Lateral System for Open Structure? 2

[Lion06]

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?

 

[rscassar]

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

 

[JAE]

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.

 

[msquared48]

Pole footings with embedded cantilevered columns.

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

 

[Ron]

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.

 

[Lion06]

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

 

[gte447f]

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.

 

[Lion06]

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

 

[JAE]

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.

 

[Lion06]

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.

 

[gte447f]

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).

 

[JAE]

I don't think it applies in SDC A.

 

[csd72]

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.

 

[hokie66]

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.

 

[MiketheEngineer]

I've done it two ways -

Cantilevered posts
Moment frame

Both have their goods and bads

 

[abusementpark]

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.

 

[minorchord2000]

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

 

[dhengr]

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.

 

[Lion06]

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.

 

[dhengr]

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.