X-BRACE DESIGN 1

[lmcan]

I was into steel tower designing and currently working on a design of warehouse steel building.

I have a brace frame(8mx 6m high) for resisting lateral wind(100kn) with X-bracing.

Brace forces are: 1) 125kn (tension only or compression only brace)

2) 51.6kn tension, 73.4kn compression ( compression-tension brace)

My questions are:

1. Do I have to design my brace for 125kn tension ONLY and ignore compression in brace , and if so what is the reason for this?
2. Or do I have to take the maximum of tension or compression ( if I assume compression-tension bracing design) and design the brace accordingly( brace forces are less in this case)?
3. What are the situations when each of the bracing system is considered?

In towers I used to design bracing for compression only as they were always higher than tension. Why building braces are designed for tension only?

Can anyone guide me in the right direction?

 

[JAE]

Some X brace frames are designed with fairly slender brace elements that effectively have very little compressive strength (such as a rod X-brace or a single angle). These are "modeled" in the analysis as being tension only members. In other words, you assume that any lateral deflection of the frame will attempt to induce compression into one of the braces and tension into the other. But the compressive brace immediately sheds this compression by buckling out of plane to avoid the force.

Thus, the other brace then has to take 100% of the lateral force in tension. You as designer simply ignore the compressive brace - you must always, however check that the compressive brace won't buckle to an extent where it reaches an inelastic stress and deforms or yields permanently.

 

[rook1]

Hello JAE

I am wondering how does the endwall windload get transferred to the x-brace. The x-brace is located at a bay that is not adjacent to the endwall but at the next bay.

Thank you very much!

 

[FalsePrecision]

Rook1,
Prefab steel building designers have 2 methods to provide an endwall wind resistance system:

1. The endwall columns are fixed-base ("flagpole"), with a large rectangular, moment-resisting baseplate. There may be 4, 6, etc. endwall columns to share the load. Each recieves virtually no gravity loads.

2. There is diagonal rod/cable bracing in the plane of the roof (just below the roof purlins) which form a rigid (braced) roof diaphragm effect to carry the endwall wind shear to the adjacent, interior braced frame, as you describe.

 

[unccpe]

Some metal building manuf. use the end wall post/columns to transfer the wind loads to purlins which in turn carry the load to the bay of x-bracing. Usually the bay that has the x-bracing also has x-bracing in the roof, that 'breaks' at the purlings that are carrying the wind load. The purlings transfer the load into the roof-rods, and down the roof to the eave/purlin. The eave purlin is a collector that transfers the load to the sidewall bracing. This way is more economical because you don't have to design moment footings/base plates, as with the "flag pole" design.

 

[FalsePrecision]

unncpe,
Are you talking about wind load normal to the short direction of building (against the endwall)? I was referring to wind load normal to the long direction (against the sidewalls). I am not a pre-fab bldg engr, so I appreciate any feedback/corrections.

 

[unccpe]

I know this is off the original subject, but...........

I used to work for a pre-engineered bldg manuf, about +6 years ago. There are many fabricators and they all do things a little different, but for the most part.....

The sidewall wind (parallel to the frames) is taken by the frames, thru moment connections and large tapered columns and rafters, tributary to there location.

The endwall wind (usually the wall that has the gable or slope) is where things differ a little. Some manuf use the flag-pole method and some use those post as simple pinned wind columns that transfer the wind reactions to the purlins at the roof and the foundation (as shear). Neither way is wrong or 'better' than the other. They each have their values and faults.

(MY OPINION!!!!)
For a pre-engineered building, it is usually more economical to avoid fixed base conditions when possible. However, it isn't always possible. (Drifts, usually determines that)

The reason isn't obvious though, because if you design the frames as fixed bases, the fabricator's price is a lot less because it doesn't take as much steel to resist the loads. However, the time and cost of the foundations (almost always) is greater than the difference of the savings in steel.

Anyone that has design a foundation for a metal building PROBABLY would agree that a big part of the cost of the building is because of the foundation. They are so light (dead load), and have hugh shear, and uplift reactions due to lateral loads (usually wind). When you fix the base those reacitons create more headaches to resolve.