I am designing 3-story Eccentric Brace Frames using the Seismic Provisions in RAM Frame and I am confused with the required strength value that is being determined for the axial design of the 1st story column.
The Member Code Check report for the column indicates that the required strength is being determine per AISC 341-10, Section D1.4a, which which notes that the required strength of columns shall be determined by 1) the load effects of Chapter F, and 2) the amplified seismic load per applicable building code.
From what I can tell, RAM checks the ultimate axial load considering the seismic load amplified by Omega (omega*1.0E = 2.0E), but it does not (edit) completely check for the axial load required by Chapter F.
In Section F3.3, the required strength for columns shall be determined assuming the the forces at the ends of the links correspond to the adjusted link shear strengths. In other words, the required axial strength should be the ultimate gravity axial load plus the summation of Vmh of the links in the stories above (which is exactly what is shown in the design examples provided in both the AISC Seismic Design Manual, as well as the SEAOC Seismic Design Manual).
RAM approaches this check differently. Instead, RAM determines an amplification factor per Link Capacity / Max Link Shear for the link at the given level, and amplifies the seismic axial load with that factor (in this example, it is 1.53). There is a note at the bottom of the Member Code Check Report that indicates that the axial design check assumes all links at the stories above yield at the same amplification level of seismic forces as the link at the given level. I am finding this assumption to be very unconservative, as the link cap/demand ratios for the links above can be much larger (1.84 and 2.94, in this example). This means the required axial strength is not properly considering the Vmh for the links at the stories above and is much smaller than required (for this example, Pu = 372 kips per RAM, but hand calcs show Pu = 478 kips).
Why would RAM implement an assumption that can be so dangerously unconservative? Why is there not a check that properly implements the requirements of AISC 341, Section F3.3?
What you are saying is correct and our approach is an oversimplification of the design. An enhancement is needed, but it's rather complex to execute. Do you do a lot of EBF designs? Just based on the models I see from users, they seem to be less common now.
Personally, this would be my first EBF design, so I am digging extra deep into the analysis and design process that RAM uses to engineer them. In doing so, I have been using an EBF design spreadsheet created and verified by our senior seismic engineer and back checking the results that RAM has been producing. In general, RAM has been producing more efficient results, aside from this column design issue.
From an engineering standpoint, it seems like it would be rather simple to adjust the design check in RAM to consider the actual adjusted shear strength values of the link beams in the stories above instead of relying on the application of an amplification factor under assumed conditions. However, from a computer software standpoint, this may well be a much harder process to execute.
Over the last few years, our office has been doing a number of steel-framed buildings on the West Coast in high-seismic areas. For these projects, a lot of them had building characteristics that best suited the use of EBFs versus other seismic systems. Until now, we have been using a combination ETABS and spreadsheets to design the lateral system for these structures, but after reviewing the seismic analysis and design capabilities of RAM Frame, I wanted to try it out.
In general, do you see many models for projects with lateral systems for high-seismic areas? What sort of systems have they been using? Are their structures just naturally better suited for systems other than EBF, or are they using other systems in lieu of EBFs?
Without actually going thru old models I would guess that BRBFs are becoming popular. Special Moment frames (with or without Side Plates) and Special Concentrically Braced Frames too.