Is my design over engineered?

[DuncanAndrews]

Hi

 

I would like to sanity check my structural engineer's design to see if his designs are over engineered.

 

To set the scene, it's a small bungalow approx 46 sqm.  We are wanting to do a loft conversion that has Velux style windows only (no dormer)

I would like comments on the below specifications:

1. The new floor joists for the loft conversion are 75 x 195mm at 40 centres

2. The trimmers for the staircase have been set at 3 x 47x195 (So 141mm in width). The three timbers are bolted together with M20 bolts.

3. The noggins between the new floor joists have been set as 3 x 47x195 (So 141mm in width).  These noggins will then support a stud wall (38 x 140mm) to support the roof.

4. 152 x 152 Steel Ridge Beam

5. For each Velux window (780 x 1180mm), triple rafters each side of the window have been specified.  So for 4 windows, 24 rafters have been specified.

 

Just wanted to check if this looks ok or seems over engineered.

 

Many Thanks

[bassanclan]

Usually double rafters around a velux, but depends on size of velux.

Has a roof truss company supplied this? If not worth getting a quote from them

[ToughButterCup]

With the very best will in the world, @DuncanAndrews, our collective genius and experience can never ever be cited as reasoned evidence to counter a qualified SE's opinion. All we can do is to advise you to do some Due Diligence among qualified and experienced SEs. 

 

All SEs aren't the same. Some experienced, some not. Some specialised, others not. Some go by the book, others engage deeply with the topic because of a special interest. Get 10 SEs in room and I bet there will be a wide range of opinion. Our SE ( a foundation raft expert) was asked to specifiy for some ICF rebar for us. Once that spec was submitted to a specialist ICF  SE, well, his laughter from  the Home Counties could be heard here in Lancashire. And saved us about £3k.

 

Ask two more SEs for a specification - and make sure one of them is experienced in working with Velux products: a truss design firm might be useful.

Do Velux have specialist SEs?  Time to get on the phone.....

[DevilDamo]

It would depend on the spans.

[George]

As said the span, location and structural form are highly relevant so there's not enough information to say.

 

The timber is thicker than normal but you say the floor is having to support the roof. And a 152x152 is the small UC section available.

 

There is almost always some room to trim but doesn't sound bonkers. 

 

 

[Gus Potter]

On 14/11/2023 at 10:55, DuncanAndrews said:

I would like to sanity check my structural engineer's design to see if his designs are over engineered.

Best thing to do would be to post some drawings and a few photos of the inside and external elevations.

 

I do a few loft conversions in the day job but your info is so limited I can't make any resonable / helpful comment unfortunately.

[Iceverge]

On 14/11/2023 at 10:55, DuncanAndrews said:

The trimmers for the staircase have been set at 3 x 47x195 (So 141mm in width). The three timbers are bolted together with M20 bolts.

 

The trouble with small projects is that it's often cheaper to slightly overspec a design to save the SEs time and fee for getting everything to the last decimal point. In any case your spec doesn't sound mad.

 

A good builder should be able to give you a feel for if the SE has designed someone sensible and maybe point you towards someone more suitable if they think yours has gone totally overboard.

 

As an aside (without having costed it) YouTube told me recently that Spaz do impact driven screws that could save a good chunk of labour to replace the M20 bolts. 

 

 

 

 

[SimonD]

On 25/11/2023 at 21:28, Iceverge said:

As an aside (without having costed it) YouTube told me recently that Spaz do impact driven screws that could save a good chunk of labour to replace the M20 bolts. 

 

Interestingly our SE specified screws for all multi-ply members: "Screw together with 5mm dia screws at 200 mm centres, staggered top and bottom with 50
mm edge distance"

 

I have to say I preferred the screwing method. BCO however, asked me to put bolts through the stair trimmers, probably because that's what he liked to see, so I now have both 🤷‍♂️

[Iceverge]

I have 2 preferred engineering methods. 

 

1. Observe what has lasted and what has failed on the farm after 60+ years . 

 

2. Jump on it and add more reinforcement until it feels strong enough. 

 

 

For connected members I would say a good dollop of glue and some nails would be more than adequate if the timber was to remain in a temperature stable environment. 

 

You would need to let the glue set before adding any load mind you. 

