a study of critical member capacities and green roof loading

Dept. of Infrastructure Engineering. Research Paper for CVEN90022,

Copyright © Syed Ahsan, Paulo Sarmiento, Anderson Liu and Nam Phan, 2015. of 35

A study of critical member capacities and green roof loading

Syed Ahsan

The University of Melbourne, Melbourne, Australia

Paulo Sarmiento


Anderson Liu


Nam Phan


Abstract: Sustainability is a major issue in the present as diminishing resources and the

decline in the environment requires conservation for future generations. The urban heat island

effect, storm water runoff and greenhouse gas emissions are all major issues faced by cities

around the world and in the future. The application of green roofs into cities can alleviate these

issues and also provide a number of additional benefits such as insulation to buildings,

improving air quality and increasing property value. Not all buildings are suitable for green roof

installation due to the limited structural capacities of critical members, such as the slab and

the beam, to support the weight of the growing medium, greenery and water; however, by

transferring the load of the green roof directly onto the columns, this issue can addressed. To

analyse this theory, information will be gathered from building plans in order to experimentally

simulate green roof loads on buildings. The final outcome of this research will determine

whether transferring the loads directly onto the columns is a viable method to overcome the

current structural limitations and also as a basis for exploring further research within this area.

Introduction



Cities around the world are looking at ways to integrate the environment into the current infrastructure and land system, as societies are becoming more aware of the potential dangers of destroying the environment and the health benefits of providing greener living surroundings. There is a limited amount of land available for green spaces as more people are moving towards the cities where there are more opportunities and activities. Green roofs have become a popular way of providing green living spaces in these major cities and highly populated areas.

Green roofs also provide long term on-going benefits such as cooling via evapotranspiration which combats the Urban Heat Island Effect, storage and filtration of storm-water runoff which subsequently prevents the overflowing of sewers and the reduction of carbon concentration in the atmosphere through sequestration and photosynthesis (Tam et al., 2011). In relation to buildings themselves, green roofs provide additional insulation which reduces energy consumption and costs, increases property value and improves the appearance of the building (Tam et al., 2011).

The Growing Green Guide (City of Melbourne, 2014) is currently investigating the potential of Melbourne’s roofs for supporting green roofs, in order to encourage and assist in green building development in Melbourne. Extensive research has already been conducted on the benefits which primarily explore environmental and financial outcomes of green roofs.

There has been extensive research conducted by The City of Melbourne, in collaboration with

GHD Consulting, in identifying if a specified building satisfies non-structural requirements in

installing a green roof or not, however, there remains a lack of information on whether a roof conversion is possible based on the building structure’s capacity to take on the extra load or

weight from installing a green roof, presenting an opportunity for further research into this area.

It is important to provide reliable structural evidence that certain buildings in Melbourne have

the load capacity to have a green roof implemented, in order to increase the environmental performance of these buildings and introduce more greenery to Melbourne.

Objectives and aims

This project aims to investigate the potential of transforming the load path distribution within current methodologies of green roof construction in identifying the most structurally feasible method of implementation onto buildings. By addressing the current structural limitations that may be present within existing green roof installation methods, the best method of meeting the growing demand of the green infrastructure industry can be identified and considered for implementation within future infrastructure. Current literature only primarily covers the general benefits of green roofs towards the environment, achieving sustainability and financial feasibility. However, there is limited literature available on the structural limitations of existing buildings to support the extra load from the green roof. Generally green roofs are installed with their load transferred to the slab, this method of installation will be referred to as the conventional green roof system. However in some cases the slab is not strong enough to support the green roof. Experimental analysis will determine whether or not transferring the loads directly onto the columns is a viable alternative, this method will be referred to as the column-based green roof system. It is hoped that a comparison between the two systems can improve the implementation of green roofs from a structural perspective and pave a pathway for future structural research within the area. The analysis will also yield information of the structural limitations of conventional green roofs when retrofitting onto existing buildings and potentially provide valuable information to combat this issue. Although the column-based green roof system may seem more complex and would require more materials if implemented in actuality, the social benefits will generally offset the



