Rigid Diaphragm and Bracing Loads
Rigid Diaphragm and Bracing Loads
(OP)
I have a 3 story building that uses the same steel brace size and configuration for it's lateral resistance(wind) from top to bottom, and has conc. floor diaphragms.
On the 3rd floor there is for example 3 braces (one on each end of the bldg. and one in the middle of the building).
On the 2nd floor, there is one at each end, one in the middle, and one that is added half way between the middle and one of the sides (say 30 feet from the middle brace).
At the first floor there are 2 at each end, one in the middle and one on each side of the middle (this time 20 feet away from the middle brace).
My question is will the diaphragm be able to distribute the accumulating force between all the braces equally on each floor or will the brace (for example the middle brace)which is lined up one on top of another, accumulate more force than the others as it would in a non rigid diaphragm?
On the 3rd floor there is for example 3 braces (one on each end of the bldg. and one in the middle of the building).
On the 2nd floor, there is one at each end, one in the middle, and one that is added half way between the middle and one of the sides (say 30 feet from the middle brace).
At the first floor there are 2 at each end, one in the middle and one on each side of the middle (this time 20 feet away from the middle brace).
My question is will the diaphragm be able to distribute the accumulating force between all the braces equally on each floor or will the brace (for example the middle brace)which is lined up one on top of another, accumulate more force than the others as it would in a non rigid diaphragm?






RE: Rigid Diaphragm and Bracing Loads
Portion of the force at the base of the 3rd floor exterior brace for example will transfer thru the diaphragm and delivered to the intermediate brace line at the lower level. Diaphragm must be checked for the uniform lateral load plus this additional shear.
RE: Rigid Diaphragm and Bracing Loads
RE: Rigid Diaphragm and Bracing Loads
Once you calculate your three forces in each brace between the third and second floor, these forces are applied at the locations of each of those three braces on the second floor diaphragm. The added lateral forces applied due to the mass of the second floor diaphragm creates a new "center of force" applied to the second floor. This totality of forces is resisted by the cummulation of the braces below, in accordance with their relative rigidities.
A floor diaphragm is pretty stiff compared to the braces - in fact, you can imagine the entire "stiff" floor deflecting laterally with minimal horizontal bowing...although there will be some. Thus, all four braces below the second level will deflect the same.....
and Hooke's Law - equal brace and equal deflection = equal force.
Now this doesn't take into account the possibility of center of mass not equalling the center of rigidity and the resulting torque on the diaphragm. That should always be included.
But I wouldn't simply direct all the upper brace forces into the lower braces that are directly aligned below. This ignores the fact that some of the force will be distributed across the diaphragm and the braces, in reality, are all working together, deflecting pretty much the same.
RE: Rigid Diaphragm and Bracing Loads
Often the assumptions include something being infinitely rigid or flexible, which often is not true. So regardless of which method is used to derive at the forces, they are merely our best "prediction" of the forces in the members. As to how close they are to reality depends on the method selected.
Traditional method for load distribution is only accurate enough for a very regular buildings with stacking braces. For multi-level rigid diaphragm buildings with irregular lateral system layout, hand calc may be done using cummulative center of mass and center of rigidity starting at the roof level and going down the structure. The result from this will still be a bit different if compared to computer output using programs like ETABS or SAP.
We can analyse something to death but if your loading is not accurate, all computer generated output is garbage.
RE: Rigid Diaphragm and Bracing Loads
Work, think, fret, worry, struggle, race, hurry, frantic, etc.
Researcher:
Show up, continue process, cup-o-java, meetings, 8 hr. days, "what budget?", etc.
Perhaps the above is like whyun's statement: either infinitely rigid or infinitely free.
RE: Rigid Diaphragm and Bracing Loads
RE: Rigid Diaphragm and Bracing Loads