Zamil steel frame bracing system, how to design economically
Zamil steel frame industrial buildings have been present in Vietnam for nearly 30 years and have quickly become popular due to their economic value and speed. So popular that people are used to calling them by their own names, like Honda motorbikes. The common noun is pre-engineered steel frame buildings (PEB – Pre Engineered Building), to refer to beveled steel frame houses, and later many brands were born and shared the Vietnamese factory market pie. Although it has been such a long time, engineers still do not necessarily understand the ins and outs of each structural detail, which can be removed or made smaller. They mainly do it out of habit, like making medicine according to a pre-made scale with this span house, this frame step, then the frame size is just like this, why calculate, blah blah. With the itchy hobby of engineers having to answer the question why (and masters are why of why 🤣…), and more importantly, see which can be reduced to save 💰 for the homeowner. This article will decode some of the so-called “structures” of PEB frame houses.
Tapered frame
This is the main load-bearing system of the house, usually flat frames parallel to each other. The reason for beveled columns and trusses of the steel frame instead of straight bars is simply to use materials reasonably. Areas that bear a lot of force due to the impact load (weight, wind, …) are made larger. Therefore, the volume of materials is reduced, saving costs and installation time. Moreover, the beveled frame also looks quite aesthetic 💎
The common concept of most engineers is to calculate according to the flat frame diagram according to Vietnamese Standards. Then arrange the bracing system, usually in the head compartments and temperature compartments according to the “structural” requirements. The structure here is understood to ensure the stability of the columns and trusses in the direction outside the frame plane. What bars and sizes are needed for the bracing system, we must calculate the stability problem of the frame.
The big problem here is that TCVN, which is translated from the Russian and former Soviet Union standards, does not have any provisions for calculating the stability of tapered frames. It is surprising for a product that has been widely used for more than 30 years. TCVN cannot be used. It is best to use the standards of developed countries, recognized by many countries and applied by many computer software, such as the US (AISC,..) or European Eurocode (EN 1993).
Here, we choose EN 1993, because of its suitability for metric units and the upcoming orientation of TCVN to switch completely to Eurocode.
Another mistake that many steel structure contractors are used to is to avoid, which is to design according to which standard, the load must be determined according to that standard. Especially wind load which greatly affects the results of steel frames. Do not “put this man’s beard on that woman’s chin” type of load then follow TCVN, calculate column size, truss according to AISC, EN… Meet the appraisal review flat back there is no legal basis to protect😨
Stability calculation principle
Unlike concrete, steel structures are often combined from thin steel plates to save materials. Therefore, the characteristic of steel structures is that they are susceptible to instability. To understand instability, we can take the following example:
Imagine you have to use both hands to squeeze a plastic brick from both ends. Can you break it? It looks so frustrating.
But what if the plastic bar is only 0.5mm thin, it looks more like a tape measure. Using both hands to squeeze it will bend and break easily. Why? The same material, same direction of force, same pressure (force per unit area) is still smaller than the strength of the plastic material, but the latter case is more susceptible to failure?
The answer is due to the phenomenon of instability. It can be seen that before failure, the plastic ruler is bent and twisted even though it is only under compressive force, while the brick in case 1 is almost not. That bending makes the pressure increase much more than just compressive pressure. The instability (buckling) acts like a multiplier on the initial load, causing the object to fail.
Purlins
The obvious function is to support the roof and wall panels, transmitting the load to the frame. But purlins also have an important function, which is to prevent the frame from becoming unstable. Roof purlins prevent the steel flange on the truss from buckling due to instability. Similarly, wall purlins increase the stability of the outer flange of the frame column.
Frame instability without purlins
Frame instability with purlins
The habitual calculation according to the flat frame diagram cannot see the role of the purlin and calculate how many purlins are needed to stabilize the frame. Obviously, we have to consider the 3D model of the entire frame system, purlins, and braces in the calculation software. It takes a bit of work, but the value brought by the savings is worth it.
The popular type of purlin in the factory today is processed by stamping thin sheets of metal into Z-shaped or C-shaped bars. This helps the bars have a very light weight but the stiffness created is great. Like the game of folding a zigzag paper, it will achieve surprising stiffness, can support heavy objects as shown below. That is also the principle of creating waves for the metal sheet.
Another shortcoming of the current TCVN is that there is no way to calculate thin-walled bars. Therefore, it is even more advisable to use the Eurocode purlin design software for synchronization.
