Capacity of a steel tension rod 2

[MJC6125]

Per AISC, what all needs to be checked when designing a steel tension rod (i.e. tie-back rod on a canopy/balcony or lateral bracing rods in a PEMB)?

The textbook I have (Steel Structures: Design and Behavior {5th Edition} by Salmons) makes it look like all you have to check is the tensile strength per AISC-J3.6 Tensile and Shear Strength of Bolts and Threaded Parts. This section of the AISC spec is based on the nominal tensile strength of the tension rod (0.75Fu or as specified), and not the yield strength (Fy). Do you also need to check the design tensile strength per AISC-D2a tensile yielding in the gross section? It looks like when the yield strength of steel is less than 0.625Fu, then the tensile strength per AISC-D2a governs, otherwise the AISC-J3.6 equation governs.

If you don't need to check the chapter D yield strength, why is that?

Lastly, what type of material do you specify for threaded rods on your drawings?

Thanks.

 

[Hokie93]

If the tension member is a fully threaded rod, the design tensile strength is determined from Section J3.6 of AISC 360-10 (assuming you are designing to IBC and AISC 360-10). If the tension member is something other than a fully threaded rod (partially threaded rod, round HSS, WT-shape, etc.), the provisions of Chapter D should be followed along with Section J3.6 (if applicable, at a threaded portion). I typically specify ASTM A36 or ASTM A572 Grade 50 for threaded rods. If more strength is needed, options include ASTM A193 Grade B7, ASTM A354, and ASTM A449.

 

[JAE]

Don't forget the connection capacities - lots of checks there...and also any turnbuckles. All those may control the overall capacity more than the rod itself.


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[MJC6125]

Agreed, definitely can’t forget the connections.

Hokie93, do you know why the yielding check can be neglected when the member is a fully threaded rod?

 

[Hokie93]

If the tension rod member is made from high-strength material (ASTM A193 Grade B7, ASTM A449, ASTM A354, etc.), the material does not have a defined yield point and strength is governed by the ultimate tensile strength. In that case, the design tensile strength would be determined per AISC 360-10 Section J3.6. For a mild steel tension rod, strictly speaking, the design tensile strength should be determined from AISC 360-10 Chapter D and would be the lesser of F_yA_g/Ω and 0.75F_uA_g/Ω. Taking an ASTM A36 rod (F_y = 36 ksi and F_u = 58 ksi), the nominal tensile strength is 21.55 ksi x A_g based on yielding and 21.75 ksi x A_g based on rupture. So, the result is essentially the same for an ASTM A36 rod whether yielding is explicitly considered or not. The 0.75 factor included in the rupture check reduces the cross-sectional area to account for the threaded portion of the rod.