Tuesday, December 11, 2007

RE: Stair stringer design for continuous kinked stringer

Conrad –


You just provided the perfect definition of an engineer: One who takes something simple and makes it complicated. I hope your not billing for all those hours when your designing stair stringers.




T. William (Bill) Allen, S.E.


Consulting Structural Engineers
V (949) 248-8588 F(949) 209-2509

-----Original Message-----
From: Conrad Harrison [mailto:sch.tectonic@bigpond.com]
Sent: Monday, December 10, 2007 8:56 PM
Subject: RE: Stair stringer design for continuous kinked stringer




Sounds like a simple plane frame. I’m assuming it’s a two element cranked beam with supports at either end, and no internal support at the crank.


If it has one pinned support and a roller support then it is determinate, and not much more complicated than a simple span beam: straighten it out for analysis if it makes it easier, adjusting the loads accordingly. If both supports pinned, then it is indeterminate then need to employ methods for indeterminate structures: slope-deflection or moment distribution etc. Alternatively may find a suitable rigid-frame formula in Kleinlogel or Roarkes, failing that see if can find a simple plane frame program.


As for the proper way: any method which produces a robust design-solution which has acceptable level of performance, minimum risk of failure, and when pushed to failure fails in an acceptable manner, is the proper way. So a simplified model can be used as long as there is compatibility of assumptions through out the design.


Therefore would treating the angle joint as a pinned joint result in collapse? Can the platform be considered as a cantilever propping the end of the stringer? Having sized members assuming pinned joints, what is the effect of inserting a redundant rigid joint? Having used a simplified model, what effect is using a more realistic behavioral model going to produce, given that the load inputs could be considered fictitious? Does the application warrant “accurate” calculation of deflection and natural frequency? Is a more accurate analytical method required for safety, or is it simply required to reduce material content and possibly robustness in the process?



Though the real solution struck me as maybe a need for getting a good rest!




Conrad Harrison

B.Tech (mfg & mech), MIIE, gradTIEAust



South Australia

From: Jim Wilson [mailto:wilsonengineers@yahoo.com]
Sent: Tuesday, 11 December 2007 02:07
To: seaint@seaint.org
Subject: Stair stringer design for continuous kinked stringer


What is the proper way to analyze/design a steel channel stair stringer when the stringer is continuous from the angled riser through to the end of the level platform?  There would be a laterally braced full pen weld at the angle point.


Is this just a simple span beam calc for the full length?  Or is there another way to model the angled section with simple methods?


Jim Wilson, PE

Stroudsburg, PA