Ensight


Changing Market Forces are Creating Structural Implications for Residential Tower Projects – Part 2. Column Cost

Written on Jun 05, 2019

By Ryan Voros

Introduction

In Part 1 of our investigation into the implications of structural design decisions on the costs of residential high-rise buildings, we looked at various column and slab configurations in a sample high-rise building. Part 1 can be found here. In Part 2, we will look specifically at columns in residential high-rises.

There are three cost parameters that directly impact overall structural costs in a concrete residential high-rise structure: reinforcing supply and install, concrete supply, and formwork (which often includes concrete placing and finishing). Based on experience, it is possible to optimize for one parameter, such as concrete volume, and still see the total cost increase. This could happen if, in an attempt to optimize concrete volume, reinforcing weight and formwork complexity is meaningfully increased to have a greater impact on total cost.

 

Column Cost Analysis

To conduct a cost analysis, a baseline set of cost values for rebar, concrete, and formwork costs is required. The following construction cost values were the assumed values for the baseline calculations. A sensitivity analysis was also conducted on the three construction unit rates to determine if the overall findings would change based on varying market conditions.

Baseline Cost Parameters

Concrete Supply Reinforcing Steel
Supply & Installation
Formwork Material & Installation,
and Concrete Placing & Finishing
MPa $/m3 $/kg $/m2
30 206 1.80 130
35 215
40 226
45 237
50 247

 

Column Capacity & Cost Analysis

The tables below show the costs for three sets of column loads and configurations for typical residential floor-to-floor height. These sets have been sorted based on the cost per kN of column capacity.

4000 kN Column Cost Analysis

Width Length Reinforcing Ratio Concrete Strength Construction Cost Cost Efficiency
mm mm % MPa $ $/kN
400 400 2 50 851 0.210
400 400 2.5 45 879 0.225
450 450 1.5 40 936 0.228
400 400 3.5 40 939 0.233
450 450 2 35 971 0.244
400 400 4 35 966 0.247
450 450 3 30 1048 0.254
500 500 1.5 30 1056 0.257
500 500 1.5 30 1056 0.257
300 900 1 35 1200 0.295
250 900 2.5 50 1279 0.309
300 900 1.5 30 1248 0.313
250 900 2.5 45 1273 0.320
250 900 3 40 1312 0.328
200 3000 2 40 3357 0.832

 
5000 kN Column Cost Analysis

Width Length Reinforcing Ratio Concrete Strength Construction Cost Cost Efficiency
mm mm % MPa $ $/kN
450 450 2 50 989 0.194
450 450 2.5 45 1024 0.205
450 450 4 35 1135 0.226
550 550 1.5 30 1192 0.240
300 900 2 40 1318 0.256
350 900 1.5 30 1342 0.256
300 900 2.5 35 1364 0.270
300 900 3 30 1412 0.284
250 1800 1 30 2042 0.414
200 3800 2 40 4211 0.819

 
6000 kN Column Cost Analysis

Width Length Reinforcing Ratio Concrete Strength Construction Cost Cost Efficiency
mm mm % MPa $ $/kN
500 500 1.5 50 1084 0.181
450 450 4 50 1153 0.188
500 500 2.5 45 1179 0.191
550 550 1.5 40 1209 0.197
500 500 3 40 1222 0.203
550 550 2 35 1261 0.212
500 500 3.5 35 1265 0.215
350 900 1.5 40 1359 0.222
550 550 3 30 1376 0.224
300 900 2.5 50 1388 0.232
350 900 2 35 1414 0.234
300 900 3 45 1436 0.242
300 900 3.5 40 1482 0.244
350 900 2.5 30 1469 0.251
300 900 4 35 1529 0.254
250 1800 1 40 2067 0.341
250 1800 1.5 35 2144 0.355
250 1800 2 30 2224 0.379
200 4500 2 40 4958 0.824

 
In general, the most cost-effective column configurations have a square aspect ratio with higher concrete strength and lower reinforcing ratio. The square columns are more cost-effective because they are impacted less by slenderness for the typical residential floor-to-floor height of 2800 mm. Rectangular columns still benefit from higher concrete strengths and lower reinforcing ratios.

When a 250 mm wide column is used, the cost is around double that of the most cost-effective column. Moreover, for a 200 mm wide column, which must be significantly longer, the construction costs are closer to five times that of the most cost-effective column even though the slenderness efficiency is about three times less. This is also due to the additional reduction factor for fire safety. A detailed breakdown of costs for the most and least cost-effective columns in the 6,000 kN category is shown in the table below. Rebar and formwork have a higher impact to the cost increase between the two options.

