Egrees Stair design per ASCE 7 Chapter 13 1

[Adams34]

Quick question about egress stair design per ASCE 7 chapter 13, how do you apply the seismic force Fp in the model? If you create a Sap 2000 model, do you just apply that concentration force at the center of gravity of the stair? Just on a single thread? at the middle the thread or what is typically done? I know the code says apply Fp laterally at C.G but it just doesn’t look right for a stair

 

[HTURKAK]

You are expected to calculate the seismic forces as per (13.3-1) for body and for fasteners separately.

Refer to Table 13.5-1 Coefficients for Architectural Components.
- Egress stairways not part of the building seismic force-resisting system

- Egress stairs and ramp fasteners and attachments .

A sketch would be useful to get better responds.




He is like a man building a house, who dug deep and laid the foundation on the rock. And when the flood arose, the stream beat vehemently against that house, and could not shake it, for it was founded on the rock..

Luke 6:48

 

[driftLimiter]

In the past I have used the term Z/H to increase the load as the mass gets higher.

A realistic load distribution must be based on mass. And your model should consider what elements are being designed.

For example, foundations and base of structure may well be designed based on load at global c.g.
But intermediate framing and lateral bracing may need a more detailed load distribution.

 

[Adams34]

Please see attachment @HTURKAK . I am assuming you have to model the riser and threads to apply Fp is both direction or how do you apply Fp in this situation?

 

[driftLimiter]

If you want to check the stringers for seismic load you might consider a distributed load equivalent to Fp.
Its an inertial force after all for a structure where the mass is distributed so much a single point load is a poor analogy.

 

[HTURKAK]

I wish you had provided a sketch showing the no. of storeys, egress stair storey , seismicity etc.
Apparently the egress stair is steel with RHS stringers .
Below is a worked example calculation based on a imaginary sample data ;
- For the stair body ;

ap=1.0 , Rp=2.50 , z=25ft, h=60 ft , Ip = 1.5, Sds = 0.8g, Wp=5000 lbs

Fp=1760 lbs Fpmax =1.6*Sds*Ip*Wp =9600 lbs Fpmin =0.3*Sds*Ip*Wp =1800 lbs ⇒ 1760<1800 so take Fp=1800 lbs .
This horizontal load should be applied to the mid point of two stringers ( 900 lbs for each one ) or may be applied UDL as @driftLimiter pointed out .

- For Fasteners of egress stairs,
ap = 2.5, Rp = 2.5, Ω0 = 2.5 z=25ft, h=60 ft , Ip = 1.5, Sds = 0.8g, Wp=5000 lbs

Fp=4400lbs Fpmax =1.6*Sds*Ip*Wp =9600 lbs Fpmin =0.3*Sds*Ip*Wp =1800 lbs ⇒1800 <4400<9600 so take Fp=4400 lbs
This load should be applied to the four connections of the stringers .
You may develop spreadsheet for multi storey bldg with egress stairs at every storey.

I hope this respond answers your post.




He is like a man building a house, who dug deep and laid the foundation on the rock. And when the flood arose, the stream beat vehemently against that house, and could not shake it, for it was founded on the rock..

Luke 6:48