In reality, the purlins are joined together at the steel frame position. Therefore, it is considered as a continuous bar, at the connection position we have 2 interlocking Z-shaped bars, so the load-bearing capacity increases significantly, in accordance with the load-bearing diagram of the purlin, as shown below.
Calculation diagram of continuous Z-purlin and instability form of purlin without purlin bracing
Purlin connection details
Corrugated Steel sheet
The main function is to cover the roof and wall. Corrugated iron sheets also have very good stiffness to withstand the load before transmitting it to the purlins and frames: weight, repair machinery, wind load, etc.
In addition, roof and wall panels also play an important role in increasing the stability of the purlin and frame system. Such a large steel plate. Once again, the habit of calculating flat frames using TCVN does not take this into account and will be biased towards safety and waste💰
The 3D design diagram of the house frame, to avoid being too complicated, will not model the corrugated iron sheets but only calculate their stiffness to increase the stiffness of the upper purlin flange as shown below. Therefore, the 3D calculation diagram in the simulation software most closely resembles the actual working of the PEB warehouse.
Stiffness model of roofing sheet on purlin
X-bracing
In case there is no X-bracing system in the 2 gable bays, the gable and middle frames are both unstable as shown below
Unstable form of the frame without X-bracing Now we construct the X-bracing on the roof in the correct structure: using D20 steel bars, which is the most popular type today. The ability to stabilize the steel truss is significantly improved, especially when bearing the wind load blowing in the longitudinal direction of the house (perpendicular to the frame plane).
Frame instability when there is X-bracing
Similarly, arranging X-bracing between the frame columns in the same bay with the roof bracing significantly improves the stability of the frame columns.
Stiff strut
When the load is too large like the 36m span frame in this example, only traditional X-bracing is not enough to maintain stability. To avoid increasing the size of the main frame and saving materials, we need to arrange additional stiff bracing (Strut). Usually made of steel, C-shaped, box, … perpendicular to the frame plane, at the positions where the end frame connects to the middle frame, the roof peak …
Unstable form of frame without stiffening
Flange Brace
Purlins and roofing sheets stabilize the upper flange of the steel truss (and the outer flange of the steel column). How do the remaining flanges stabilize? Flange Brace is often used. It is very effective in keeping the lower flange of the truss and the inner flange of the frame column from buckling due to instability. The difference between the frame with and without flange braces can be clearly seen in the figure below.
Frame Load Capacity Without Flange Bracing
Frame Load Capacity With Flange Bracing
In the above figures, the % is the ratio of the load to the load-bearing capacity, taking into account the stability of the frame. When there is no wing bracing, the load exceeds the stability of the frame (greater than 100% in areas with large loads), the structure becomes unstable.
The wing bracing is usually made of small angle steel bars, connecting the wing under the truss to the purlin. It is surprising that supporting the fragile purlin can significantly improve the stability of the wing under the frame truss.
If the cross-sectional height of the truss or column is greater than 1m, it is necessary to arrange diagonal wing bracing on both sides of the truss, but normally only one side is needed to save money. According to habit, contractors often construct butterfly bracing at all locations with purlins. But stability calculations show that it is not necessary, and costs can be greatly reduced by only arranging bracing at locations that are prone to instability, as shown in the example below. Once again, knowledge reigns supreme in helping 🎩The boss save a lot of unnecessary money.
No need to arrange wing bracing at every purlin to avoid waste
Purlin bracing
The main function is to stabilize the purlin. As we know, purlins are made from very thin steel plates, so they are more susceptible to buckling and instability than the steel plates of the main frame.
The detailed stability calculation of purlins shows that it is only necessary to use purlin bracing when the purlin span (frame step) is large enough (over 8.5m), or the roof load has a sudden change to ensure stability.
Whether 1 or 2 bracing points are needed, the size of the purlin bracing (usually using round steel rods with a diameter of 12mm) is also calculated in detail by the software. The figure below illustrates an example with and without purlin bracing for a span of 11m with 2 bracing points.
Instability of purlins without purlin bracing
Instability of purlins with purlin bracing
Okay, so what we have always considered “Details” actually plays an important role in ensuring the safety of steel structures. Reading this far, you may find it funny, because this simple house has been built for 30 years and you are used to it, why do you need to dig into each steel bar like that? 😯 In addition to the passion for answering the question Why, engineers can apply this knowledge to a more practical goal: saving money for the 🎩 homeowner. Grasping the basic principles can then be applied to more airy architectures using steel structures, instead of heavy concrete blocks.
Once again, I am not abusing English words because I am a foreigner. Those are effective keywords for Google on this topic for you to research further, compared to the limited Vietnamese resources available today.
