Strength of laminated timber 115x115

[magnus_kall]

Hello!
I am planning to build a pergola using glulam posts measuring 115x115mm. How far apart can the posts be without them warping over the years? They are available in lengths up to 5 meters. Can I put a support at each end and expect it to stay straight?

There will be no additional load in the form of a roof or similar - only sailcloth.

 

[scorp1on]

Long-term deformation = instantaneous deformation × (1+kdef)

Where

instantaneous deformation, w = 5qL^4/384×E×I

Where q is the load in N/m from the self-weight of the beam. E is the modulus of elasticity, I is the moment of inertia (bh^3/12), and L is the span.

Kdef is a modification factor related to the climate class, i.e., how exposed the structure is.

You calculate q as b×h×the density of the wood × g where g is the gravitational acceleration.

For example, when looking generally at deformation limitations of floor joists, the recommendations are that the instantaneous deformations should not exceed L/500 and the long-term deformation should not exceed L/300, but you can determine these as you wish.

 

[Stefan1972]

If you lay it down between two supports, it will probably sag quite a bit just from its own weight.

 

[scorp1on]

If you want to count those deformations, see my previous post.