portal one side braced other

[londonkid]

hi how do you determine the effective stiffness in ordor to calculate how horizontal load is shared if in a structure you have portal on one side and braced frame the other (in the same plane obviously)

 

[msquared48]

Put a unit lateral load to each and see how much each deforms laterally. The stiffness will be in inverse proportion to the amount of deflection seen. In other words, the more deflection, the less the stiffness.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[sdz]

msquared48 is right. I would expect that the braced frame would take most of the load so you could conservatively assume it takes 100% for sizing of bracing members.
Do you have an analysis packege available?

 

[rscassar]

Build a model up in an analysis program, apply a unit load and see what proportion of that unit load is taken by the portal column and brace respectively.

As a quick estimate, the lateral stiffness of a single portal column (pinned bottom, free translation and fixed rotation top) is 3*E*I/L^3. Whereas the lateral stiffness of the brace is A*E/L*(sin(angle))^2 (I haven't looked at a textbook for those equations, they are off the top of my head so their could be errors).

The brace component of the lateral stiffness will more than likely be the major contributor.

 

[ToadJones]

Have you considered treating the portal frame as a lean-to?
I am guessing that maybe one frame or the other is an existing structure and you are adding to it?

 

[paddingtongreen]

Why build models? before computers that was a no-brainer, unless the portal members were massive, the bracing carries the load, period.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[oldrunner]

I've kind of always look at it this way: You have to break the brace before the portal starts to work.