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Question #1: Capacities of Composite Columns
Where can I best obtain the axial compressive capacities of concrete-filled HSS columns?
Answer #1: The current design of composite members is prescribed by Chapter I of ANSI/AISC 360-10, which is a free download from www.aisc.org. Further information for designers is available from the current AISC “Steel Construction Manual”, 14th. edition, 2011, also available from the AISC website. In Tables 4-13 to 4-20 of this Manual, available strengths in axial compression (for both LRFD and ASD) are given for square, rectangular and round HSS to ASTM A500 Grade B, and pipe to ASTM A53 Grade B, filled with 4-ksi or 5-ksi normal density concrete.
Question #2: HSS Capacity with Holes
I have designed a tube header for a door that is taking both horizontal wind and also a vertical load at the midspan of the header. My customer is now requesting to allow for 2" diameter holes @ 2'-0" o.c. along the top and side of the tubing. I am told this is for insulation. Taking out this much section property concerns me and I am not sure quite how to check this. What checks would you recommend? Do you think I should be concerned? I am thinking first off that they should space the holes apart further. Suggestions?
Answer #2:The implications of two-inch diameter holes, at two-foot centers, will very much depend on the size of your HSS (presumably square or rectangular) and the magnitude of the applied loadings.
If holes are being put in both the side and the top of the HSS it would seem prudent to offset them, if possible, so that two holes do not occur at the same cross-section.
It seems like you have a bi-axial bending situation under wind + gravity loads. You could calculate the reduced cross-sectional properties of the HSS (with portions of steel removed) and use these properties at the critical cross-sections (points of maximum bending moment and maximum shear force). You should be checking the reduced bending capacity, the shear capacity (which is not normally critical but it may be, if parts of the web are removed) and the deflection.
Question #3: Use of Cast Steel Nodes for Bridges
We would like to propose a tubular truss steel bridge for regular highway vehicles with cast steel nodes to one of our clients in Alberta, Canada. The steel nodes would have to be fairly large in size (expected pipe sizes are up to 1 meter in diameter) with a thickness that would likely be 50 mm or more. We would also need adequate fracture toughness and strength. What manufacturers in Canada and the US would produce cast steel nodes for such a project? And how many are there outside North America?
At this stage, we would like to know if proposing this concept would mean being dependent on a single manufacturer. We are afraid that this may lead to delivery delays and/or premium cost. If you are not the right address for these questions, please let me know who I should contact to obtain this information.
Answer #3: Your concern about sole-sourcing is quite appropriate. There are a number of steel casters in North America that are capable of supplying such nodes and there are two issues to address: (i) the engineering (structural design under multiple load cases, product castability, specification of appropriate properties and finishes, etc.); and (ii) the manufacturing, integrity and delivery of the castings. I have copied this to Mr. Carlos de Oliveira, at Cast Connex Corporation ( www.castconnex.com ), who are the industry leaders in steel castings for construction in North America. Mr. de Oliveira has been required to deal with competitive bidding on some of his casting projects, hence he can suggest alternate suppliers.
I know that, on a recent custom casting job for a large atrium of a high-rise building in Toronto, Cast Connex was commissioned to perform the engineering design and then the actual manufacturing was bidded competitively.
Good to hear that you are planning on using steel tubes for the bridge – a very elegant solution.
These answers to these questions were prepared by Dr. Jeffrey Packer. While it is believed to be accurate, it has not been prepared for conventional use as an engineering or construction document and should not be used or relied upon for any specific application without competent professional examination and verification of its accuracy, suitability, and applicability by a licensed engineer, architect or other professional. Dr. Packer disclaims any liability arising from information provided by others, or from the unauthorized use of the information contained in this document.