Thoughts on a beam

^woody^ said:
So no brittle finishes, or designing flexible or isolated cladding means it passes? :rolleyes:
Click to expand...
No, because one of the criteria for serviceability is that the beam must not deflect excessively so as to look unsightly or to cause alarm to the occupants.
So if the ceiling was spanning -say- 3m and consisted only of 2 x1 battens supporting a finish of PVC sheeting, the sheeting will not fail, but the beam will because it has deflected too much (it's own weight would probably result in excessive deflection).
 
1mm more than the acceptable criteria would fail the beam then, but no one would notice this.

Like I said, deflection is not a failure of the beam.
 
@ Woods; a steel beam spanning - say - 4m, should have a live-load deflection of no more than span/360 (11.1mm) if it supports a brittle finish.
If it deflected an additional 1mm, no one would notice and - with luck - a plaster ceiling might not develop a crack.
But technically, the beam has failed, though only in terms of serviceability, not structurally.
 
Isn't that the difference between failing and being fit for purpose??

If a beam doesn't comply with the table below then it's failed the limitations of the serviceability limit state. But the beam might not be close to reaching its elastic limit.

Dead load deflection shouldn't matter as beams can be cambered to take out the dead load deflection on bigger projects. On smaller ones where cambering is uneconomical I usually design to L/250 for combined dead and live load.

upload_2022-5-10_16-14-19.png

upload_2022-5-10_16-15-3.png


upload_2022-5-10_16-17-15.png
 
Thanks everyone I really appreciate the feedback. Notch7 you have understood perfectly, it's the point loads that concern me, and as you say the beam isn't spanning the entire width. Which incidentally is 7m, 7.2 with the overhang and 3.4m width internally to the wall 3.75m with the overhang. Thanks for the warning though Dereekoo, best to shout if you think something's dangerous, but even I wouldn't be stupid enough to try and span 7m like this :)

I take your point about the firings as well Notch7, I will need to ensure they fall away from the centre of each lantern at the back and also toward the front. The reason for all the C24 170x45 lumber everywhere is that I know someone with some excess at a good price so it's cost effective for me to use it where I can, and also I like the idea of the extra meat on the front wall that is helping to support the beam. The extra depth of insulation won't be a bad thing either. I was toying with the idea of a brick pillar at the one end so the beam can rest on that and the garage walls well as the front stud wall but I'd rather avoid it if I can.

Anyway, I've had a bash at a couple of drawings to show the carbuncle I plan to build, please keep in mind its only roughly to scale, the studs are all way thicker than they would be but you wouldn't see them if I didn't do that, or see where they are doubling up. Also for some reason I seem to have done it without the use of a pencil sharpener ??
I hope I can work the technology and upload them now!

Thanks again all.
IMG_0007.PNG
IMG_0009.PNG
 
PS. Forgot to respond about the deflection issue, thanks for bringing it up. I'm in the the camp here of as long as it is serviceable and safe then a few cracks or whatever in the finish is not an issue. This is going to be a rough and ready sun/garden room. Dogs, kids, mess, who knows what? I may not even plaster it but go for something a bit more rugged like cladding, which I assume should be a bit more forgiving of any slight movement.

Anyway, thanks again for the heads up.
 
I like your plan generally, but you will be crippling a lot of rafters with those lanterns. Why not go with something like 3 no. Velux MK10s That way you only need to cripple 3 rafters. And another bonus is you can open them in the summer.
 
The point loads from the trimmers are less of an issue than if you had no lanterns and full length joints.
The trimmers move the majority of the load away from the centre of the spans in this instance, thereby reducing both the bending moment and the total deflection. Shear will stay the same.

You might struggle to find a single length at 7.2m long though, in which case the member won’t quite behave as continuous. Shouldn’t matter though - stagger the joints and position atop the double studs,perhaps notching the members so that the ends sit on top of each other.

Would need to do a calc to confirm if what you propose would work but I’d imagine it won’t be far off.
 
Thanks Ronny, I suspect I'll only have 4.8m lengths, so I had planned on staggering scarf joints before bolting the whole mess together. If I can source, or more importantly think I can handle and manoeuvre a 7m steel plate to make a fitch, I might just to add a bit of extra strength. Belt & braces never hurt :D

I could make the lanterns slightly longer so that their trimmers do land directly on the window and door double studs, but my gut tells me that won't make a huge difference, and then of course I get into extra costs for the lanterns and even more cripples.

The other thing I did wonder about is ordering a glulam beam. I think it would be my preferred approach, but the issue I have would be one of access and getting it in place. At least with my Frankenstein approach I can get all the parts into position fairly easily, build it, then just have one lift to get it in place, lots of help muscle wise or hire a couple of genies if I have to.

Thanks again for all the advice.
 
TeachMeObiWan said:
The other thing I did wonder about is ordering a glulam beam. I think it would be my preferred approach
Click to expand...

I always used glulam to build orangery roofs - generally 90 x 315 for external gutter jobs.

The biggest Glulam I used was a pair of 8.7m glulam beams 140 x 630 section - needed a genie for that!

I used to buy them from glulam in Southampton

they used to be about £800 a cubic metre - which is around £25 a metre for a 90 x 405
 
You must log in or register to reply here.
Back
Top