7.4 Results
7.4.4 N -RMC modelling
Optimum Outrigger Location 10% Lower to Base
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0 0.2 0.4 0.6 0.8 1
Value of ω
Deflection reduction efficiency %
Figure 3.15 Deflection reduction efficiency with outriggers 10% lower than the optimum location
3.3.3 Factors affecting the efficiency of an outrigger-braced structure
A parametric study has been carried out to investigate the factors that affect the efficiency of an outrigger-braced structure in terms of top drift and moment reduction. In general, these can be categorized into several main factors, including:
• Building height;
• Core properties, such as size, thickness and concrete strength;
• Outrigger-braced column properties, such as size and concrete strength; and
• Distance between both outrigger-braced columns
1-Outrigger
2-Outrigger
3-Outrigger
4-Outrigger
5-Outrigger
3.3.3.1 Height of structure
It is necessary to investigate the height of a structure to show its relationship to the efficiency of an outrigger-braced system. By adopting the example described in Section 3.3, the height of the structure is changed to 500m while all the other structural parameters remain the same. Graphs are plotted as per Figure 3.16 to show the deflection reduction efficiency and per Figure 3.17 to show the moment reduction efficiency.
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0 0.2 0.4 0.6 0.8 1
Value of ω
Deflection reduction efficiency %
Figure 3.16 Deflection reduction efficiency with total height of 500m
A comparison of Figures 3.12 and 3.16 for the deflection reduction efficiency, and of Figures 3.13 and 3.17 for the moment reduction efficiency clearly shows that both graphs give a similar result. There is no change in reduction efficiency in all the graphs and this leads to the conclusion that height has no relationship to the efficiency of the outrigger structures.
1-Outrigger
2-Outrigger
3-Outrigger
4-Outrigger
5-Outrigger
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0 0.2 0.4 0.6 0.8 1
Value of ω
Moment reduction efficiency %
Figure 3.17 Moment reduction efficiency with total height of 500m
3.3.3.2 Core properties
Core properties have to be investigated in order to show their relationship to the efficiency of an outrigger-braced structure. In this case, by adopting the example in Section 3.3, a stiffer core is required. Therefore, the core moment of inertia is changed from 1000m4 to 2000m4 and concrete strength is changed from 65MPa to 80MPa; the other structural information remains the same. Graphs are plotted as per Figure 3.18 to show the deflection reduction efficiency, and per Figure 3.19 to show the moment reduction efficiency.
1-Outrigger
2-Outrigger
3-Outrigger
4-Outrigger
5-Outrigger
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0 0.2 0.4 0.6 0.8 1
Value of ω
Deflection reduction efficiency %
Figure 3.18 Deflection reduction efficiency with different core properties
A comparison of Figures 3.12 and 3.18 for the deflection reduction efficiency, and of Figures 3.13 and 3.19 for the moment reduction efficiency clearly shows that Figures 3.18 and 3.19 yield lower efficiency in an outrigger-braced system. This may be due to the fact that the stronger core attracts more forces and moments, causing the smaller forces to be redistributed to the outrigger-braced core-to-column. Because of this, it can be concluded that a stiffer core with an increase of core properties or an increase in concrete strength will decrease the efficiency of the outrigger structures. It is important for designers to assess the structural performance of a building, in terms of investigating the core properties and column properties, before choosing an outrigger-braced system as the lateral system for a structure.
1-Outrigger
2-Outrigger
3-Outrigger
4-Outrigger
5-Outrigger
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0 0.2 0.4 0.6 0.8 1
Value of ω
Deflection reduction efficiency %
Figure 3.19 Moment reduction efficiency with different core properties
3.3.3.3 Outrigger-braced column properties
The structural properties of an outrigger-braced column have a direct relationship with the performance of an outrigger-braced system in a building. In this case, a weaker column will be adopted, changing the column size from 1.5m x 3m to 0.5m x 1m, and the concrete strength from 65MPa to 40Mpa. All other structural information remain the same. Graphs are plotted as per Figure 3.20 to show the deflection reduction efficiency, and per Figure 3.21 to show the moment reduction efficiency.
.
1-Outrigger
2-Outrigger
3-Outrigger
4-Outrigger
5-Outrigger
1-Outrigger
2-Outrigger
3-Outrigger
4-Outrigger
5-Outrigger
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0 0.2 0.4 0.6 0.8 1
Value of ω
Deflection reduction efficiency %
Figure 3.20 Deflection reduction efficiency with column changed
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0 0.2 0.4 0.6 0.8 1
Value of ω
Moment reduction efficiency %
Figure 3.21 Moment reduction efficiency with column changed
1-Outrigger
2-Outrigger
3-Outrigger
4-Outrigger
5-Outrigger
Clearly, Figures 3.20 and 3.21 show that the weaker outrigger-braced columns with lower concrete strength and column size yield lower efficiency of the whole outrigger-braced system in comparison with Figures 3.12 and 3.13. With a decrease of column size of 50%
and concrete strength of 25%, the drift reduction efficiency has dropped from 85% to 65%, and the moment reduction efficiency has reduced from 72% to 58% if the outriggers are considered stiff. So, wherever possible, larger or stronger columns are preferred in this lateral system in order to reduce the top drift and core moment significantly.
3.3.3.4 Clear distance between outrigger-braced column
A clear distance between outrigger-braced columns is investigated to show its relationship to the efficiency of an outrigger-braced system. Using the same example as in Section 3.3.2, the clear distance between outrigger-braced columns will be modified from 36m to 25m, with an approximate reduction of 30% in terms of the distance. All other structural information remain the same.
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0 0.2 0.4 0.6 0.8 1
Value of ω
Deflection reduction efficiency %
Figure 3.22 Drift reduction efficiency with column clear distance changed
1-Outrigger
2-Outrigger
3-Outrigger
4-Outrigger
5-Outrigger
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0 0.2 0.4 0.6 0.8 1
Value of ω
Moment reduction efficiency %
Figure 3.23 Moment reduction efficiency with column clear distance changed
A comparison of Figures 3.12 and 3.22 for the deflection reduction efficiency, and Figures 3.13 and 3.23 for the moment reduction efficiency, shows that an outrigger-braced structure with a shorter clear distance between columns would yield lower efficiency for the whole outrigger-braced system. With a decrease of clear distance between columns of 30%, the drift reduction efficiency has dropped from 84% to 67%, and moment reduction efficiency has reduced from 73% to 58% if the outriggers are considered stiff: i.e. when ω =0. The clear distance between the outrigger-braced columns plays an important role in increasing or decreasing the efficiency of an outrigger-braced system in a structure. It is suggested that enlarging the clear distance between columns can reduce the top drift and core moment significantly.