Exterior Standalone Stair

[McMichael05]

I am designing an exterior stair, so its subject to wind loads as well as seismic. For the seismic design, i am looking at Chapter 15 of ASCE 7-16 for nonbuilding structures similar to buildings. This stair isnt part of the lateral system and isnt detailed to attach to the building, so it has to support itself. Looking at Table 15.4-1, i am trying to justify the structure type as a steel ordinary moment frame with R=3.5. It does have special detailing requirements in AISC 314. When i looked through this, it has a section for OMF with a knee brace system which is what i have. It says to design the beam to column connections, the braces, and the brace end connections for the forces required to develop 1.1RyMp/as (alpha sub s) of the beam or column. Is the Ry the R value from table 15.4-1 (R=3.5)? Also i believe the alpha sub s=1 for LRFD and 1.5 for ASD, is that correct?
[URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1632411028/tips/OMF_Knee_Brace_ep2rmb.pdf[/url]
[URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1632411047/tips/Stair_Graphic_catfod.pdf[/url]

 

[McMichael05]

Update: Ry is not the R value from the table it is the ratio of the expected yield stress to the specified minimum yield stress. Also, if anyone could provide a design example thatd be helpful. I have the value now but it is in k-ft and im wondering how to put that into a tension/compression couple or some sort of kip load to check the connections.

 

[HTURKAK]

I have copy and pasted the definitions of Ry and αs from AISC 341-16 ( section= symbols).

Ry = Ratio of the expected yield stress to the specified minimum yield stress, Fy

αs = LRFD-ASD force level adjustment factor = 1.0 for LRFD and 1.5 for ASD .

-Just for curious: what is the Seismic Design Category ?
- The use of knee bracing or K-bracing is not a good idea ( say ; at least IMO) since Knee bracing exerts large unbalanced force into a column..

 

[McMichael05]

Seismic Design Category is C. It is permitted under the Ordinary Moment Frame, but i totally understand. Fortunately its not very heavy, only grating for dead load other than self weight and its all open so just wind pressure on the face of the members so there isnt a ton of lateral loading and thus axial loading going through the braces.

 

[HTURKAK]

In this case yes... you may use Knee bracing ..

You should follow the procedure defined at AISC 341-16 ( OMF Knee-Brace Systems )

 

[SJBombero]

My preferred procedure for a non-building moment frame is to use R=1. This will eliminate having to design for the plastic moment of the beam or column.