The numerical model emulated the overall force-displacement relationship, as well as the essential deformation patterns and stress concentrations observed during the physical test. Nevertheless, a mesh sensitivity analysis was carried out in order to discover the optimal mesh size for the braces along the width/depth and longitudinal directions considering the strain amplitudes, out-of-plane displacements, deformed shapes (e.g. local buckling) and computational time. Fig. 3.15 illustrates the mesh adopted for the first story braces in the simulated SCBF. As mentioned previously, the bracing members were meshed using two and seven elements across their thickness and width, respectively. The filleted corners of the square hollow sections were divided into three elements. For the sake of the comparison, only the mesh sizes along the length and flat width/depth directions were altered,
respectively, for the mesh sensitivity analysis while the number of elements in the filleted corners and in the through the wall thickness (tw) were remained constant. It was essential to consider the mesh size in each direction as a fraction of the brace dimension of concern. Therefore, with the intention of proposing a general range for the optimal mesh size of
axially-loaded members, the simulation cases were established based on the normalized mesh size.
Figure 3.15. Meshing of the square hollow bracing in the simulated SCBF
Two sets of simulations were created as follows:
(a) The uniform longitudinal mesh size was adjusted in terms of the actual mesh size (l) divided by the distance from slotted end to slotted end (L). Table 3.6 presents the simulation cases with changing l/L values from 0.15% to 1.25%.
Table 3.6. Parametric study on the longitudinal mesh size Model Name
Element size* Normalized element size
t (in) b (in) l (in) Max. Aspect
Ratio** t/tw B/b l/L l/L=1.25% 0.175 0.658 1.00 5.71 0.500 7.00 1.25% l/L=1.00% 0.80 4.57 1.00% l/L=0.50% 0.40 3.76 0.50% l/L=0.20% 0.16 4.11 0.20% l/L=0.15% 0.12 5.48 0.15%
*D=B=4.604”; tw=0.349”; L≈80”. **The maximum of the ratio of any two dimensions of the element.
(b) The aim of the second simulation group was to reveal how the mesh size in the flat width/depth direction affects the results. For this purpose, the ratio of flat section width (B)
l b
t A Linear BrickElement
Side view L Flat width (B) Flat d epth (D) Wall thickness (tw) Section view
over element width (b) were altered by dividing the flat width of the section into different number of elements while the other two element dimensions (t and l) were held constant, as given in Table 3.7. Note that the number of elements used in the flat width and flat depth directions was identical for each simulation case.
Table 3.7. Parametric study on the mesh size through flat width/depth Model
Name
Element size Normalized element size
t (in) b (in) l (in) Max. Aspect
Ratio t/tw B/b l/L B/b=7 0.175 0.658 0.80 4.57 0.500 7.00 1.00% B/b=8 0.576 8.00 B/b=10 0.512 9.00 B/b=12 0.384 12.00 *D=B=4.604”; tw=0.349”; L≈80”
The simulation results were compared by means of the local buckling deformation and peak axial plastic strain at the plastic hinge location, load-displacement response of the frame and out-of-plane displacement of the braces. Fig. 3.16 illustrates the extraction process of the peak axial strain values for each simulation case. As shown in Fig. 3.16(a), (b) and (c), the maximum axial strain was obtained from the mid-section of the first story brace in
compression. The peak axial strain readings of all cases were acquired from a node close to the filleted corners, in which the initial cracking of the tested brace occurred (Fig. 3.16d). Note that the red dot given in Fig. 3.16(d) represents the element node with the maximum plastic strain amplitude. However, the peak axial plastic strain values given in Figs. 3.17 and 3.18 were extracted from the integration point of an element that shares the red dotted node.
(a) SCBF (b) Buckled brace
(c) Section A-A (d) Peak axial plastic strain location Figure 3.16. Illustration of plastic strain data processing
Fig. 3.17 presents the cyclic response of the frame, normalized peak axial plastic strain (axial strain divided by the maximum axial strain obtained from the simulation group) and out-of-plane displacement of the braces. Based on the results of the first simulation group, the following can be discussed:
(1) The global response of the simulated frame with a normalized longitudinal mesh size of 1.25% was slightly different than that of the other four cases. Still, the variation in the mesh size in axial direction of the braces did not play an important role in overall cyclic response of the SCBF (Fig. 3.17a).
(2) The peak out-of-plane displacement increased linearly as the mesh size decreased. Despite the increase, the peak out-of-plane displacements of the frames did not substantially
A
A
Max. axial plastic strain obtained from
differ from each other. The maximum difference between the cases with the largest and smallest mesh sizes was about 7% (Fig. 3.17b).
(3) As seen in Fig. 3.17(c), the peak axial strain followed a trilinear path as the mesh size refined. First, the normalized peak strain amplitude showed a rapid increase from 30.1% to 69.5% when the normalized mesh size reduced from 1.25% to 1.00%. Then, the strain
amplitude linearly increased from 69.5% and reached its peak value (100.0%) and dropped to 90.4% when the mesh size further decreased to 0.15% of the length. In other words, the axial strain amplitude and local deformations at the plastic hinge location were strongly influenced by the changes in the longitudinal mesh size.
The results of the second simulation set are compared in Fig. 3.18. It appears that the cyclic response of the frames were virtually identical (Fig. 3.18a). Furthermore, similar to the first simulation group, the maximum difference between the peak out-of-plane displacements of the bracings was on the order of 2% (Fig. 3.18b). Most notably, as can be interpreted from Fig. 3.18(c), the variation in the mesh size in the width direction did not affect the local buckling shape of the mid-section significantly. Even though there exist slight changes in the peak out-of-plane displacement and axial plastic strain amplitudes, it seems that the
improvement in the observed local and global responses with a finer mesh size was quite negligible compared to its computational cost.
(a) Base shear vs. roof displacement (b) Peak out-of-plane displacement of braces
(c) Peak axial plastic strain and local buckling shapes of braces
Figure 3.17. Comparison of local and global responses of the frames with various longitudinal mesh sizes.
l/L=0.20% l/L=0.50%
l/L=1.00%
l/L=0.15%
Front view of the plastic hinge location after out- of-plane buckling
Buckled brace
View of the mid-section of the brace Max. Axial
(a) Base shear vs. roof displacement (b) Peak out-of-plane displacement of braces
(c) Peak axial plastic strain and local buckling shapes of braces
Figure 3.18. Comparison of local and global responses of the frames with various mesh sizes in width/depth direction.