costs if a government incentive program similar to the program in Germany (Köhler & Keeley, 2005) is implemented in Australia. The column-based green roof system offers a method of construction that will generally be more cost-effective than strengthening the slabs and beams, as in indicated in the Growing Green Guide (City of Melbourne, 2014) The objective of this investigation includes: • Exploring the principal structural member capabilities of buildings supporting green roofs

found amongst particular buildings in Melbourne. • Comparing the structural feasibility between the conventional green roof system and the

column-based green roof system. • Providing an indication of the additional required loads that will need to be resisted through

comparison between the two systems. Ideally, the analysis and results will assist future developers in confidently designing their green roofs within a structural context, by observing load paths in accommodating the self-weight and traffic loads on the green roof, identifying the consequential structural impacts of such loads on the critical members of each green roof system and consequently, choosing the most appropriate green roof installation method for the given project’s context.

Literature Review

The literature review focuses on two main areas: the background of green roofs and the structural factors behind green roof implementation. The purpose of focusing on these

specific sections is to provide a general context, an idea of green roofs and the structural

aspects that will be considered in the model.

Background of green roofs

How a green roof works

Green roofs function by providing shade for roof surfaces and providing benefits through evapotranspiration as the plants blocks sunlight from reaching the underlying roof membrane; the paper also conducted a study in Florida comparing two buildings, one with a green roof and one with an adjacent light-coloured roof, where results returned a lower maximum surface temperature of 27°C for the building with the green roof (Environmental Protection Authority , 2013). This evidence provided a justification that vegetation can be used to lower building surface temperatures due to shading, where the cooler temperature from the surface is transmitted to the environment. The combination of this process and evapotranspiration provides a cooling effect on the surrounding air (Environmental Protection Authority , 2013). The structure and the components of a green roof can be seen in Figure 1.



Figure 1: Typical green roof design - Source: (State of Victoria: The Department of Environment and Primary

Industries, 2014):

Financial Benefits

Chan and Chow (2013) conducted a study on the energy and economic performance of green

roofs under future climatic conditions in Hong Kong. The method involved a computer modelling simulation using an experimental setup of a green roof on a commercial building in

Hong Kong, which is validated by comparison of experimental results between the modelling

of that same building using EnergyPlus, an energy and thermal load simulation program. Chan

and Chow (2013) used the program to simulate energy performance of buildings under future climatic conditions and two emission scenarios. It was found that under the study’s proposed

conditions, green roofs could reduce the A/C energy consumption by 2.4 to 10 per cent per

year, and after considering other financial factors, it was estimated that the cost payback

period of a green roof was ten years. The results, however, could vary depending on the initial installation costs and changes in the interest rate, oil price, and electricity price.

The Growing Green Guide (City of Melbourne, 2014) also suggests an alternative method of

accommodating the weight of people, plants, substrate, and other items by strengthening the green roof. Costs associated with strengthening range from $450 to $650 per metre squared

to strengthen the concrete roof slab under trafficable areas, $240 per metre squared to

strengthen a steel roof and the provision of additional column supports and foundations

range from $2500 to $7500 each.

Environmental benefits

The Urban Heat Island effect is a current issue that is hazardous to society as most of the

population resides in urban areas. The implementation of green roofs will reduce the Urban

Heat Island Effect through shading and evapotranspiration processes. According to a

modelling study in Toronto, it was examined that increasing the amount of green roofs by 50 per cent within a city will cool the entire city by 0.1°C to 0.8°C (Environmental Protection



Authority , 2013). Based on this study, this supports the claim that having more vegetation in

urban areas will allow the air to be cooled, which assists in combating the Urban Heat Island

effect (Jenrick, 2005).

Green roof types and associated loads

Wilkinson & Reed (2009) states there are two primary types of green roof design; intensive and extensive. Intensive green roofs are primarily more robust structures than its counterpart; traditional-style roof gardens consisting of plants, trees and shrubs (Tam et al., 2011). They are generally designed to hold traffic loads and can provide many social benefits, although regular care in maintenance and irrigation is required, making it a labour demanding structure (Worden et al., 2004). Extensive green roofs are light-weight (Liu, 2012); (lightweight plants such as succulents and moss are generally used (Cutlip, 2006). They are less labour demanding and produce significant environmental benefits, including stormwater mitigation and providing protection for building components (Tam et al., 2011), these roofs are generally inaccessible and so, provide minor social benefits.

