ASTM A500 Grade B (Fy = 46 ksi) has traditionally been specified for rectangular HSS. Recently, the use of ASTM A500 Grade C and ASTM A1085 (Fy = 50 ksi) have become more popular. The purpose of this tech note is to summarize how values for Ry (ratio of the expected yield stress to the specified minimum yield stress) are tracked between RAM Structural System and RAM Connection and discuss issues that can occur with each of these HSS material specifications in RAM Connection when seismic provisions are used.
AISC 341-10 tabulates a single value of Ry (Ry = 1.4) for HSS that applies to both ASTM A500 Grade B and Grade C. A value for ASTM A1085 is not tabulated. In AISC 341-16, separate values are listed for ASTM A500 Grade B (Ry = 1.4), ASTM A500 Grade C (Ry = 1.3), and ASTM A1085 (Ry = 1.25).
In RAM Structural System, the Ry value is determined as follows:
HSS and TS Sections: Ry = 1.4
Pipe Section: Ry =1.6
For all other members, Ry is based on the yield strength:
Fy = 36 ksi: Ry = 1.5
Fy = 42 ksi: Ry = 1.3
All Others: Ry =1.1
When a RAM Structural System modeled is imported into RAM Connection, a single material is imported for each steel Fy. The Ry associated with each of these materials is based on the Fy assumptions above. For example, a steel member defined with Fy = 36 ksi in RAM Structural System is imported with a material labeled “RamFy 36” with Fy = 36 ksi and Ry = 1.5. The Ry exceptions for HSS and pipe sections noted above are not considered during import. The materials and corresponding Ry values that are imported into the RAM Connection materials database from RAM Structural System are listed below:
RamFy 36: Ry = 1.5
RamFy 42: Ry = 1.3
RamFy 46: Ry = 1.1
RamFy 50: Ry = 1.1
The values above are problematic for HSS and pipe sections. For example, a rectangular HSS brace specified as ASTM A500 Grade B would be modeled with Fy = 46 ksi in RAM Structural System. In RAM Frame – Steel mode – Seismic Provisions, these sections are identified as HSS and designed using Ry = 1.4. When imported into RAM Connection, however, these members are associated with the “RamFy 46” material, and the connections are designed using Ry = 1.1. Since it is not possible to assign a new material to members in RAM Connection for RAM Structural System, most users work around the Ry issue by manually changing the Ry associated with the “RamFy 46” material in the RAM Connection material database.
Manually changing the Ry value in the database does not work well if A500 Grade C or A1085 is specified for the rectangular brace. Members associated with these specifications are modeled with Fy = 50 ksi in RAM Structural System. Members, such as wide flange beams and columns, that are associated with other material specifications like ASTM A992 also are modeled with Fy = 50 ksi in RAM Structural System. After importing these members into RAM Connection, each of these members are associated with the “RamFy 50” material, which uses Ry = 1.1. This Ry is correct for the wide flange members, but a different Ry should be used for the HSS members. The best work around in this case is to manually change the material selection in the Connection Pad:
Unfortunately, material changes made in the Connection Pad are not saved when closing out of the window. This change would need to be made each time the connection is reviewed in the Connection Pad.