Expansion of single storey steel frame industrial building? 5

[IJR]

I have this bad habit of designing steel portals (one storey), clad them with sandwich panels on roof and walls and brace the other direction(or say brace the portals) with X-bracings at suitable locations where doors wont interrupt. You call this wind bracing.

Apparently I have been making a mistake by bracing the ends of a building.

Some strict authorities have warned me against that and they have forced me to put wall bracings at the middle of the building. Reason is,this one will make the unbraced length free to expand and contract, while my bad habit tends to restrain the building against that.

Makes sense. But I still love my old habit, and dont mind carrying out thermal expansion calculations on steel, (though I dont think I will be able to do that with cladding
!)

Any further discussion on this subject is highly appreciated.

Respects a l'infinity
IJR

 

[JAE]

I have seen the warning against X bracing both ends of a frame for exterior steel such as open crane runways. For buildings, if you brace at either end, but not both, you are not creating a pair of braces that will resist thermal movements, but you collector/drag strut will increase in axial force.

In buildings, where you are generally insulated against large thermal movements once the building is erected, placing bracing at one end shouldn't pose a huge problem.

For real long structures, perhaps a centered brace would be preferred just to keep the collector forces under control and limit total lateral deflection.

 

[idl]

I agree with JAE. For very long buildings, however, I would incorporate expansion joints and have an X-brace installed in each building section. The same goes for long crane runways. The length of each section is determined by the amount of the expected movement and the expansion joint type. No problem.

 

[phuduhudu]

I usually do try to put my braced bay somewhere near the middle of a series of portals to limit the lateral deflections at the ends but it's not always possible due to openings or other client requirements. For buildability it is best if the contractor puts up two portals which are the braced portals and because he normally likes to start at one end and work to the other a contractor might prefer a braced end bay. With a normal portal you have four planes to brace and it is best if these are all between the same two portals for the above reasons but this is again not always possible. I have offset braced planes from each other but then temporary stability requires consideration. Carl Bauer

 

[dik]

I usually try to get the X-bracing in a middle panel, but not always (It's nice to have 'square' corners though <G>) Also with steel structures, I also like to have proper expansion joints at approx. 300' max. With eccentric building loading (for seismic and otherwise), non-symmetric bracing can pick up additional forces due to torsion; this can also have an impact on connecting the deck diaphragm to the bracing elements. (Partially in gest something I keep in mind is that once it yields the first time, it generally behaves elastically).

I generally try to avoid bracing at two locations on the same wall surface without having a joint between.

Your approach to have the end panel braced and review the amount of thermal movement between erection and 'steady state' is valid. With winter erection, I've even had the X-bracing 're-done' once steady conditions have been reached (a matter of checking the the cladding and girts don't interfere). If the frame is within the insulated envelope, there should be little movement. This may be a concern if the building is 'mothballed'; although this is part of my standard drawing notes, I usually notify the owner accordingly (I use a standard MS Word template document to inform the owner/client of the stipulated design loadings).

Had one project, a 600' exterior conveyor gallery (temperatures ranging from -35C to +35C (with sun), sitting on concrete bin walls. This was braced with an HSS strut at one end. Our firm was responsible for the cantilevered concrete separating walls and base supports for the conveyor frame and another firm for the conveyor framing. I suggested (via memo) that they should re-think the conveyor bracing because the thermal movement would be several inches. The base plate assembly had to be designed to accommodate the rotation of the HSS conveyor supports and the other consultant was informed of the column tilting. The other consultant didn't want the frame revised, and the conveyor assembly had to be redone a couple of years later (really happy that I sent the bracing critique in written form). The 28' high cantilevered walls, designed for the additional horizontal loading stayed put.

In closing, interesting that the building 'dude' wants to pick up liability by insisting on how the design should be done!

 

[Arad75]

in gable frames, the most critical point in design is lateral bracing.dsigners almost design bracings only for the wind load,on the other hand in design of gable frame, the coeficient of effective length of columns is assumed equal to 1,this means bracing is so effective and stiffened which would provide lateral bracing for the columns.so in current LRFD code ( Not in ASD Edition 5 ),the stiffness and strength of lateral bracings are limmited to provide the excepted restrain against lateral buckling of columns.
due to this code ( for more information read Steel structure stability, Author Galambus ), u can see sometime neither thermal nor wind loads are critical to choose number of bays to brace, but also the buckling load of columns are.