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Lateral System for Open Structure?
2

Lateral System for Open Structure?

Lateral System for Open Structure?

(OP)
What kind of lateral system would you use for an open structure that is really just posts with a roof?  This is a structure to cover a patio?  It can't lean on anything.  

My first thought was knee braces, but the posts are steel clad in wood.  So my next thought goes to simple cantilevered columns.  My only hesitation is that the footings will end up being very large for a large uplift and overturning moment.

I know there's not an easy way to get a reliable moment connection between a steel post and a glulam beam, so are cantilevered columns my only other choice?

RE: Lateral System for Open Structure?

I have provided cantilevered columns or a combination of cantilever and frame action.

RE: Lateral System for Open Structure?

Depending on the size of your structure, cantilevered columns can work - the footing situation can be dealt with sometimes by using a layout of stiff underground grade beams that help provide the bending stiffness at the base of the columns.  

Sort of like creating a square/rectangular layout of grade beams to match the square/rectangular layout of roof edge beams - all can be moment connections and create a competent framework.

The grade beams take the moment so the footings don't have to.

RE: Lateral System for Open Structure?

Pole footings with embedded cantilevered columns.

Mike McCann
MMC Engineering
Motto:  KISS
Motivation:  Don't ask

RE: Lateral System for Open Structure?

Lion06...those are your two choices with variants.

I do a lot of open structures like this and usually opt for cantilevered columns because of the "look".  Yep...the footings get huge, particularly here where the wind blows.

Because the moment induced in the footings is usually high, you have to be concerned about variable contact with the soil.  Not such a big deal where the soil bearing capacity is high, but in areas where the bearing capacity is relatively low (below 2ksf)it can be an issue that will further increase your footing size.

JAE's solution is good, but the objection I usually get from contractors is higher excavation costs and higher labor for rebar placement (typical contractor whining).  Nevertheless, the owners often listen, so its back to big footings or drilled piers.

RE: Lateral System for Open Structure?

(OP)
Thanks, guys!  Good to know I wasn't way off base.

RE: Lateral System for Open Structure?

For cantilevered column seismic force resisting systems, don't forget ASCE 7 section 12.2.5.2 (2005)... the axial load on individual cantilever columns cannot exceed 15% of the axial design strength or axial allowable stess.  I wasn't always aware of this requirement.  It can make a huge difference in the required column size.  It seems seriously conservative, but I assume it is intended to guard against large p-delta effects.

RE: Lateral System for Open Structure?

(OP)
It's actually in a high wind region, so seismic isn't even close to controlling.

RE: Lateral System for Open Structure?

Lion06 - probably not a big deal but even if the wind "controls" you may still need to apply the 15% rule for the seismic load combinations depending on how high your SDC is.

 

RE: Lateral System for Open Structure?

(OP)
I realized my last post wasn't terribly clear after I hit "submit".  

A more clear statement would have been, "I'm in a high wind region with cantilevered columns.  The axial load on the columns is almost negligible."

It's less than 5% of the axial capacity.

RE: Lateral System for Open Structure?

JAE, please clarify... doesn't the 15% rule apply regardless of SDC and regardless of wind "controlling"?

In this case, 5% is less than 15%, so it is a non-issue (i.e. the 15% is satisfied).

RE: Lateral System for Open Structure?

I don't think it applies in SDC A.

 

RE: Lateral System for Open Structure?

Cantilevered columns is the standard way these things are done in australia, often using timber posts.

You can encase the columns in a bored pile or sonner tube to reduce excavation.

RE: Lateral System for Open Structure?

Providing moment capacity at the base is a good thing, but you should also be able to get some top frame action in at least one direction.  If using laminated beams as indicated, you could provide a cap plate on the steel column with through bolts to the beam.  Not completely rigid, but neither is the footing.

RE: Lateral System for Open Structure?

I've done it two ways -

  Cantilevered posts
  Moment frame

Both have their goods and bads

RE: Lateral System for Open Structure?

Quote:

My only hesitation is that the footings will end up being very large for a large uplift and overturning moment.

