Industrial Facilities Roof System 3

[aapl2k7]

I've noticed that the roof system used in industrial facilities (by industrial facilities i mean one story, large span frames), usually have braces at the roof level; i read on the book "Structural Concepts and Systems for Architects and Engineers" by T.Y. Lin & S.D. Stotesbury that these braces are placed to provide longitudinal rigidity (i supose that it acts as a diaphragm at the roof level), the thing is that in my country (Nicaragua), usually these braces are bars, and also i've seen several diferent "patterns" for these braces, for example some are placed sort of like a chess board, others are placed in every bay and others only at end walls and the middle bay.

In my opinion, these braces should be L shaped sections (to resist tension and compression forces), and i believe that the placing of these braces are in function on how rigid the structure should be or how much displacement i want the structure to be limited to, i would like to read other engineers opinions about it to use the correct approach when designing these type of structures. Thanks.

 

[JAE]

I think that most of the horizontal "diaphragm" bracing that occurs in roof systems are tension-only braces. In fact, I cannot recall ever seeing a tension-compression brace system in a roof.

Even if you went with angles, the compression axial load capacity would still be quite limited.

 

[hokie66]

I think it is important that you understand the difference between braces and a diaphragm. True, bracing in the roof plane usually has the same purpose as a diaphragm, but the way it works is different. A diaphragm usually consists of steel or plywood decking material which spans between the secondary roof framing members to carry gravity and uplift loads, and also distributes lateral loads from the loaded building faces to orthogonal vertical bracing elements. A diaphragm should support the roof membrame, not serve this function as well.

Bracing does not carry vertical loads other than self weight. It is there strictly for the purpose of distributing the horizontal forces. It is designed as a horizontal truss system, and can consist of both tension only members such as your rods and also members which are capable of taking compression forces. Compression members can be circular or rectangular hollow sections or rolled wide flange sections. Angles are not very efficient compression members in a roof system, but are often used for tension members.

It is difficult to discuss a general design approach for bracing in a forum such as this, other than to say that the bracing system should be designed as a truss, and you just need to follow the load path right through your structure to make sure it gets to the ground.

Feel free to follow up with any specific questions you may have.

 

[csd72]

aapl2k7,
You are correct in that the bracing provides stability in one direction whereas the portal frames provide the stability in the perpendicular direction.

Rods are a typical way of providing the diagonal braces for these, largely because these are much cheaper than any other option. When angles are used they are generally used as tension only members also but they have the benefit of higher capacity and greater stiffness.

Hollow sections are much more expensive than either of these options due to the large amount of detailing work required for connections. I would avoid using these for diagonals and only use them for the required compression struts.

 

[aapl2k7]

Thanks for your answers.

I agree with you all that even with angles, the compression capacity of the brace is very limited, but for some reason i rather use them instead of rods, unless the span is too large, probably because rods can be tensioned in a more appropiate way, but if the braces are used as tension only elements, why not use cables?.

hokie66, when i said that the roof bracing acts as a diaphragm, i meant what you explained, that it distributes horizontal forces to the vertical members, i should have expressed myself better, you also said that it should be designed as a truss, this means it is used to distribute horizontal forces in both directions(in plane and out of plane)?, csd72 says it provides bracing for one direction while the frames provides stability for the in plane forces, am i right csd72?.

Finally i want to ask the three of you about the "patterns", i've uploaded a pdf file to explain myself.

 

[hokie66]

It is possible to use cables, but you need to be careful with elongations. Whichever type bracing you use, elongation of the individual element (PL/AE) will cause lateral deflection of the structure, which needs to be checked against your control criteria.

csd72 was referring to a building which is framed with portal frames in one direction (normally the short direction) and braced in the other. But you can have a braced building in both directions.

Now to your patterns:

Each configuration will work in the longitudinal direction. The member forces will be greater in the scheme with only three bays braced. The three bay system, or probably only the end bays actually, would be typical of longitudinal bracing in a portal frame building. If your loads are shared between the braced bays, the central strut needs to take whatever force is shared.

Only the "chess board pattern" works to distribute the long wall forces to bracing in the end walls. This is because the other schemes do not have continuity of forces in the roof truss.