bearjew
Structural
- Apr 2, 2015
- 27
Hi everyone. I am in the process of designing a laced column system for a new bldg. The new bldg houses a magnet crane, and we are to follow TR13 requirements for the design of its support. I have available to me AISC DG7, AISE TR13, and ASCE "Structural Design of Industrial Buildings", all of which discuss combined columns and the different methodologies for designing them. However there are very few worked examples, and even fewer to be found on the web (that I can find). I am having some trouble understanding certain aspects of the design, so I would like to detail out how I am tackling the design to see if anyone can offer me some guidance.
My approach so far has been:
[ul]
[li]Model the frame in RISA 2D, treating the lower segment of the column as a combined section and the upper as independent (with regards to MOI). See attached picture.[/li]
[li]Analyze the frame with due regard for the TR13 load combinations. The wind load in my attached sketch is per ASCE7-10.[/li]
[li]My RISA 2D model outputs a moment at the base of my column, which I divide by my column spacing to determine a force couple which is applied as axial load to each of my columns. Each column is then designed for this axial load + its own axial load (from either bldg or crane loads). The bldg column also has some bending in it from either wind above the lacing or where the side thrust has to travel to the lacing.[/li]
[li]Design the lacing members for a total load of (side thrust + 2.5% max column axial load)/cos(lacing angle).[/li]
[li]Design the top and bottom stay members for the (side thrust + 2.5% max column axial load)[/li]
[/ul]
However the deflections outputted to me by RISA assume that my lower column section acts perfectly as a combined section. In reality, there would be some distortion of the frame via shear deformation and elongation of the lacing members. How do I account for this in my design?
I would like to design my lacing and stay members to be sufficiently stiff so that I can ignore these effects, but am unsure of what forces to design for to ensure composite behavior.
This is quickly turning into a wall of text. I can offer any additional information necessary, but would just like to get a discussion started on what I could be doing differently for a more efficient design or to get past this speedbump.
My approach so far has been:
[ul]
[li]Model the frame in RISA 2D, treating the lower segment of the column as a combined section and the upper as independent (with regards to MOI). See attached picture.[/li]
[li]Analyze the frame with due regard for the TR13 load combinations. The wind load in my attached sketch is per ASCE7-10.[/li]
[li]My RISA 2D model outputs a moment at the base of my column, which I divide by my column spacing to determine a force couple which is applied as axial load to each of my columns. Each column is then designed for this axial load + its own axial load (from either bldg or crane loads). The bldg column also has some bending in it from either wind above the lacing or where the side thrust has to travel to the lacing.[/li]
[li]Design the lacing members for a total load of (side thrust + 2.5% max column axial load)/cos(lacing angle).[/li]
[li]Design the top and bottom stay members for the (side thrust + 2.5% max column axial load)[/li]
[/ul]
However the deflections outputted to me by RISA assume that my lower column section acts perfectly as a combined section. In reality, there would be some distortion of the frame via shear deformation and elongation of the lacing members. How do I account for this in my design?
I would like to design my lacing and stay members to be sufficiently stiff so that I can ignore these effects, but am unsure of what forces to design for to ensure composite behavior.
This is quickly turning into a wall of text. I can offer any additional information necessary, but would just like to get a discussion started on what I could be doing differently for a more efficient design or to get past this speedbump.