Farshid aMIRI
Civil/Environmental
Hi.i modeled a 4 story structure to check some procedures Etabs is using to design a building. In doing so I came across something I couldn't fathom .plz help me if you know what Etabs is actually doing. In all Versions of Etabs 9.74 to even the new version of 2018, the same thing happened. Assume a special sway frame that you want to design its beams. Load combinations are divided into two major categories one that just includes gravity loads and the other both gravity and seismic. For designing the flexural rebars of a beam it just considers the max moments for each station along the length of the beam ( for shear design though it uses some criteria let's say from ACI318.14.). The Max negative moment is usually calculated from those seismic included combinations at the start and the end of the beam span while the Max positive moment is the result of either gravity combinations or seismic ones depending on how gravity loads are being applied to the span as well as the length of span itself(this moment is used to design the bottom rebars at the mid-span in special sway moment frames if this value is less than a quarter of the negative moment at the corners you need to apply 0.25M corners but if not you need to use it itself. Here my problem shows up. In my case, the latter scenario happens where due to heavy gravity loads 1.2DL+1.6LL brings about a positive moment which is greater than what induced by 1.2DL+1.0LL+1.0EQ.
In design sheets that Etabs shows at the end of the design this value is not what combination itself shows but the name indicates the exact gravity combination. for instance, the mid-span moment in load combo is 11.92 t.m which is greater than 0.25M from corners so that the number itself should be used but 16.05 t.m is used to design the bottom flexural rebars at the mid-span.
Plz, notify me if you know what this is all about. I posted my related photos and Etabs file to further clarify my points.
Best Regards, Farshid Amiri.
In design sheets that Etabs shows at the end of the design this value is not what combination itself shows but the name indicates the exact gravity combination. for instance, the mid-span moment in load combo is 11.92 t.m which is greater than 0.25M from corners so that the number itself should be used but 16.05 t.m is used to design the bottom flexural rebars at the mid-span.
Plz, notify me if you know what this is all about. I posted my related photos and Etabs file to further clarify my points.
Best Regards, Farshid Amiri.
