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Part 2 - Changing Market Forces Are Creating Structural Implications for Residential Tower Projects - Column Cost

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.

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