Base Plate Design 1

[medeek]

I have a customer looking to install steel HSS posts rather than the typical PT wood posts (ie. 6x6). Not sure why, I think steel in this setting is ugly but I suppose you could wrap it and never have to look at it again.

I'm not real familiar with steel posts and baseplate design so I am looking for some resources or guidance on the subject.

My primary questions are:

1.) Baseplate size relative to post (square) size.
2.) Baseplate thickness
3.) Anchor Bolt size (assuming 4 bolts, one at each corner). My gut feeling is (4) 1/2" bolts is going to more than adequate for any lateral or uplift requirements.
4.) Bolt edge spacing: Is there some rule of thumb or AISC rule that governs this?

As for the actual sizing of the square post I have some good example problems to refer to and eventually build a spreadsheet calculator.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[kingnero]

some interesting reading material:

http://www.towernx.com/downloads/Technical_Manual_MP_BasePL.pdf

http://www.fusionpoint.be

http://be.linkedin.com/in/fusionpoint

 

[bridgebuster]

Take a look at the attached AISC design examples; not much to designing a post or a base plate.

 

[KootK]

1) in the absence of constraints, base plate larger than post by 3" on all sides.

2) 3/4" plate.

3) 3/4" bolts if cast in. 1/2" if post installed.

4) 1 1/2" from centre of bolts[pre][/pre] to all edges.

Those are the rule of thumb minimums that I see. Detailed that way, the connection will look normal and nobody will whine about excess. Within the normal range of variation, there isn't much cost associated with this stuff anyhow. Obviously, your calc'd values trump where they indicate a demand for more.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[XR250]

2) 3/4" plate. Detailed that way, the connection will look normal and nobody will whine about excess.

Sheseh, maybe in Canada :> Seems excessive unless there is a huge uplift or moment - which is unlikely in a residential setting where a 6x6 would normally be used.
The fabricator would much rather use 1/4" plate as he can easily punch the holes, if desired, and make an easier weld to a 3/16" column.
In my area, they will whine about anything and things being "over-engineered" so I must be mindful about these things or they will move on to the next engineer.

 

[KootK]

Well, there's a regional aspect to most things. It was also similar to my recommendation in the northern US states where I've practiced. I do go 1/2" base plates for non-primary stuff from time to time.

Hearing about your regional norms makes me grateful for mine. I don't bother with HSS wall thicknesses less than 1/4". The only "real column", 1/4" base plates that I've seen were in a pre-engineered metal building (forensic). All around the perimeters, the base plates were curled upwards, out of contact with the grout somehow. This prompted a little FEM to find an explanation. I believe that baseplates don't distribute load nearly so well as we assume until some serious local concrete crushing takes place. To keep things elastic in service, you really just want to count on a few plate thicknesses either side of the plates imparting the load.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[jayrod12]

KootK's tips for baseplates are identical to how I design mine. I will, on lightly loaded columns, consider 5/8" but never thinner.

The steel guys around here are really pushing for typical baseplates, i.e. same dimensions and hole locations for all if possible. There was a mall built here recently that every column had the identical baseplate, it wasn't our job but apparently made fabrication and installation a breeze.

 

[medeek]

Thank-you for all of your responses. I should have checked the AISC design examples more thoroughly, sure enough J.7 is a baseplate design example. The only difference between that example and my case is the post/column is a wide flange. The plate thickness calcs appear to use equations specific to Wide flange profile and not a HSS profile. I will need to dig into this further.

In the example they also used a 3" distance for the bolts on each side or a 1.5" edge distance.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[jimstructures]

Speaking from a PEMB background 3/8" thick baseplates for lightly loaded columns is common.

Jim

 

[bridgebuster]

Example K12 is shows an HSS base plate design

 

[XR250]

Well, there's a regional aspect to most things. It was also similar to my recommendation in the northern US states where I've practiced. I do go 1/2" base plates for non-primary stuff from time to time.

Hearing about your regional norms makes me grateful for mine. I don't bother with HSS wall thicknesses less than 1/4". The only "real column", 1/4" base plates that I've seen were in a pre-engineered metal building (forensic). All around the perimeters, the base plates were curled upwards, out of contact with the grout somehow. This prompted a little FEM to find an explanation. I believe that baseplates don't distribute load nearly so well as we assume until some serious local concrete crushing takes place. To keep things elastic in service, you really just want to count on a few plate thicknesses either side of the plates imparting the load.

For a situation that the OP is referring to, the 1/4" seems to work fine as well as 3/16" wall columns. A lot of these columns get set by hand and I am happy to make them lighter if possible. I do go with thicker base plates when there is wood bearing below as I want to keep the load distributed well. Lets face it, for most residential loads in which a 6x6 would be used, it likely does not matter.
I did a job residential job recently where I had 8 columns with 1" base plates as they were flag-poled. The fabricator ended up ordering the base plates online pre-drilled. Said it saves him a bunch of money.