[SteamyTea]

On 14/11/2023 at 10:55, DuncanAndrews said:

Just wanted to check if this looks ok or seems over engineered

It might be, but engineering is not all about catastrophic failure. Acceptable deflection is important as a constant tiny movement can cause problems i.e. rattling or leaks.

 

On 25/11/2023 at 21:28, Iceverge said:

As an aside

 

16 hours ago, SimonD said:

"Screw together with 5mm dia screws at 200 mm centres, staggered top and bottom with 50
mm edge distance"

 

16 hours ago, SimonD said:

BCO however, asked me to put bolts through the stair trimmers

 

Ah, the old screw and bolt debate.

Regardless of what others say, screws and bolts are different things, that work in a different way, not a case of one is better than the other, they are different and should not, in isolation, be compared to each other.

[SimonD]

3 hours ago, SteamyTea said:

Ah, the old screw and bolt debate.

Regardless of what others say, screws and bolts are different things, that work in a different way, not a case of one is better than the other, they are different and should not, in isolation, be compared to each other.

 

Yes, absolutely agree. My point wasn't that one was better than the other. I followed the specification of my SE, who is a specialist in timber frame buildings, and especially glulam/steel hybrids, which is what we've got for our main structure. My main point was that despite the SE report, specifications and design, BCO still asked for bolts, which in my view is not correct but seems like an argument I could simply not get into because I had so many other things to contend with.

 

However, organisations like NHBC allow for nailing or bolting, for example, and don't make any distinction between those differences other than to say the design should be followed.

 

To me it's a bit like the debate between using screws to assemble a timber frame. I've seen builders locally ignore the design documents suggesting that screws are better than nails. Two carpenter/builders I spoke to about our project said they didn't nail (because that's old school apparently) but would screw it all together using 'high' quality Spax......

[SteamyTea]

5 minutes ago, SimonD said:

organisations like NHBC allow for nailing or bolting

I never used to think much to nails, mainly because I am a clumsy (expletive deleted)er and and kept bashing my fingers.

But then I learnt about them and they are fantastic fixings for timber.

 

It is odd that your BCO does not trust the SEs calculations, even odder that builders, old school or no, think they can override the SE design.

[SimonD]

22 minutes ago, SteamyTea said:

But then I learnt about them and they are fantastic fixings for timber.

 

My absolute favourite tool of the build, one of a growing collection. Does everything from 90mm ring shank framing nails to full head stainless for cladding. Still need a hammer, mind you.:

 

[Iceverge]

I love hammering nails. 

 

As I understand it they are less prone to sudden sheer failure than screws. 

 

You can get special screws now that are fit to replace nails in joist hangers for example. These might be the "high" quality space screws mentioned above.

 

As for the BCO disregarding the SEs design, they will probably become believers after the screwed joint is still in place in 50 years. Frustrating for you but that's the way most knowledge on building seems to become accepted. 

 

 

[saveasteading]

28 minutes ago, SteamyTea said:

It is odd that your BCO does not trust the SEs calculations, even odder that builders..

The less you know, the less you know that you don't know.

I had an apprentice bco once tell me he didn't like something. 

I'll get you an Engineer's letter for your files says I (me but he didnt know that).

"Don't bother, they'll write anything you ask them to."

Now somebody taught him that. A failed bco that had become a teacher perhaps. I never saw him again, so somebody was perhaps less restrained than I.

 

Day release bco. 3 or 4 years uni for the SE. Then more exams for the chartedhip.

[joe90]

1 hour ago, SteamyTea said:

It is odd that your BCO does not trust the SEs calculations,

I once had a BCO say he didn’t like an SE,s calcs so told him to prove it was wrong!,!, he told me he was going to get their “in house” SE to look at it, never heard back and I carried on.

[ToughButterCup]

1 hour ago, saveasteading said:

The less you know, the less you know that you don't know.

...

 

In other words, Dunning Kruger.

@saveasteading - when talking about this to students (The less you know, the less you know that you don't know) , I found that they had to think hard about how to 'read' that dictum using the comparator - less...less.

Since I also had to find a way to teach them good written style, I found positivity worked better hence;

 

... The More You Know The More You Realize You Don't Know ... 

Realising the essential truth of that is a bit depressing.  AA Milne comes to mind. The six year old seems stuck in Dunning Kruger's jaw.