Wood & Steel mix construction issues? 2

[structurebeton]

I am designing a 3 story residential house, with a combination of steel columns, steel beams to support floors, with a combination of wood bearing walls and other wood wall components.

Is there any issues I should be concerned, such as when I will specify 40 ft tall single columns...which will need lateral bracing at each floor level...any suggestions on how to do that with wood elements?

Should shrinkage differential between the two materials be a concern?

Any comments appreciated. Thanks.

 

[SlideRuleEra]

Forty foot wood columns (and longer) can be fabricated from standard dimensional lumber as built-up columns with mechanical fasteners. See Page 204, of Chapter 3, in "Wood Structural Design Data", a free .pdf download at this American Wood Council link

http://www.awc.org/Standards/wsdd.html

It works, this is how many of the famous wooden roller coasters at amusement parks are constructed.

http://www.SlideRuleEra.net

 

[RARWOOD]

SlideRuleEra

It is not clear to me how you would construct a 40' long column out standard dimension lumber. Chapter 3 in "Wood Structural Design Data" only appears to address space columns and mechanically laminated columns. My understanding is the use of either space columns or mechanically laminated columns would require the use of full length pieces.

The method you are describing sounds like the 40' columns are made up by overlapping shorter lengths of lumber.

I have seen a discussions in an older design book that seem to imply that short pieces could be joined together with butt joints to create a longer column. This method seems to rely on the use of overlapping lengths on the outside of the compression area to assure the column acts as one unit. The book doesn't present enough information, so I am not clear on what they actually were describing.

 

[SlideRuleEra]

RARSWC - You are right, as many times as I have read that short article, I never picked up on the statement that all the laminations had to be the same lenght... a Slide Rule Error.

For longer mechanically laminated columns with spliced laminations, the Forest Products Lab has done a fair amount work. Here are a few references:
"Bending Properties of Reinforced and Unreinforced Spliced Nail-Laminated Posts"

http://www.fpl.fs.fed.us/documnts/fplrp/fplrp503.pdf

"Bending Properties of Four-Layer Nail-Laminated Posts"

http://www.fpl.fs.fed.us/documnts/fplrp/fplrp528.pdf

"Modeling Vertically Mechanically Laminated Lumber"

http://www.fpl.fs.fed.us/documnts/pdf1989/bohnh89a.pdf

An authority on this subject (and author of the above documents) is Dr. David Bohnhoff at the University of Wisconsin. His website shows other publications

http://bse.wisc.edu/profiles/faculty/bohnhoff.html

Although I have not seen it, the American Society of Agricultural and Biological Engineers offers a document titled "Design Requirements and Bending Properties for Mechanically Laminated Columns". From the on-line summary it seems to cover columns where there is a maximum of one end joint per lamination

http://asae.frymulti.com/abstract.asp?aid=14899&t=2

http://www.SlideRuleEra.net

 

[SlideRuleEra]

Forget to include this direct link to Dr. Bohnhoff's pubs, most are free .pdf downloads

http://bse.wisc.edu/bohnhoff/publications.htm

http://www.SlideRuleEra.net

 

[RARWOOD]

SlideRuleEra

I had forgot about the work done in the area of pole building. Good information!!!!

 

[MillR]

If your question was how do you brace a steel column with wood elements (rather than how to get a 40' wood column), a wood diaphragm can do this. Use a wood nailer on the column to help the detailing. You can calculate the required force and add enough blocking to distribute the load into the diaphragm.

 

[MiketheEngineer]

NOT only is shrinkage a problem - season to seasno dimensional changes will haunt you -- cracked drywall, ceiling-wall separation - moulding pulling away, etc.

The steel will not shrink or grow at all - unless you have wide temperature variations.

New wood will definitely shrink the first year or two and then going from winter to summer and back - you will see movement - esp in something 40' tall.

Can you "stack" wood columns and use your floor diaphragms
to provide lateral support??

 

[ckar]

MillR how do you put a nailer on a 3 1/2" wood pipe?

 

[massengineer]

In the book "understanding wood" by a UMASS professor (I forget his name) he comments on houses where the chimney has been used as a point to connect some of the upper framing or roofing. he calculates how just the shrinkage in width (across the grain) of sills and horizontal pieces causes problems even before allowing for a little lengthwise change in length of wood (vertical studs). This analysis would be equally valid for a steel vertical member, would it not?

 

[canstruct]

I am designing a 3 storied wooden house. Walls from basement to level at second floor as more or less ok, but
walls from second floor to roof are not on the wall below. According to the Canadian COde, partitions can be offset
600mm from the walls below. If almost all the walls except
only a stretches of 4 and 5 ft in either direction go continuous because these are the stair well and architect have no choice. Please tell me how can i support the walls from second floor to roof and how is the lateral system for wind works if the walls are not on each other.

 

[msquared48]

ckar:

"3 1/2" wood pipe"? I assume you mean a steel pipe column. If I need a nailer, I use a tube section with Nelson Sill plate anchors. Works well for me.

I do not like mixing steel with wood in residences, but with higher end homes, it is unavoidable. Stay away from dimension lumber wherever possible and go with TJI's, glulams, parallams, micropams and the like. Most (90%) of the shrinkage occurs across the depth of dimension limber, as with joists and plates. The use of TJI's for the joists will minimize that.

Mike McCann
McCann Engineering

 

[AlmostPE]

If you have a long stl. column, you only design it as if it were only 1 story high (assuming each floor is same height). Usually you put the splice in the joist space. Then I would put 2x blkg around the column underneath the plywood and nail it to the plywood to brace the beam laterally.