Avoiding Sagging on 7m Pergola Beam, What to Use? Wood/Steel/Aluminium etc?

blobert

Hi Guys,

Quick question. We're doing some work on our garden and the person that drew up the plans is no longer available, if you look in the pics (below) you'll see there is a pergola that spans about 7m+

The plan was to build it from wood, but I dont think think the beam at the front will work as the span will be too much and it will sag. The one at the back should be fine as it's sitting on top of a wall for half of the span.

What would be the best way of doing this. Guy doing the work says if we were to use wood we'd need support beams which we need to avoid.

Would a 7m aluminium or other metal beam do the job here in terms of not sagging/needing support

If so where would I go about getting such a thing and do you have any idea of the cost? Have failed to find anything online but possibly not even searching for correct thing!

Basically we have the wood ready to go but I think it will look bad with support beams so if there is a better solution I need to find one quickly.

Thanks in advance for any suggestions!

homewardbound11

I'd clad an rsj.

blobert

homewardbound11 wrote: »

I'd clad an rsj.

Thanks for that, you mean somthing like this: http://www.adverts.ie/other-building-materials/steel-rsj-h-iron-girder-i-beam/10452905

My worry is that might be too heavy, at 7m+ it could be 300kg+, not going to be somthing 2 guys can lift in place and there is no access to our back garden other than through house

blobert

Just as a follow on from your suggestion I contacted these guys: http://www.dkennedysteel.ie/steel-guide/

The thinnest beam they do is a 120mm x 60mm x 7 metres Steel Universal Beam for about €80: http://www.dkennedysteel.ie/steel-beams/

My question is will that sag also, they said they would not know?

CJhaughey

Use a couple of jacks to lift it into place incrementally.

blobert

CJhaughey wrote: »

Use a couple of jacks to lift it into place incrementally.

Thanks, it seems if I used a lighter beam it would not be as heavy

The question is what is the thinnest steel beam that won't bend over a 7m span if such a thing is possible?

Alternatively maybe there could be a steal beam coming out from the back wall to support the wooden beam about half way across that would not be too visible? ie you could have something to help prevent sagging while not having wooden columns going down into garden in the middle which is what we're looking to avoid.

Bonzo Delaney

How about a double lapped 9x2 bolted top and bottom at 18inch centers. As in an 18 ft 9x2 in the Center then 2 more outside that bolted and fill in the inside with another 9x2 it'd be like laminating up a beam use toothed washers in between the beams and keep the high side up

CJhaughey

I think those plans have been drawn without much attention to detail.
a 7m span of any kind of wood except maybe Gluelam will deflect considerably.
Steel will also deflect over a span that large unless its large and consequently heavy.
The part that would concern me is the freestanding pole that supports the corner, this wll be subject to a lot of load and there seems to be no triangulation on any part of the structure to prevent racking movement.
Wind loading would need to be factored in as well IMHO.

swailer

Just support it with the narrower pieces, let them carry the beam instead of the beam carrying them

Bonzo Delaney

swailer wrote: »

Just support it with the narrower pieces, let them carry the beam instead of the beam carrying them

Are you looking at the picture the right way up ???

swailer

Yea, support it out from the wall instead of on the length. The length is immaterial, it's like a cantelever

Bonzo Delaney

swailer wrote: »

Yea, support it out from the wall instead of on the length. The length is immaterial, it's like a cantelever

Had to read that twice I suppose if the short lengths were strapped down to the back of the feature wall they'd act as a cantilever for half or the way and take a good bit of the spring out the straps would need to be fixed at least 3-4 block courses down to work

tabby aspreme

Have you spoken to any of the steel purloin suppliers to see what they might have.

Calahonda52

OP: You say
Basically we have the wood ready

What exactly do you have ready?

Are all the walls in place?

What is the budget for overruns?

Metric Tensor

If only there was a professional who was qualified to design and size members and connections like these and who could calculate in advance the deflection that would occur in each!

kieran.

Steel/timber composite Flitch beam wood be an structural and aesthetic cost effective solution.

blobert

Thanks Guys,

Ideally looking for a quick solution as we're working on it at the moment.

At the moment we have 4.5 x 9cm x long wooden beams to build it from.

Everything in the mockups exists other than the pergola although the feature wall is about 40cm less wide which increases the span.

I'm thinking somthing that comes out and supports the 7m span would be easiest. I suspect a heavier steel/glulam etc beam is going to present it's own problems

Any suggestions on how to maybe brace or even use somthing like this: https://www.erico.com/catalog/variants/PH34303m.jpg (though if it was attached to the back wall it would need to be at an angle (pergola is higher than back wall)

Thanks again for all the suggestions, I really appreciate them!

JimmyMW

Metric Tensor wrote: »

If only there was a professional who was qualified to design and size members and connections like these and who could calculate in advance the deflection that would occur in each!

Brilliant

Calahonda52

4.5 x 9cm
=
45 by 90 mm
=
1 3/4" by 3 1/2''

please tell me you are mistaken

Bonzo Delaney

Calahonda52 wrote: »

4.5 x 9cm
=
45 by 90 mm
=
1 3/4" by 3 1/2''

please tell me you are mistaken

I don't think so it sounds like 4x2pao to me.
Just thinking a loud would a glue lam beam made up of these with the joints overlapped by 6ft glued and bolted to make a beam of roughly 9x4 take that distance.

Calahonda52

Yes 4 by 2 but how he expects to get 22 feet with 4 by 2 is beyond me.

I dod some rough cals and even with a 1/240 deflection it would need to be 12" to 14" deep [ might bring deflection back to 1/320 or so]
A simple SS pole in the middle or even two at 3rd points would work wonders, but not for the ego at play here.
As noted earlier the bending moment on the skinny up right will be massive.

blobert

Thanks Guys,

Yes, there is a pile of 4 x 2's out in the back, though I'm assuming the upright (on the left in pics) will be somthing thicker.

This pergola will not need to support anything other than it's weight, and if it's built out of 4 x 2's it's not going to be that heavy so I'm thinking rather than trying to make somthing much more heavy/substantial it might be better to build a lighter one and try to support it, preferably without resorting to having uprights going down in middle of lawn.

The whole span is 6.8m, and from edge of feature wall to shed is 4.2m so I'm not sure if with a couple of supports like this: https://www.erico.com/catalog/variants/PH34303m.jpg (just in L shape) attached to feature wall if it would hold up for the 4.2m bit.

I suppose we could always try and add a support upright later if it did sag

Any further suggestions would be much appreciated! Thanks