| Product(s): | SACS |
| Version(s): | All |
| Area: | Joint Can |
Problem
How to derive brace limit stress as half of capacity of brace in case of segmented members in SACS?
Solution
Let us try to simulate the scenario to walk through the calculation procedure with a test model as shown below.
Hence, C/S area for segment 1 (21m segment length) = 306305 mm2
And C/S area for segment 2 (1m segment length) = 246300 mm2
Now, let us prepare a jcninp file.
In JCNOPT line, Column 8, call “EQK” option. This option will provide us “Member Capacity Report” in SACS lst file. SACS will calculate 100% strength for earthquake-case joint checks. But we can obtain Euler & Inelastic Buckling Capacity and Stress from the “Member Capacity Report”.
Now, in “Detailed Report” tab for the brace (BRA) member in Post view file of “Review/ Redesign”, an effective length is reported under the section “Member Properties”. Since, I am using segmented member, different Leff shall be reported for different segments. This effective length is determined from Pcr calculation of segmented elements which requires iteration procedure. SACS internally calculate the Pcr of a segmented member and reports the corresponding Leff. In our analysis output, following are the Leff reported for segment 1 and segment 2 of the brace member.
Now, let us use 21.93m as effective length for segment 1 and calculate Fa from API RP 2A (Section 3.2.2), then Fa will be 320.628 MPa. The same is calculated in SACS as shown below.
Similarly, for segment 2, Fa calculated using effective length of 19.76m is 323.448 MPa. And SACS reports the same value as shown below.
Hence, the full capacity reported in SACS “Member Capacity Report” matches with the API calculations.
Now, in the “Member Capacity Report”, for multiple segments, SACS is reporting corresponding inelastic buckling load, which SACS calculates internally using iteration method as per “Theory of Elastic Stability by Timoshenko”. Out of these loads, SACS will consider the least one for the purpose of strength design. When this selected load is divided by the segment (which is connected to chord) area, the result will be reported as “Brace Axial Capacity” in SACS lst file.
Since, I was using the 100% strength (using the command EQK in JCNOPT line), I need to factor this capacity to obtain the desired value of 50% Brace Limit Stress which is 130.04 MPa and matching with SACS output as shown below.