Square vs Long Column Cost Comparison

Width Length Reinforcing Ratio Concrete Strength Concrete Cost Rebar Cost Formwork Cost Total Cost Cost Efficiency
mm mm % MPa $ $ $ $ $/kN
500 500 1.5 50 173 183 728 1084 0.181
200 4500 2 40 519 1018 3422 4958 0.824
Ratio 3.00 5.55 4.70 4.57 4.54

 

Cost Parameter Sensitivity Analysis

The findings above were derived from the baseline cost parameters and it should be considered whether changes to these baseline costs would impact the general findings. The three cost parameters were altered using multipliers and the 6,000 kN column set was used in the analysis. The table below shows the changes to the order based on cost-efficiency and the baseline values.

Cost Parameter Sensitivity Analysis

Multiplier Applied to Baseline Cost Parameters
Concrete Rebar Formwork Order Compared to Baseline
0.5 1 1 1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19
1 0.5 1 1,2,3,5,4,7,6,9,8,10,13,11,12,15,14,16,17,18,19
1 1 0.5 1,4,3,2,5,6,8,7,9,11,10,12,13,14,15,16,17,18,19
2 1 1 1,2,3,4,5,7,6,9,8,10,11,12,13,15,14,16,17,18,19
1 2 1 1,4,3,2,5,6,8,7,11,10,9,12,14,13,15,16,17,18,19
1 1 2 1,2,3,5,4,7,6,9,8,10,11,13,12,15,14,16,17,18,19
0.25 0.5 1 1,2,3,5,4,7,6,9,8,10,11,12,13,15,14,16,17,18,19
0.5 0.25 1 1,2,3,5,7,4,9,6,8,10,13,12,15,11,14,16,17,18,19
1 0.25 0.5 2,1,3,5,7,4,6,9,8,10,13,11,12,15,14,16,17,18,19
1 0.5 0.25 1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19
0.5 1 0.25 1,4,3,8,6,2,5,11,10,7,9,12,14,13,15,16,17,18,19
0.25 1 0.5 1,4,3,2,6,5,8,7,11,10,9,12,14,13,15,16,17,18,19

 
The overall trends generally remained the same, with the most cost-effective columns being the square columns and trending towards higher concrete strengths and lower reinforcing ratios. The order of some of the column arrangements in the mid-range changed but overall trends remained valid. The results from using the multiplier values of 1.0 concrete, 0.25 reinforcing, and 0.5 formwork are in the table below.

6000 kN Column Cost Analysis

Width
mm
Length
mm
Reinforcing Ratio
%
Concrete Strength
MPa
Construction Cost
$
Cost Efficiency
$/kN
Previous Order from Baseline Analysis
450 450 4 50 557 0.191 2
500 500 1.5 50 583 0.098 1
500 500 2.5 45 601 0.098 3
500 500 3 40 606 0.101 5
500 500 3.5 35 611 0.104 7
550 550 1.5 40 646 0.106 4
550 550 2 35 652 0.110 6
550 550 3 30 675 0.110 9
350 900 1.5 40 717 0.117 8
300 900 2.5 50 706 0.118 10
300 900 3.5 40 717 0.118 13
350 900 2 35 723 0.120 11
300 900 3 45 712 0.120 12
300 900 4 35 722 0.120 15
350 900 2.5 30 731 0.125 14
250 1800 1 40 1103 0.182 16
250 1800 1.5 35 1112 0.184 17
250 1800 2 30 1124 0.191 18
200 4500 2 40 2484 0.413 19

 

Conclusions

Generally, square columns with higher concrete strength and lower reinforcing ratios are the most cost-effective. While square columns are more cost-effective than rectangular columns, rectangular columns are often preferred on the typical floors of residential high-rise buildings. This is because it is easier to hide rectangular columns in suite walls and limit their impact on the suite layout. In addition, it is common for architects to request columns as thin as possible. While this is possible from a code and safety standpoint, the costs can quickly rise significantly with thinner columns. Finally, it was seen through the sensitivity analysis that the general findings are valid for all reasonable market conditions.

 


 
About Ryan Voros

Ryan is an Associate at Entuitive with over 10 years of structural engineering experience in design, project management, and construction administration including reinforced concrete, post–tensioned concrete, structural steel, and wood-frame construction. Ryan’s experience covers multiple sectors, including large mixed-use developments valued at over $250 million.

Read more about Ryan Voros >

 

Ryan Voros P.Eng., LEED® AP
ryan.voros@entuitive.com

 

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