When assessing the differences between the two, intensive roofs generally impose larger impacts on the development procedure, since they are heavier (Table 1), costly and require care in maintenance and irrigation as mentioned before (Cutlip, 2006; Ngan, 2004; Jenrick, 2005). The two green roof types also requires further structural support to cater for additional loads (Koppany, 2002).

Table 1: Typical design loadings found on the two primary green roof types - Source: Section 2.5 Green Roofs. Knauf Insulation. (n.d.). 1st ed. Merseyside.

Green roof type Weight (kPa)

Extensive 0.59 - 1.50

Intensive 1.96 - 5.40

Additionally, trafficable loads should also be considered in identifying associated loads with a green roof. FLL Guidelines (2002) outlines a range of vegetation types, from sedum-moss

herbaceous plants to small trees, and loads, from 0.1 kPa to 0.6 kPa, which can typically be

expected to be found on a green roof. Standards Australia (1971) also indicates that for

the typical Australian building built between 1971-1989, the traffic live load rating is indicated to be 3.0 kPa, while newer buildings have a higher traffic live load rating of 4.0 kPa (Standards

Australia, 2002). This recent rise in live load rating is explained in Castleton et al. (2010),

which outlines that due to technological advancements in assessing structural capabilities of

buildings, buildings that are less than 30 year olds can be designed for more accurate capacities. Hence, it is suggested that older buildings have been over-designed within the

context of minimum design requirements and should be able to accommodate larger green

roof loadings.



Summary of Structural Aspects

Melbourne CBD studies and suitability of building types for green roof installation

A visual survey of buildings was conducted by Wilkinson & Reed (2009) in the Melbourne CBD district to determine their potential for green roof installation based on the following criteria: position of the building, location, orientation of roof, height above ground, pitch, weight limitations, plant types, sustainability of components and level of maintenance required. Wilkinson & Reed (2009) indicated approximately 15 per cent of commercial buildings were predicted to be suitable for green roof implementation; from an observation of 526 total buildings, 78 buildings were found to be suitable for green roof addition, implying there is very limited potential for green roof addition in the Melbourne CBD.

Wilkinson & Reed (2009) also found that 61 per cent of commercial buildings in the Melbourne CBD possess concrete framing which are more suited for extensive green roof implementation. Although a building roof’s structural capacity may be adequate for green roof installation, extensive overshadowing on adjoining buildings must be considered, which may potentially make green roof implementation unsuitable for the specified building (Wilkinson & Reed, 2009). Kincaid (2003) also states that non-adjoining buildings are generally more beneficial for green roof implementation due to increased implementation accessibility and reduced disturbance to neighbouring buildings.

Though Wilkinson & Reed (2009) did conduct an extensive analysis of potential buildings that can be installed with green roofs, the study primarily considered non-structural aspects, limiting the scope of the study. Only a small number of such buildings with considerations to physical characteristics are suitable for the installation of green roof. Concrete buildings were found to have the most potential compared to brickwork and stone frames. Most roofs are either made of concrete slabs or lightweight steel decking where steel decking has a limited amount of capacity. This study provides an estimated figure of the amount of buildings that may be unsuitable due to non-structural factors. According to GHD Consulting’s (2015) Rooftop Validation and Scoring Matrix (Appendix G), the use of several non-structural factors in assessing a green roof’s implementation feasibility is required before considering matters associated within a purely structural context. Non-structurally, it has been identified that within the University of Melbourne, Parkville campus, the Doug McDonell building has been deemed very suitable for green roof implementation while the Baillieu library has been deemed unsuitable using the Scoring Matrix provided by GHD consulting.

Load Positioning

Sofi, Zhong, Lumantarna and Cameron (2014) explored the consequential load impact of green roofs on a building’s columns. The study involved categorising buildings based on their structural attributes and then selecting a sample of buildings within the University of Melbourne for design and placement of modelled columns and beams. Space GASS modelling was then used for design analysis purposes, which was further reassured through manual calculations conducted in determining the ultimate strength capacities of these columns and beams, in adherence with the structural capacity guidelines set out in Australian Standards for Concrete Structures (Australian Standards 3600, 2009). Based on this paper, it is justified that columns are able to accommodate for additional loads.

