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Bracings. Typologies. FEM

August , 29th 2018 | Author: (@Prontubeam_en) Read: 2600 times

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This article, as its name suggests, talks about the different bracing types that exist. Its aim is to compare the effectiveness of each one of them using finite element models.

Source: http://aceroarenas.blogspot.com.es/2012/02/contraventeos-cruz-de-san-andres.html

 

Let's start by giving a simple definition:

A bracing is the structural element used to brace, that is, to stiffen or stabilize the structure preventing or partially limiting the displacements / deformations thereof.

There are several types of bracing: X (San Andrés cross), K, inverted V and different forms of these previously stated.

To verify its effectiveness we will resort to brace the following structure in the last frame and to compare / comment the possible advantages / differences of each one as well as the variations in the displacements.

The structure is loaded with 10kN / m simulating a lateral load, it has HEB200 columns, IPE160 longitudinal beams and all types of bracing will be modeled as HEB100. It is a purely academic example to compare the effectiveness of the bracing, it does not pretend to be a real structure.

We begin studying the displacements of the structure without bracings. As we can see, it has about 2.2 meters of displacement in the upper part of the structure. Initially, this displacement would be inadmissible.

Then we check the ratio at which the bending bars tested with the EC-3 are working. Again, in the following figure, we can see that in the base the maximum bending admitted by the profiles used is exceeded.

It is also interesting to check how compression stresses are transmitted to the structure, so we show how this transmission occurs in the initial configuration (we see a structure tending to overturn, with tensions in the left columns and compressions in the right ones.)

 

X-shaped cross-brace

If we repeat the same analysis performed for the brace-less structure but this time with X-shaped cross bracings we observe an incredible improvement.

We start noticing that now the maximum displacement does not occur in the top lintel, but in the local area where the loads are applied.

The maximum displacement in the upper part of the structure is 8mm, that is to say around 1% of the displacement of the structure without bracings. This incredible improvement has been obtained only adding a few kilos of steel to the structure which could be compensated, or even could lighten the structure, simply lowering the previous existing profiles.

We analyze the ratios to see how they have varied to look for the possibility of lowering the profiles. We see that except for the local area of application of loads, the column’s ratios have dropped from 1.2 and 0.8 to ratios of 0.1, that is, the columns do not work under bending stresses anymore.

Instead, this is how our braced structure works in compression. We observe a structure with a totally horizontal transmission of efforts until we make them descend through the bracings. Now there are no tensions on the column on the left, but instead we sacrificed the columns on the right. We will have to be aware of possible buckling effects.

I show a comparison of the axial loads in both models which I think is very interesting:

As I mentioned, the compressions could multiply up to by 5.

 

V-inverted bracings

As it has happened with the X-shaped bracing, the maximum displacements in the structure occur locally in the column where the load is applied.

We are going to skip that area and take a look at the rest of the structure. In the following image we have changed the scale to see the displacements in the structure in a greater detail. In the upper lintel it now has a displacement of 13mm, 5mm more than with the X-shaped bracing. The difference is not very big but it could make our structure fail due to the column drift.

What we do see is a considerable increase in the bending loads of the beams where the peak of the inverted V of the bracings is placed. The sign of the bending loads has changed from positive to negative and the ratio has doubled. We see that although the bending loads in the columns have increased a bit (it is a less rigid structure which forces the columns to bear a bigger stress in the joint) they are still not remarkable, as it happened with the X-shaped bracing.

In terms of compressions / tensions, we look at how the compressions travel and analyze the change.

However the behavior regarding axial loads is totally different. The columns are significantly unsolicited (from 165kN to 96kN) while it is the bracings those that now bear the largest compression load.

 

Spider web bracings

We may also find the case where the length of a V-bracing is very long or that due to any geometrical reason (i.e. a door) you need to design a bracing with a different geometry that allows to overcome the impediments. In this case we may find useful the spider web bracing (it is known by several names, this is one of them).

Analyzing again the displacements and obviating those displacements produced locally in the load application area, we can see that the most affected area is the upper beam, with a displacement of 12mm.

We have a displacement a bit smaller than with the V-inverted bracing, since we have reduced them from 13mm to 12mm.

Regarding the ratios of the bending beams according to the EC-3, there is no noticeable difference, both of which behave in a similar way. The advantage of this typology is that it allows us to increase the number of turns if the geometry forces us to do so.

Regarding the axial forces, we cannot appreciate a special difference either, because the way of transmitting efforts seems similar to the inverted V

 

Conclusions

- Bracing a structure allows us, as we have seen, to reduce enormously its displacements while allowing us to save material

- A braced structure causes the columns to stop working as corbels, since there are more rigid triangulated elements that make them work with tension / compression thus transmitting the efforts to the foundation instead of as bending forces to the columns.

- We have seen that the X-shaped bracings are the ones that stiffen the most the structure, showing a more logical and uniform behavior of the structure

- We have verified that other types such as the V-inverted shape give good results although it can penalize the beam where the V of the bracing is supported

- When the bracings are very long, we can split them by means of triangulations using the bracing in the form of a spider web. It allows us to adapt, like those of V, to different geometries in which, for example, we need to save a door in the middle of the span.

- It is important to highlight that this is a purely theoretical exercise in which a building configuration is studied. Playing with different profiles and depending on the loads, we could obtain other results.

Before reaching the end I would like to encourage you all to leave your professional opinions in the comments area, assessing which bracings work best. If you want me to add any comment to the article with some calculation / typology that you have calculated, contact [email protected] and attach it.

I want to finish with an image that I have found in the social networks about the steel haunches prepared to make a joint that looks like it will be well braced

Article written by Carlos Corral, the author of Prontubeam.com and Prontubeam.com/en

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About the author
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Carlos Corral . MEng Civil Engineering from the Politécnica university of Madrid. Speciality: Structural engineer. Owner and programer of Prontubeam.com and Prontubeam.com/en.
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