Determining the load in a diagonal brace in a steel frame?
Determining the load in a diagonal brace in a steel frame?
(OP)
Hi there,
I am trying to determine the axial load in a diagonal brace in a frame. I thought the method to calculate the load would simply be as shown in the attached image, which gives an axial tension load of about 120 kN. However when I modelled the same frame in Staad pro to verify, it only gave me an axial compression force of 0.6 kN for the bracing member. The Staad pro model gave the same results as my hand calculations for the axial compression loads of the external frame members, just not the bracing member, which is why I am a bit confused. Any help would be appreciated.
I am trying to determine the axial load in a diagonal brace in a frame. I thought the method to calculate the load would simply be as shown in the attached image, which gives an axial tension load of about 120 kN. However when I modelled the same frame in Staad pro to verify, it only gave me an axial compression force of 0.6 kN for the bracing member. The Staad pro model gave the same results as my hand calculations for the axial compression loads of the external frame members, just not the bracing member, which is why I am a bit confused. Any help would be appreciated.






RE: Determining the load in a diagonal brace in a steel frame?
I fail to see in any arrangement like this how you would ever get a tension load in the brace.
RE: Determining the load in a diagonal brace in a steel frame?
If the frame is responding to an applied vertical load on each column, the diagonal member will tend to be in compression but the upper beam will not have a compression of 110kN.
You need to show the applied forces on the frame and ensure that each joint has the appropriate degree of freedom.
BA
RE: Determining the load in a diagonal brace in a steel frame?
RE: Determining the load in a diagonal brace in a steel frame?
RE: Determining the load in a diagonal brace in a steel frame?
RE: Determining the load in a diagonal brace in a steel frame?
^ For the avoidance of doubt. The axial compression forces are acting on each member as shown above and the supports are all pinned.
RE: Determining the load in a diagonal brace in a steel frame?
RE: Determining the load in a diagonal brace in a steel frame?
When I modelled it, the only support I included was at the vertex of the cables (as a pinned support).
I ran a few load cases and only seemed to get a force in the bracing member when the corner loads were unbalanced (i.e. two corners having higher downward loads than the other two) and cause the frame to tilt. I was getting max approximate loads in the bracing member of about 50kN compression and tension depending on which side of the frame tilted. Although I somewhat trust the outputs that STAAD is giving me for the bracing loads I would still like to learn a method to double check using some form of hand calculations.
RE: Determining the load in a diagonal brace in a steel frame?
The above statement does not remove all doubt. Where are the supports? If each of the four corners is deemed to be a pinned support, then there are no forces in any of the members.
Alternatively, if every joint is pinned, the structure is unstable.
There should be one pin and one roller and the two beams must be continuous with the diagonal pinned to them.
BA
RE: Determining the load in a diagonal brace in a steel frame?
BA
RE: Determining the load in a diagonal brace in a steel frame?
RE: Determining the load in a diagonal brace in a steel frame?
For an unbalanced situation like you mentioned you need to run a nonlinear analysis. I made up some numbers and tried it myself and much to my surprise the deflections actually made sense. There was still no significant load in the brace (under 1 kN). I had to cheat a little bit and give the slings some bending stiffness to keep it stable but they didn't pick up much moment. Maybe if I would have used more steps or put in some soft springs might could have got it to work without doing this, but, hey, what you want for nothing?
RE: Determining the load in a diagonal brace in a steel frame?
Hmmm. What you say makes sense, so if there is no force in the diagonal brace, it rotates slightly in plan and the frame racks slightly to compensate for the axial shortening of the outer members, i.e. the corners are no longer right angles.
BA
RE: Determining the load in a diagonal brace in a steel frame?
BA
RE: Determining the load in a diagonal brace in a steel frame?
Here's an exaggerated view of the frame racking even when the braces go to the corner. I'm impressed that you figured that out.
RE: Determining the load in a diagonal brace in a steel frame?
I believe that this is a secondary effect and will result in a very small compressive strain in the diagonal bracing member.
BA
RE: Determining the load in a diagonal brace in a steel frame?
BA