Very large sheds to design - design philosophy for stability?
Very large sheds to design - design philosophy for stability?
(OP)
Hello all,
I have four large warehouses to design - each 300m long x 90m wide x 15m high to eaves (5 deg roof slope). Construction will be standard lightweight steel clad frames. Large, uninterrupted internal spaces are required so I need to maximize internal column spacing. I am looking at different options for the roof framing, but will most likely be four-span frames (22.5m span) at say 7m centers along the length of the building. Transfer frames (truss or beams) supporting the main frames will span longitudinally between internal columns, giving a column spacing of 21m.
My question is regarding stability. Generally with this type of structure I would obviously portalise the frames, but I am thinking it may be more efficient to adopt a fully braced system, particularly given the height of the columns. The additional horizontal roof bracing would obviously be additional steel, but would it be offset by the saving in reduced perimeter column sizes and also the beam / column connections (pinned connections opposed to rigid)?
Obviously I can do some preliminary numbers for both options and do a tonnage / cost comparison, but does anyone here have any experience with structures of this size and can offer an opinion on the best approach?
Also, a number of areas will have large span cranes so horizontal deflection limits will need to be tighter than normal.
Thanks
I have four large warehouses to design - each 300m long x 90m wide x 15m high to eaves (5 deg roof slope). Construction will be standard lightweight steel clad frames. Large, uninterrupted internal spaces are required so I need to maximize internal column spacing. I am looking at different options for the roof framing, but will most likely be four-span frames (22.5m span) at say 7m centers along the length of the building. Transfer frames (truss or beams) supporting the main frames will span longitudinally between internal columns, giving a column spacing of 21m.
My question is regarding stability. Generally with this type of structure I would obviously portalise the frames, but I am thinking it may be more efficient to adopt a fully braced system, particularly given the height of the columns. The additional horizontal roof bracing would obviously be additional steel, but would it be offset by the saving in reduced perimeter column sizes and also the beam / column connections (pinned connections opposed to rigid)?
Obviously I can do some preliminary numbers for both options and do a tonnage / cost comparison, but does anyone here have any experience with structures of this size and can offer an opinion on the best approach?
Also, a number of areas will have large span cranes so horizontal deflection limits will need to be tighter than normal.
Thanks






RE: Very large sheds to design - design philosophy for stability?
Have you also considered using concrete perimetral frames and internal columns and steel trusses? I've seen this system being used quite efficiently in a couple of large industrial buildings.
RE: Very large sheds to design - design philosophy for stability?
Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin
RE: Very large sheds to design - design philosophy for stability?
As noted by avscorreia, to implement a fully braced system longitudinally would likely require intermediate frame locations that would serve as anchors for the bracing. Those will be very difficult to control deflection on as now instead of one bay of lateral load, they will be carrying 6-8 bays of load. That is unlikely to be doable with cross bracing somewhere to transfer those loads to the ground. Perhaps some sort of outriggers or buttresses outside of the building if the inside must remain clear for the crane aisles. Probably easier to bite the bullet and design each frame to take its own lateral forces. A small set of braces along each eave in the roof plane could be helpful in balancing out any lateral crane loads that are applied to the frames so that instead of one frame taking the entire lateral load of the crane system, a set of 3 or 5 frames could distribute the load out from the single load application point. Unfortunately, the same concept doesn't work for getting rid of wind loads that will likely be significant on a 15m high structure.
RE: Very large sheds to design - design philosophy for stability?
thanks
RE: Very large sheds to design - design philosophy for stability?
Secondly, it means you can keep deflection limits for the building reasonable and foundation conditions reasonable, and then design the crane as a separate system so you can control your spans and supports differently.
If the crane covers the entire square footage of the building that may not make sense, but it's nice to give separate structural systems a try if it's possible.
RE: Very large sheds to design - design philosophy for stability?
Dik
RE: Very large sheds to design - design philosophy for stability?
In the 300m direction I would brace the external walls and provide a band of bracing in the roof at these locations. Add enough bracing so that bracing forces in the roof and walls and resulting foundation loads are reasonable without heavily influencing the strength sizes of the portals in the other direction or needing ridiculous foundations.
Pay attention to the crane pointers by others, the deflections to find the frames will be governed by these depending on the size of the crane.
I have seen one building years ago that had large external props from the top of the columns to the ground at each frame, so Bracing doesn't have to be internally in the building is for it impacts on layout.
RE: Very large sheds to design - design philosophy for stability?
Dik
RE: Very large sheds to design - design philosophy for stability?
I've seen people add giant openings or decide they're going to rip the roof apart to crane something into industrial buildings. With bracing you know that a halfway competent tradesman will know it's important (doesn't mean I haven't seen bracing cut). With diaphragms, it's a lot less obvious. It's the same reason you may want to avoid using the floor slab to tie the foundations together in an industrial building. Someone can come along later to install an equipment foundation or a trench or something and cut critical structure because it's not obvious that it's important.
With a warehouse I expect the roof situation is probably okay, but you never know.
RE: Very large sheds to design - design philosophy for stability?
Dik
RE: Very large sheds to design - design philosophy for stability?
RE: Very large sheds to design - design philosophy for stability?
Regarding bracing, and some column lines with bracing on the interior of the bldg., etc., I think you have to have a serious talk with your client. Hell they all want them to be 300m by 90m, with no interior columns, so they can do anything anyplace, certainly no x-bracing every third column line, they just couldn’t work around that. This really requires some interior usage study and traffic flow study as part of the design. After all, they do want their bldg. to stay standing in a light wind, or EQ or snow loading. The starting point... We all have no problem designing a 20m by 20m shed to withstand reasonable lateral loads. But then, since the 20x20 works, most clients don’t understand that you can’t just stack these 20x20 bays side by side, 3 wide and 20 long, and eliminating all the end walls, all but two, when these end walls provide all the lateral resistance. Nor do they understand that there is just no way to make that 300m long roof diaphragm work. You have to explain that to them, and convince them that you can provide a workable design, with some fixed openings, and with some bracing at every third or forth column line. Additionally, in that large a facility, their insurance company or the local BO and bldg. code may require fire walls at some reasonable intervals in the length of the bldg. A way you might save them some construction time and money is to consider panelizing the roof system on that large a group of bldgs. You build (mass produce) roof panels, 4-5-6 stl. jsts. wide with decking installed on the ground, and lift these panelized roof sections into place, as you move down the bldg. The crane just backs down the length of the bldg. and the stl. jsts. and decking flow in from either side where smaller equipment helps with the panel assembly.
RE: Very large sheds to design - design philosophy for stability?
Dik