Wind Braces and a Question

I have never used cables as permanent cross bracing in a structure. They are adequate and easy to install as temporary braces, but solid shapes are much better as permanent braces. A pair of steel angles passing back to back are very good. You can bolt them together at the crossing point to prevent them from slapping each other in a stiff wind. The end connections are very simple also.

For larger wind forces, symmetrical sections such as HSS or WF are very good and can be connected with standard connections.

Steel rods can also be used and are the choice of most pre-engineered building manufacturers because they are efficient. There are special hardware devices to connect them at the ends. If they are not connected at the crossing point, they will tend to "slap" each other in fluctuating wind conditions but, structurally, they do the job.

Steel cables are not as good as solid shapes for permanent bracing because they stretch too much under load. After stretching, they retain their strain, so tend to be loose for subsequent wind loads.

Frame action of the primary members is the best way to deal with seismic effects, but carrying lateral loads by flexure requires much larger sections and is not as cost effective as developing the forces by axial stress (i.e. cross bracing). In some parts of the world, however, building codes require that a stipulated percentage of the lateral forces be developed by frame action because it makes the building more ductile when exposed to seismic forces.

solid bars are not always preferable to cables. cables have the following advantages:

- they can have extremely high working stresses (due to the deep drawing processes that work harden them during manufacture) and so can be much lighter in appearance compared to standard solid sections (although there are some high performance steels and stainless steels that have very high yield strengths)

- they can be prestressed (care should be taken during the prestressing procedure) and so create some elegant solutions where the superimposed tension in the system enables what would normally be considered a tension element to accept forces that would normally give rise to tension in the cable.

- installation can be easier due to access for bendable cables getting around tight corners prior to installation.

however, the steel sections do have the very serious advantages of cost, simplicity and that the industry knows about them.

given the size of the sections that you are putting together and their relation to one another, unless you have some aesthetic reason to choose cables, the simplest solution is the back to back angles tied together in the middle to stop slapping due to load reversal i.e. wind blows and then doesn't - ties are in tension and then aren't. they are cheap easy to install and everyone knows how to do a reasonable job on them.

i would say that not all cables stretch under load permanently. for permanent bracing you should be choosing parallel strand cable and not helically wound cable. the helically wound cable has an effective modulus of elasticity that is less than the parrallel strand due to the twist creating a longer length of stretch for the same overall length of cable (it is the helix geometry that is responsible for this).

i have not come across cables that have shown creep over time; at least not if correctly installed. the cables may become loose due to slip in the main structure connections occurring after final installation of the cables. only a small movement in the connection can take away all of the stress in the cable.

with respect to seismic loads, you can also think of using flexible foundations to reduce the effects of earthquakes. it really does depend on what type of earthquake we are designing for, the structure we have and the ground conditions.

Cable braces are used in the context of tall radio antennae masts and used to be used for railway signal posts. There must be standards or codes-of-practice for these things locally?

Consider the cross-sectional area of the structure as presented to the wind. Given the maximum estimated windspeed and the rigidity of the structure, estimate the maximum thrust upon the structure. Given the thrust to be contained, estimate the tension in a cable brace or a number of cable braces. Determine whether the cross-sectional area of a single cable brace, or a number of them, is adequate. Then make a strategic decision.

I'll agree with everything said above, and also add the fact that under certain circumstances cables with sufficiently rated turnbuckles can also do the job quite well. They make it simple to achieve the proper tension, and also to take up any slack, should they loosen for whatever reason. But without knowing what you're constructing, it's difficult to determine if this would be appropriate.

In these steel framed buildings, I have seen both cables and metal rods used. Rods are actually easier, because they usually consist of two rods threaded (and reverse threaded) and connected in the center, such that you can adjust the threaded connector to increase the tension when needed. Both systems are typically for tension only, neither is designed to take any compression load. The connection is critical, for both the rods and cables, since they operate under tension. I understand how thew cables are connected. How would you connect the Rods?