 

[racookpe1978]

If the "usual post" is only a 6x6 pressure treated wood post, then 3/4 thick plate is way excessive.
1/4 plate is adequate for the baseplate thickness for a lightly-loaded wood column placed onto a concrete foundation, concrete slab, (or embedded concrete post in the ground), UNLESS the baseplate is on the ground itself. Most wooden guardrail-type posts are smaller than 6x6 (extending up around porches or decks, for example) , but handrails/guardrails baseplates are 1/4 thick, extending 2 inch outside the vertical.

 

[KootK]

For a situation that the OP is referring to, the 1/4" seems to work fine as well as 3/16" wall columns.

I don't doubt that it does. Everything under the sun has some capacity and, therefore, some suitable application. What I do doubt, under certain contractual models, is whether or not the effort expended in refining the design is justified. Cui bono? The design of a thin base plate is no herculean effort but, for me, involves:

1) 10 minutes to run/re-run a spreadsheet.
2) 10 minutes to add a row to my column design schedule.
3) 5 minutes on site to remind myself what those thin base plates where supposed to be.

Depending on who does the work, that's maybe $50 worth of billable time. Hopefully, it's being expended on behalf of half a dozen columns in stead of just one. So maybe a unit cost of $10/column. $50 is not a huge some but, then, it's my money. And what I would prefer here is a column base design that I really don't bother to design at all. Seriously. So, if I'm to expend the effort, I want someone who would fall under the heading of "client" for me to experience a tangible benefit. Let's say, for the sake of argument, that this is the difference between a 1/2" 12 x 12 plate and a 1/4" one. So that's what, 10 lbs of steel and $25 per base plate on a column? Ish.

1) Is a $15 net savings per column worth pursuing if you're the PEMB guy and it's all your $$$. Yup. Just business economics.

2) Is a $15 net savings worth pursuing if your client is the contractor AND the savings will actually get passed along from his good buddy the steel fabricator to him? Probably.

3) Is a $15 net savings worth pursuing if your client is the owner/architect and any steel fabrication savings will likely just go to the steel fabricator and maybe the contractor without ever actually being passed along to my client, the owner? Nope. In this case, I'm just throwing away money and increasing potential liability a bit. I guess I'm reducing waste which has environmental benefits but I'd be lying through my teeth if I pretended that had any real impact on my decision making process.

For most of my projects, I'm working under #3. When fabricators estimate the cost of the post for my projects, they price the connections as just a generic % add without even really looking at what I've done unless it's highly atypical in some way. Yeah, over time, fabricators might give me a better deal on that percentage if I've got a reputation for doing things lighter than everybody else. But that's a long slog and of no real benefit to today's project.

Perhaps the economics are different if this kind of steel is your "usual" steel and skinny base plates are the norm rather than the exception.

Lets face it, for most residential loads in which a 6x6 would be used, it likely does not matter.

Agreed. Unless lateral loads are involved, I'd say that a 6x6 HSS is likely to have grossly more capacity than generally required for most residential applications. That's why I'll usually try to lighten up my columns by using a smaller size at 1/4" thickness. Something in the 3"-5" range.

For steel used outside the building envelope -- particularly steel that will be hidden from view and/or in a coastal environment -- I like to have a bit of sacrificial wall thickness available in case my post sees some unanticipated corrosion over the course of it's lifetime. The Canadian steel code limits steel used outdoors to 3/16" for just this reason. So that's one entity's opinion of where that line ought to be.




I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[XR250]

I don't doubt that it does. Everything under the sun has some capacity and, therefore, some suitable application. What I do doubt, under certain contractual models, is whether or not the effort expended in refining the design is justified. Cui bono? The design of a thin base plate is no herculean effort but, for me, involves:

1) 10 minutes to run/re-run a spreadsheet.
2) 10 minutes to add a row to my column design schedule.
3) 5 minutes on site to remind myself what those thin base plates where supposed to be.

I guess if you are actually gonna check it - then yea, that makes sense. I don't even bother for most residential loads. Reminds me about the recent thread regarding highly loaded metal studs bearing on a bottom track.

I do agree with your sacrificial metal thickness thought.

 

[KootK]

If I'm going to play in the residential sandbox, then perhaps I need to expend the effort required to get myself set up with a few "no design" 1/4" base plates. Maybe I'll go 1/8" and see if I can scoop some market share.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[txeng91]

Obviously the base plate thickness seems to be up for debate. I'd probably go 1/2" min for this application. I will say sometimes on smaller things like this, embedded plates are sometimes preferred for aesthetic and constructability purposes. In that case you should go 5/8" min to prevent the field welding of the column from heating up and cracking the surrounding concrete.

 

[medeek]

It does seem that the baseplate thickness is all over the place.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[KootK]

Since you're coming at this anew in your work, I think that the rational course of action would be to develop a tool that would allow you to quickly evaluate base plates of the 1/4" variety and then work that into your standards. That seems like the best way to maximize your competitiveness. I have such a tool but it's per Canadian codes.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.