Atrium Framing

[slickdeals]

I have a situation where I have a glass + polycarbonate roof supported by steel trusses.

The trusses span about 12m and are spaced at 3m o.c. Since I don't have any diaphragm at the level of the glass, I am planning to introduce horizontal X bracing to function as a diaphragm.

This atrium is at Level 4 of a 35 story building. There will be structure on all 4 sides rising above this glass roof. As such I won't have to worry about any wind loading. I plan to have a sliding connection on one end so that these trusses don't form part of any lateral resistance.

My questions are:
1. How much force should this X bracing be designed for? I think the primary role is to prevent compression flange movement. There will be infill members spanning 3m between trusses to support the glass.

2. Will any bridging be required in between the trusses? I won't have any net uplift but I like to brace the bottom chords.

3. Any other items to be considered other than deflections to be compatible with that of glass?

Thanks.

 

[ishvaaag]

Except that vents assure an extremely fast dissipation of any differential of pressure between the interior and the exterior (say, with significant perimetral ventilation) and even with vents transiently, your roof still will see upward forces from suction. Yesterday I read of one big hole in mining in Siberia upon which flying is forbidden because of the wind draft forces causes the planes be forced downwards; essentially, when the wind blows over a hole a depression is formed, and this depression draws the currents towards its center, taking the objects, the plane over the mining hole in the siberian example, and your roof in your building towards such center of the depression; a chimney stack effect in your case (even the pressure may be estimated by chimney's effect).

So I wouldn't discount uplift, suction and anchors at supports.

Respect bracing, I would lay it as you have in mind ... including it in your structural model. This way you will see the actual expected behaviour, and not the simplified one. Diagonals, in braced frames, and here in your triangulation scheme, do take loads from the main structural system, becoming part of it, and the description of the structure as designed as a space structure gives a more accurate view of what happens. You have as well the benefits of complete definition of your solicitations for the secondary members in this setup. If the braces are selected as not needing themselves bracing against LTB/FTB, say a pipe, or tubes in the flat, you will have everything in sight -bracing effects included- after analysis in direct design, not needing any application of any notional bracing force, since you, now, are getting autostability effects through which are main, analyzed, structurally bracing members.

 

[msquared48]

Slickdeals: In addition to the previous...

Not knowing the specific requirements for your region of the world, I would nevertheless definitely be concerned about any seismic forces due to the dead weight of the atrium. The X bracing needs to transfer these forces back, either to the building structure, or some other lateral resisting mechanism.

Mike McCann
MMC Engineering

 

[hokie66]

Is the structure all around the atrium all tied together or are there several structures? This would determine how much movement you have to design for. Are the cross members connected to the tower or just to the trusses? If to the tower, then one end of those members should have a guided slide bearing similar to the trusses.

The diagonal bracing doesn't have to take much force to maintain the alignment. Probably depends on the detailing and what looks right more than any force requirement.

Hard to say about wind on that roof, but I would tend to design the structure to resist as much applied uplift as gravity load.

 

[ishvaaag]

I would love to add that the upwards force in one such closed roof may include:

1. The inner pressure coming from any significant opening at the lower levels fron which such pressure can be built.

2. The difference of temperature (density effect) of the air inside and outside

3. Venturi's effect from the wind going atop at some velocity where at lower levels can be assumed to be zero or close to zero.

Plus flutter effects can have a say in some cases. This and effects 1 and 3 are usually cared for in suction wind by codes ... I mean, for this lower level roof I quite likely wouldn't use less than the suction if such roof as at the top level; this may be somewhat conservative in that it will be assuming a rigid transmission of the wind suction force down the void within the building. Respect component 2 you may evaluate the buoyancy in an archimedian evaluation, evaluating the buoyancy and spreading the value all over the roof surface. Component 2 will be almost despicable for nearly every case, even for very big atrium volumes, given the small density of air.

 

[csd72]

Slick,

There will be uplift on this roof an it could be quite significant as some of it will be the result of the wind suction at the 35 storey high roof. There is also the potential positive internal pressure in the building that would add to the uplift.

I disagree that this suction will dissipate by the time it gets to ground level. by what means does this happen?

Secondly, if the 4 buildings are tied together with existing cross beams and cannot move independently then I would not see the need to introduce movement joints in this atrium unless the size of either is sufficient to warrant this.