Small Free Standing Structure 1

[Wbruseski]

I am designing a gazebo structure for the wind loads of Florida. The gazebo will generally be 8-sided with screen as an option. I don't have a problem getting the wind loads, but have some questions on the design.

What are the accepted assumptions for a free standing strucure? For an open structure, is the windward pressure applied to the area of the posts and the gabled roof?
I obviously need fixed bases to handle moment. Is the only way to imbed the posts in concrete? Is core-drilling existing slabs then refilling acceptable? Any type of screwed or anchored connection considered fixed for concrete or wood deck?

Also, I came across a set of plans that has this statement:
"ROOF SYSTEM
RIDGED TRIANGULAR MODULES CONNECT TO FORM A CONIC ROOF SYSTEM. THE COMPRESSION RING IS INTEGRAL TO THE CONIC DESIGN AND ALLOWS RESULTANT FORCES TO BE PERPENDICULAR TO GRADE."
What is a "compression ring" and how does it allow the resultant forces to be perpendicular to grade?

Any insight to this design would be appreciated. Thanks.

 

[ishvaaag]

What are the accepted assumptions for a free standing structure? For an open structure, is the windward pressure applied to the area of the posts and the gabled roof?

In Spain you would have to effectively consider the forces in the planes, as correspond to open and closed structure.

In any case the on projection forces should also be checked here, just in case.


I obviously need fixed bases to handle moment.

It seems you need at least pinned connections. You may want fixed.

Is the only way to imbed the posts in concrete? Is core-drilling existing slabs then refilling acceptable? Any type of screwed or anchored connection considered fixed for concrete or wood deck?

Normally till 4 bolts most times it would be better not consider fixity; in any case many and I myself I have assumed fixity with 4 bolts. Other thing is that the behaviour under wind is higly dynamic and depending upon the deatil becomes quite theoretical many times...and even worse things may happen at baseplate, typically the bolts breaking, a mix of corrosion, fatigue and dynamic load.

What is a "compression ring" and how does it allow the resultant forces to be perpendicular to grade?

You seem to describe one trunk of pyramid with members at edges. The compression ring I only see atop the trunk, for the inclined edge members would push outwards (for gravity) and then it will be more a tension ring (the members at the base will be in tension). As long this ring is in tension, the inclined members outwards push is cancelled and for gravity loads you only have vertical reactions.

Then simply putting hinges at corners at base will show inclined reactions under wind.

 

[JAE]

For an open structure, the codes provide increased factors for the wind forces applied to the building. Your roof would take the code specified loads as indicated, which would primarily be upward in nature (perpendicular to the surfaces of the roof).

The vertical posts would also take some wind load and this is a little more problematic. It would be incorrect to use windward wall forces on a single vertical post. The best estimate would be to use the magnified wind forces (such as those for rectangular truss elements).

Your final result would be a very large upward force, with a lesser amount of lateral.

Now the above is for a truly "open" structure. With screens across all the openings, you would get a considerable drag on the building. At some point, the screens would be blow away (somewhere below the code design wind load). This is where your engineering judgment steps in. I would design first for an enclosed building using a wind speed limit equal to the point where the wind would blow the screens off. Then, another design case would be the code wind speed without the screens.

As far as the posts are concerned, if you are depending on them for bending, then usually some form of embedment is required and the concrete base should be also designed to take the applied moment from the posts. To just drop a post into a 5" slab won't work.

Another thought....what about knee braces at the tops? This is how the traditional "Americana" city park gazebos were built years ago. The knees were ornamented to fit into the aesthetic desaign of the gazebo.

 

[Wbruseski]

Thanks for the replies.

Ishvaaag, you say I only need pinned connections. Then am I correct to say that I would need to design the roof-wall connections as moment connections? I really prefer to use a base with anchor bolt connections to the concrete and the post thru-bolted to the base. I don't feel comfortable with a moment connection assumed there.

JAE, the knee braces would help with the moment connection at the roof-wall correct? Any side-effects of using the knee braces at the corners? The roof has a 6:12 slope. Does that affect the knee brace design?

 

[JAE]

As long as your horizontal eave beam can take the vertical reaction, the slope of the roof shouldn't affect the knee braces too much. Now as the beam deflects, there will be some axial load placed on the knees and with an octagonal layout, the knees would tend to push the column outward somewhat.

I would simply model the entire framework in a 3D analysis. If you don't have a 3D program, you could still work with a 2D program and manually work through the loads on the columns.

I guess the above tells you that the knee braces primarily affect the columns by adding bending to them. The knees essentially make the 90 degree intersection between column and beam rigid, much like a moment connection. When the lateral forces are applied, the knees keep the connection at right angles and force the columns to resist the lateral in bending.

 

[ishvaaag]

The advice of making a 3D analysis is best.

Respect absolute need of the joints between walls and roof be rigid, there's not. Rigidity of course wil add to safety, and the analysis will indicate if we need to modify something, like put corner braces. The octagonal trunk of pyramid can be stiff enough to sustain without problematic distortion the general deformation under wind, in which case yo can bolt walls to roof. If not, X cable bracing the facets at the octogonal trunk could help, but the trapecial facet with small lenght of member at oculus provides almost the same. Of somewhat more effect could be cables in the base of the octagon for triangulation, yet at 1/12 slope scarce thing we will be adding if the oculus or lantern hole is small.

So one analysis may indicate you best how to preceed.

 

[Wbruseski]

Thanks again for the replies.

I ran the gazebo on a 3D structural analysis program.

Here's what I found:

The vertical loads are all OK.

For the horizontal wind load, the deflection at the top of the column exceeds L/60 (for screen enclosures) using pinned base connections. The deflections are OK with fixed bases. The client is against knee braces.

Is there any other way to stiffen up the gazebo without going to columns buried in concrete. The client wants this to be versatile to concrete or wood decks. The columns are 2" x 4" hollow aluminum self-mating beams.

Any suggestions are appreciated. Thanks

 

[ishvaaag]

X or V cable or strut bracing in the vertical walls will add somewhat. If the plygon atop shows disgusting distortion, adding star shaped or other bracing at the plane atop may help as well.

 

[JAE]

Most of your deflection is probably due to bending in the columns. And your column deflection is very sensitive to the base fixity (as you determined in your model). To fix the base of the column, you must, somehow, transfer the column bending moment to the supporting base.

If you client wants to be able to just set his gazebo on basically any kind of base, then you really cannot count on developing fixity at the bottom of the columns since you will not know, in advance, what the base is, what the base can take (in terms of moment) and what the base is even made out of. Therefore, you cannot design a moment connection.

With that, you simply must either increase the size of your columns to limit deflection, and/or establish a moment connection at the top of the columns. The knee brace will create that connection AND reduce the length of your column to limit deflections.

I would also think that gazebos, with open sides and no brittle material, would be capable of a greater lateral deflection than most buildings.

 

[haynewp]

Did you take into account the screen you mentioned earlier in your 3D model? If wind is blowing into the screen, it can't blow off. Only on the suction side.