Peck & Kuhn (2003) further reassured the findings of Sofi et al. (2014) by highlighting the positive impact of placing more structurally demanding loads, such as small trees (Philippi,

2002), in close proximity or directly above columns or load bearing walls, since they are



better able to accommodate the compressive load impacts of green roofs and minimise

beam and slab bending. The study infers that the limiting factor to potential green roof

implementation on a building’s roof may be due to its limited structural capacities in its beams or slabs.

Structural Durability

The literature shows that for older buildings, durability factors need to be considered such as the corrosion of the reinforcement and the weakening of the concrete from exposure to the environment when considering the capacity of structural members of existing buildings. Almusallam et al. (1996) conducted a study on the effects of corrosion on the flexural behaviour of concrete slabs, with the proportion of corrosion measured by the loss in weight of the reinforcement. It found that a corrosion level of up to 29 per cent in the reinforced concrete caused a large reduction in the flexural strength of slabs, while a corrosion level of 60 per cent made the strength of the reinforced concrete slab similar to plain-concrete slabs. A study conducted by Ismail et al. (2010) on the compressive strength loss of concrete

structure due to long-term exposure to weathering effects discovered more than a quarter of compressive strength was lost within five years of continuous exposure. The weathering

effects were due to rainfall, wetting and drying, temperature changes, ground moisture and

chemicals in the soil. The loss of compressive strength from 32.1 kN/m2 to 23.35 kN/m2 was

observed over a 5 year period.

Conclusion from literature

The literature explored covers a wide range of studies that have been done on the different types of green roofs and their associated benefits, particularly of the environmental, social and economic nature, however there is limited information and a knowledge gap in regards to the structural considerations associated with the implementation of a green roof.

It was found that based on visual surveys, most commercial buildings in Melbourne were unsuitable for supporting a green roof primarily due to overshadowing, however for buildings that are suitable, there is a lack of technical analysis on the capacity of building structures to support extra loading. This extra loading can also vary depending on if an intensive or extensive green roof is installed, where the more lightweight extensive green roof is more desirable. This is also further reassured by GHD Consulting’s model, where only non-structural factors of a broad category are used in determining if a building is viable for green roof addition, excluding purely structural-based factors that may be important.

Determining the critical member structural capacities and load distributions are important when considering green roof installation as the literature indicated a case where existing building structures had impaired members, which caused a significant loss in durability and load capacity. This is a crucial consideration when modelling the structure to accurately represent the actual load capacity. Although these impaired columns could be strengthened, it would result in high costs which would make installing a green roof less economically feasible in this case.

However, it has also been found that within the Melbourne area, existing buildings’ columns are able to safely accommodate for green roof loadings given that they are situated in close

proximity above the columns or load-bearing walls within the buildings. This should be

considered during modelling in not only ensuring that the building roof’s critical structural

members are able to handle the green roof loadings in the first place, but to also provide a fair



comparison that can be made between the conventional green roof system and the column-

based green roof system in analysing load impacts on critical roof structural members.

Methodology and Method

A computer model and simulation will be used to impose green roof loadings, found from the

literature, onto a building’s roof attributed with detailed structural plans within the Parkville

campus of the University of Melbourne, by comparing data on loads and critical member capacities between two different green roof system models to determine the adequacy and

required extra capacity. This data will be utilised in determining the most structurally feasible

green roof model for implementation. The analysis will provide a range of structural limitations

present in existing and future buildings and provide an alternative means of bypassing these limitations. Due to time and resource constraints, the method outlined below assumes a

simplified model but ensures that the modelling and analysis are as realistic as possible in

adequately addressing the objectives of this study.