Especially if the geotechnical engineer does not give you any allowable adhesion on the sides of the footing.

RE: Lateral System for Open Structure?

I just recently designed an open octagonal shape ramada.  I used steel framing for the posts and roof members (HSS members) and I fixed all the columne bases with square footings and moment connection base plates.  I was debating whether to use circular drilled concrete piers but the square footings were cheaper.  

Good Luck

RE: Lateral System for Open Structure?

Lion06:
What size are the glulams, width, depth and length?  What size and shape are the columns?  What are the actual forces and moments?  Hokie kinda stole my thunder.  But, to extend his thought of moments at the top and bot. of the columns; you could dap shear plates into the top and bot. faces of the glulams, then the glulams fit/mate up under a long cap plate on top of the columns, and fit/mate on top of seat angles with long horiz. legs, and lag screws complete the connection.  You develop a moment equal to T & C, the shear plate cap'ys. times the glulam depth, much better than any bolted connection.  You could put shear plates in the sides of the glulams and mate them to side plates off the column, and through bolt, but you have a less effective lever arm on the bolt groups.

RE: Lateral System for Open Structure?

(OP)
Glulams are 3-1/2" x 16" spanning 25'.  The posts are HSS 4x4's.  There is 7 kips of uplift with an 8 k-ft moment.    

RE: Lateral System for Open Structure?

Use a cap pl. and a seat pl. 16" below the cap pl., welded to the column.  The pls. should be pre-drilled for timber rivets at your design spacing.  T & C will be the sum of the timber rivet cap'ys. and d = 16", for the moment cap'y.

RE: Lateral System for Open Structure?

By the way, having read a few of your other posts lately, you have my permission to take the rest of the evening off, unless you are getting time and a half for this work.

RE: Lateral System for Open Structure?

(OP)
Ha!  That's funny!  I put in 9 hours today.  I'll put in another probably 4 tomorrow.  That would be 13 UNpaid hours this weekend on top of the extra 8 I put in during the week.   

RE: Lateral System for Open Structure?

That's not funny, but it reminded me of my early days in a small design office.  Did get a nice bonus at year end, though, until one year...

RE: Lateral System for Open Structure?

Lion06...don't know all your conditions, but your uplift seems high relative to the moment.  Your structure is relatively heavy for an open structure.

I'm guessing your footings are in the 4' square range and will have to be fairly thick for OT moment resistance.

What wind area is this in?

RE: Lateral System for Open Structure?

(OP)
Ron-
It's a 120 mph wind area.  The OT moment isn't terribly high because there's only 1'-6" of actual wind surface for the wind to act on laterally.  So I essentially have 11' cantilevered columns with around 700# acting on the most highly loaded one.  Also, the lateral wind load is less than the uplift.

I actually have the footings at 6'-6" square x 1'-6" thick to get stability under control (with the 0.6DL + WL combination).  The bearing pressure is extremely low, even in the non-uplift condition.   

RE: Lateral System for Open Structure?

Looked back at your orig. post and since you're not transferring any beam moment...makes sense.

 

RE: Lateral System for Open Structure?

Using some frame action or knee braces would certainly help, no?

I think you could use some HSS4 knee braces that were also clad in wood.
Your HSS is already wider than the glulam which might look a little funny architecturally (man, did I just say that?)

You could add knee braces with HSS 4's or 4x6 timber then, cover the entire knee brace, column, beam connection with wood, so you'd essentially have a knee brace connection that has sheathing on it for a really rigid connection. It may also look nice architecturally (ouch, hate that word).

I have built a few pavilions like this in the past (builder style, no calcs, just build it for the homeowner).
The large timber posts were sitting on piers and the bottom of the post was slotted and a welded up "T" shape was bolted to the pier with the stem sticking up. The post was slotted to accept the "T" stem and bolted thru with a single large pin/bolt. The base was truly pinned and the stability was all from the knee braced joints. Again, this was cowboy style and maybe the calcs wouldnt have checked out, but they are still there after 10 yrs or so.
Mostly just throwing ideas out there...
I'll see if I can find some pictures.  

RE: Lateral System for Open Structure?

