P-Delta Effect
P-Delta Effect
(OP)
What is the "P-Delta" Effect?
How is it related to Bracing for Structural Stability?
How is it related to Bracing for Structural Stability?
When was the last time you drove down the highway without seeing a commercial truck hauling goods?
Download nowINTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS Come Join Us!Are you an
Engineering professional? Join Eng-Tips Forums!
*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail. Posting GuidelinesJobs |
|
RE: P-Delta Effect
With a lateral load, a building is usually displaced by a little bit; this can be a matter of, for discussion, say an inch. This is the 'delta'
The entire vertical load caused by the weight of the building and the occupants, snow, equipment,etc. is now applied to the supports by this offset of an inch. This is the 'P'.
The entire weight times the one inch displacement is considered as an additional 'overturning moment'. This is the 'P-delta'.
This overturning moment has the same effect as increasing the lateral load by an amount 'P-delta' divided by the storey height.
RE: P-Delta Effect
dik's answer is also correct with regard to a system of members. It is basically, additional loads on members (or the system) due to deflection from the applied loads.
If you do not apply the P-Delta condition to your member or system, you are required to determine the "moment magnification" of each member under axial load. The moment magnification equation(s) are derived from that differential equation mentioned above. Depending upon the characteristics of the member and the loading, the magnification ranges from negligible to significant.
RE: P-Delta Effect
First - there is the frame sidesway condition that dik and rowe discuss above.
Second - the individual column may deflect along its length due to frame deflection or lateral loads applied directly to the column. Think of an "S" shaped deformed column in frame.
Now along the length of the column there is axial compression that can add additional, secondary moments to the column itself. This is also a PDelta effect that must be considered.
An analogy would be taking a bow (from a bow and arrow) and setting one end on the ground, and the other straight above. Due to the curvature of the bow, if you push down, the bow bends even more.