof, run the services and ventilate either by an air gap of by way of a membrane, sarking and almost full depth insulation between the rafters.. much food for thought!