Assisting struts under compression

[ajd-brown]

I am designing a simple truss structure...

consider a member of a truss under a compression of xN.

As x increases, the member will tend to want to buckle.

eulers strut buckling formula states that the force a member buckles at is inversely proportional to the square of the length of the member.

so halving the length will multiply this load by 4.

I'm certain this isn't a new proposition but I thought Id say it anyway...

putting an normally unloaded strut perpendicular to the midpoint of a member under compression, to effectively half the length of the member, as this unloaded strut will constrain the member and prevent it from buckling in the centre? Am i correct? obviously there will be a slight load in the perpendicular strut, but not considerable (i hope).

I hope I have explained this fairly well and i would just like some feedback as to whether this is a flawed theory? I understand it wouldn't be as effective as truly halving the length of the member but, should increase the buckling load somewhat?

 

[Studiot]

Yes your thinking is sound. Propping a member partway along its length increases its resistance to buckling. This method is also used to stiffen other compression members such as the webs of I (and other) sections. They are then called web stiffeners.

However there is one thing you should take note of.
Buckling can occur in any direction, subject to the cross section of the member, so your stiffening strut could also need to act as a tie in your truss in order to prevent buckling.
Indeed, during fabrication, erection, lifting and even normal service life (eg due to wind loads) the stresses in truss members may reverse, so all members should be stability checked against anticipated activity in this direction.

go well

 

[ajd-brown]

great that's what i was hoping to hear, so if i constrained the movement in both the two perpendicular axis to the member, it is much less likely to buckle, have you any estimation as to how much it would increase the resistance to buckling?

luckily this is only for a model bridge, so I hope those factors are limited because of its lack of weight?

thanks for your reply!

 

[Studiot]

Well your truss members are hinged and both ends so the effective length for the Euler theory you mention will equal the actual length. The tendency and resistance to buckling will then depend upon the member section properties and as well. This will set a critical slenderness ration you should aim to be below. For ordinary steel this ratio should be less than 1:100. There will be local code requirements based on this.

The attachment shows how to derive your own safe area.