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Laced Column Design

Laced Column Design

Laced Column Design

(OP)
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:
  • 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.
  • Analyze the frame with due regard for the TR13 load combinations. The wind load in my attached sketch is per ASCE7-10.
  • 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.
  • Design the lacing members for a total load of (side thrust + 2.5% max column axial load)/cos(lacing angle).
  • Design the top and bottom stay members for the (side thrust + 2.5% max column axial load)
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.

RE: Laced Column Design

Why not treat each built up column as a frame composed of individual members instead of assuming it is a perfect combined section? That should take into account all deformations. If you still want to use the 2.5% rule as a minimum requirement for design of lacing members, that's okay even if not needed.

BA

RE: Laced Column Design

A couple of items that look unusual, compared to designs that we have done, and seen in recent years. I'm not used to seeing the crane column base fixed - I'm not sure how much advantage that buys you, and at a premium foundation and base-plate details cost. And the lacing seems very "old-school" in the concept sketch. We mostly see segments of wide-flanged sections tying the two column sections together. I hope that you do find the best answer to your particular setup though.
Dave

Thaidavid

RE: Laced Column Design

(OP)

Quote (thaidavid40)

A couple of items that look unusual, compared to designs that we have done, and seen in recent years. I'm not used to seeing the crane column base fixed - I'm not sure how much advantage that buys you, and at a premium foundation and base-plate details cost. And the lacing seems very "old-school" in the concept sketch. We mostly see segments of wide-flanged sections tying the two column sections together. I hope that you do find the best answer to your particular setup though.
Dave

Thank you for your response. You're correct, the crane column likely does not need to be fixed.

Regarding the lacing design, I do remember reading in DG7 that fabricators prefer W-shapes to the lacing angles. This ship has not sailed yet so I have time to explore alternatives. Do you have any design resources regarding this approach?

RE: Laced Column Design

Don't have any resources to point to specifically. Choose a minor-axis bracing interval that gets you the allowable crane column capacity that you like, and then choose a W-section with a web thickness thick enough to take the brace's axial force (no web crippling). If your program's model doesn't give you a brace axial force to design to, use 2% of the maximum crane column load for its axial design force.

Thaidavid

RE: Laced Column Design

Since you're using Risa why not explicitly model the lacing between columns? Seem like you wouldn't have to mess around with effective moment of inertia or under/over-estimating deflections using the combined moment of inertia. I've never used wide flanges as bracing but it seems like an interesting and simple approach. though if your frames also receive wind loads it seems like the angle lacing will give you a better and possible stiffer frame for resisting these loads.

RE: Laced Column Design

Quote (T Bat)

Since you're using Risa why not explicitly model the lacing between columns?

Agreed!

BA

RE: Laced Column Design

Several Months ago, AISC offered a 9 week "night school" program which I attended. It consisted of the design process of A 60' tall x 120' wide two bay industrial building that housed two 50 ton OH cranes. Perhaps you can down load the pages relevant to your question...

Working from memory, I recall that the base(s) were pinned and the top was fixed... which is typical for most all industrial buildings that house cranes (ie PEMB's - almost always moment conn up high). The design did include the sister column arrangement.. the cols were braced to one another with some form of WF section, but there were some alternates...

Wow - 30 kip lat load??? That's a big hoist - but since magnet duty, it will be very high speed

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