Oak pergola beam span question and joint method

[Babystep7]

Hi all, I'm about to build a pergola in our garden and am scratching my head a bit regarding the max span length I can do, before it sags. And also the jointing method to use for the top frame. My Wife's quite specific on what she likes and the one she's settled on is 4.5m x 2.5m. This will have 4x 100mmx100mm oak posts. But she wants the top frame (front, back and sides) in the same size timber, so all 100mmx100mm. So, my questions are;- A) can I span a 100mm square beam 4.5m with no support, other than at either end? The 2x 4.5m lengths will have no load placed upon them.

B) What joint method would be suitable for the top beams resting on the posts? I was thinking mitred butt joint.

If I've missed anything out, or more info is required, please ask. Hope you can help.

Thanks

Rich

 

[freddiemercurystwin]

You can but they will probably sag.

 

[Babystep7]

You can but they will probably sag.

Hi, thanks for the reply.

Yeah, that's my worry. Do you think 3.5m x 2.5m would be more suitable?

 

[Dereekoo]

You can but they will probably sag.

well all joists sag its just a case of what is acceptable. running the calcs on a c24 joist with just selfweight load taken into account gives a deflection 4.7mm which is well under the 13.5mm allowable in normal construction
Since oak is denser and hence the selfweight larger the deflection would increase by 2mm even though strength wise its stronger

 

[foxhole]

Buying 100mm square oak in those lengths will be a struggle and will probably already have some type of deflection/twist to it,

 

[Babystep7]

So, we've gone back to the shortlisted garden designs and ( I think) we've decided on doing a 3.8m x 3m pergola. So, it's still a reasonable size, but hopefully won't cause too much sag.
I've been looking on a website called uk-timber. They look pretty reasonable and will do custom cuts and PAR. It will be green oak, if that makes any difference?

 

[^woody^]

Deflection is only relevant to internal timber with decorative finishes.

How the beam is cut from the tree trunk will have a massive impact on whether it bows or bows a lot. You can't assess this unless you specify the type of cut you want . Merchants can't normally accommodate this as they just sell what they get.

Similarly, the constant wetting and drying and sunshine on the timber in situ will promote bowing under its self weight as the grain gets affected.

Selecting rectangular sections instead of square will reduce any bowing.

A tenon or housing joint would be best for the connections.

 

[Dereekoo]

Deflection is only relevant to internal timber with decorative finishes.

Unless you don't want a banana shaped wood structure from an aesthetics point of view

How the beam is cut from the tree trunk will have a massive impact on whether it bows or bows a lot. You can't assess this unless you specify the type of cut you want . Merchants can't normally accommodate this as they just sell what they get.

Would have thought you can only cut down the trunk to get a length of joist?

Similarly, the constant wetting and drying and sunshine on the timber in situ will promote bowing under its self weight as the grain gets affected.
.

Guess that's why there is a service 3 designation for timber design (strength/deflection) when its open to the elements

A tenon or housing joint would be best for the connections.

joist on top of post with a 6" nail