Bending and axial together: a braced frame
Step 8's portal frame carried its loading predominantly through bending. The lateral stability and stiffness of the frame relied on the bending stiffness of the columns. This made the frame susceptible to large sway deflections when the frame experienced lateral wind loading.
In this step, we'll model a two-storey frame, but this time, we won't rely on the bending stiffness of the columns to resist lateral loads. Instead, we'll model another common stability solution - diagonal braces modelled using truss members. This will create a stiff bay where the lateral loads are transmitted back down to the foundations through axial loading of the bracing members. This is our first mixed model that contains both types of structural members.
1.0 The idea
Wind loading on a frame is simulated using a horizontal force applied at each floor level. One of our main tasks is to ensure that this loading has a safe load path back into the foundations of the structure. In an unbraced frame, it gets there by portal action.
We ensure that the beams and columns have sufficient capacity to bend and that the connections between beams and columns are rotationally stiff enough to transmit moment. This is exactly the behaviour we saw in step 8.
This works well, but as a frame becomes taller and more slender, this approach to lateral stability quickly reaches its limit as the lateral sway deflections grow beyond acceptable limits. Also, ensuring every connection in a multi-storey steel frame is stiff enough to transmit moment is a tall (and expensive) order.
Implementing cross bracing is a much more efficient way of achieving lateral stability and the required sway stiffness. Instead of making the columns bend in response to the wind, the lateral loads are directed into the bracing elements as axial forces, a much more efficient load-carrying mechanism.
2.0 The model
Our model has two bays, each spanning 6 m, and two storeys, each 4 m high. The columns are UC 254 x 254 x 73 and the four beams are UB 356 x 171 x 51. All of the beams and columns are frame members with rigid joints between them. In the right-hand bay, we have X-braces: each of the four bracing elements is a CHS 88.9 x 5.0 member. There are two load cases: , 15 kN/m on every beam, and , a 20 kN point load applied to the left-hand column at each floor. These loads are arranged into two combinations, ULS and SLS.