Sectional Area Sizing with Fu per ASD
Sectional Area Sizing with Fu per ASD
(OP)
Anyone know how tension member sectional areas came to be sized with the ultimate strength, Fu, with a factor of safety = 2, while the middle of the member's area is sized with Fy with a FS = 1.5?
Area net req'd at connections = Tensile Load / (Fu/2)
Area gross req'd btwn. conn's = Tensile Load / (Fy/1.5)
My guess is that the rupture stress, Fu, the stress at which the connection bolts will tear thru the member ends, was just divided by 2 to get the allowable stress. But why is the middle section based on yield stress and not ultimate as well? (Or vice-versa.) For A36, Fu/2 = 29ksi; Fy/1.5 = 24 ksi, so the connection should yield first since it is designed closer to Fy. Is there an advantage to that? Any corrections and pointers would be greatly appreciated.
Area net req'd at connections = Tensile Load / (Fu/2)
Area gross req'd btwn. conn's = Tensile Load / (Fy/1.5)
My guess is that the rupture stress, Fu, the stress at which the connection bolts will tear thru the member ends, was just divided by 2 to get the allowable stress. But why is the middle section based on yield stress and not ultimate as well? (Or vice-versa.) For A36, Fu/2 = 29ksi; Fy/1.5 = 24 ksi, so the connection should yield first since it is designed closer to Fy. Is there an advantage to that? Any corrections and pointers would be greatly appreciated.






RE: Sectional Area Sizing with Fu per ASD
The reason for the two different limit states is that typically the gross strength of a member would well exceed FyAg due to strain hardening - however the large elongations induced by the yielding through the entire section would probably render the member ineffective for its intended purpose thus "failing" it (though not actually falling down). The actual member tearing mode of rupture will occur along the plane of least resistance (at the bolt holes) and requires the member to actually break thus the use of the ultimate tension stress Fu.