By the way Lion-
The more you work for free on the weekends, the deeper you are digging your hole. There is some saying I have heard guys use that goes something like "the more you do, the more you they'll give you to do". I always thought that was a lazy man's comment, but it is true in many ways. In the end, the result is really just a very bad personal life.
I love engineering, but my work is a far off distant second or third to family. There will always be another job somewhere. Other things aren't quite as easy to fix...
I'll shut up now.  

RE: Lateral System for Open Structure?

Lion06, how much dead load (unfactored) do you have on the columns?

RE: Lateral System for Open Structure?

TJ..I'll refrain from hijacking this thread to respond to your second post, but if it ever shows up in the "soft" forums...pack a lunch.

RE: Lateral System for Open Structure?

(OP)
Toad,

I hear you.  I'm at a bit of a loss for what to do.  I'm trying to learn new software while in the midst of a huge design project.  Then I get more stuff piled on.  

The software, even in the opinion of the developer is difficult, and no one in the office knows it.  I'm the guinea pig and am supposed to disseminate the information.

I feel like a heel saying, "sorry, this just isn't going to get done".  This goes back to a recent comment someone had about the different personality types in engineering.

RE: Lateral System for Open Structure?

(OP)
There's about 400# max DL on the most heavily loaded post.

RE: Lateral System for Open Structure?

Ron-
I didn't mean offend anyone.
Sorry for the hi-jack. The topic gets me going.
I don't know what "soft" forums are.
Hope I didn't break the rules here.  

RE: Lateral System for Open Structure?

I could be way off base here, but for a column with just 400# load, a footing that is 6'-6" in plan seems unreasonable. I don't doubt the numbers at all, but it seems like a moment frame or knee braced frame would be more economical.
I guess I look at it like a structure with really stiff legs that you're just trying to keep from blowing away.  

RE: Lateral System for Open Structure?

(OP)
Toad,
I think Ron is referring to the "How to Improve Myself to Get Ahead at Work" forum.  I do appreciate the input, though.

RE: Lateral System for Open Structure?

(OP)
It's not the 400# DL that's the problem, it's the 7 K uplift.

RE: Lateral System for Open Structure?

yea, dumb by me...sorry.

RE: Lateral System for Open Structure?

how do you approach the 0.6 Dead term in the combination as far as factoring the footing weight itself? Do you account for soil overtop the footing?
I know there is some debate over this.
Do you actually multiply the weight of the footing by 0.6?  

RE: Lateral System for Open Structure?

(OP)
I do multiply the footing self weight and the souls overburden by 0.6.  I also did consider the soil overburden in this case.   I typically ignore it unless I feel the footings are just getting too big by not including it, which is why I included it here.

RE: Lateral System for Open Structure?

Quote:

Ha!  That's funny!  I put in 9 hours today.  I'll put in another probably 4 tomorrow.  That would be 13 UNpaid hours this weekend on top of the extra 8 I put in during the week.

Quote:

I hear you.  I'm at a bit of a loss for what to do.  I'm trying to learn new software while in the midst of a huge design project.  Then I get more stuff piled on.  

The software, even in the opinion of the developer is difficult, and no one in the office knows it.  I'm the guinea pig and am supposed to disseminate the information.

I feel like a heel saying, "sorry, this just isn't going to get done".  This goes back to a recent comment someone had about the different personality types in engineering.

They must serve some potent kool-aid where you work.

RE: Lateral System for Open Structure?

(OP)
I pucker up every time I see it coming out.

RE: Lateral System for Open Structure?

Without reading the above posts (usually a mistake) -

I believe woodworks recently did a webinar on open air structures and most of the structures used embedded pole/cantilever columns (use embedded pole calc or treat it like a soldier pile and use broms method).  I can try to find the slides for it if you would like it.  I don't remember if being too informative as it was more for architects.

 

EIT

RE: Lateral System for Open Structure?

It does not seem your loads are high enough to warrant this, but I thought I would mention the handy Simpson Strong Tie steel moment frame solution.  I tried out their software package, and if the frames were a more readily available in our area, I would spec them a lot.

Brad

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