Selection of appropriate building

The Electrical and Electronic Building (Building Number: 193) within the Parkville campus of the University of Melbourne will be selected for the purpose of analysis as it is the only campus building for which accurate structural plans were able to be acquired. By taking a conservative approach, the building will be idealised by using the following design parameters and assumptions for the model: • The compressive strength of concrete will be assumed to be 50 MPa • The core will be simplified as a displacement fixed nodes • All T beams within the building will be replaced by concrete slabs of 300mm thickness • Pin joint connections are modelled for member connections • Fixed connections are assumed between all ground-level building columns and the

foundation underneath As the two green roof models depend on different member sizes, the assumptions impose a limitation to the investigation in terms of maintaining consistency in design. However, this limitation can be taken into account by analysing the relationship between differences in member sizes and the design loadings imposed by each green roof model, allowing a fair assessment of the structural feasibility for both models.

Determination of member location and sizes

The structural drawings are limited in their accuracy and clarity as they are photocopied from

the original documents, making it hard to determine the actual member location and sizes.

This made it difficult to determine actual member location and sizes, so an adoption of the

following measures will be undertaken to address for these limitations. Determination of the locations of critical members will be conducted through referring to the Architectural CAD drawings found in Appendix A, which uses a CAD scale to determine the relative locations of

these critical members. AutoCAD measurements will then be used to measure the lengths

and sizes of members by comparing these lengths with the scale of the drawing in order to ensure an accurate estimation is made, while also further double checking the measured

AutoCAD lengths and sizes with structural drawings.



Space GASS modelling

The two green roof systems will be modelled using Space GASS - a structural analysis

program to determine the loads on critical members. The nodes will be modelled based on AutoCAD measurements of the building’s columns and wall locations found in the structural

plans. From the initial nodes, a second set of nodes will be generated at an inter-storey height

of 3 metres, a simplified arbitrary value selected based on the average between two inter-

storey heights given in the structural plans, for five more floors. Columns and beams will be then connected through the use of these nodes. Sectional and material properties used for the beams can be found in Appendix B2.

Modelling of core structures

Through reference to the Architectural CAD drawings, it was observed that there were three cores within the Electrical and Electronic Building (Building Number: 193). The design of these cores will be simplified by modelling the two end cores as displacement fixed nodes whilst ignoring the core towards the centre of the building (Appendix D5). The two models will also be simplified by only considering vertical loadings, as lateral loads are assumed to be resisted by the two cores modelled and additionally, the emphasis is on vertical green roof load impacts on critical members within the building. The limitation associated with this approach is that the vertical displacement of the core may displace away from the connections to the roof slab and beams, contributing to different load distributions. This may result in significant differences between the moments in the slabs and the beams surrounding the cores. A plate is to be generated above the nodes to simulate a slab with a thickness of 300mm. A mesh will also be formed within the plate to improve the accuracy of the finite analysis modelling in Space GASS. The initial meshing will involve the application of a dummy member to split the sections into rectangular shapes along with the removal of intermediate nodes, which will ensure the connectivity of the section plates for the whole structure. The plates will then be drawn with boundaries located on the corner nodes of the split where the dummy members were removed. To ensure the building structure is connected, the connectivity function within Space GASS will be used to check each member connection. These tests are conducted to ensure the validity of the model and reduce possible errors. The model is then generated for five floors and repetitions of the tests will be performed. The sixth floor, containing a smaller plant room, will be ignored in the modelling due to time constraints and additional complexity that would otherwise be imposed on the modelling and analysis.

Loads imposed on model

The design loadings on the building includes the green roof loading, dead loads and the accessibility live load rating of 3 kPa (Standards Australia, 1971). The dead load will be modelled using the self-weight function, while other dead load components, such as furniture and partitions, were not included to simplify the model. Including these components would lead to the model inaccurately representing the building due to the capacity of members to load on these members ratio of the original building not matching the models. The reason is that the model can no longer represent the original building due to the change in member detailing between the two models causing differences in deflection relative to the original building’s deflection. As it is known that the deflection is proportional to the load distributed to each member, the capacity to load ratio of the original building no longer matches the model, resulting in the scaling of the model failing to represent the original building’s loading and deflection profiles.



Therefore, this study is limited to the comparison of the differences between the loading impact of the two green roof systems on the building’s critical members. As other dead load values are not known, any arbitrary value could be used although this may potentially alter the results if the arbitrary value is too high, as it may result in the buckling of the model’s columns. The dead loads could also be chosen to be any arbitrary value and kept consistent for the purposes of fairly comparing structural impacts between the two green roof systems. However, dead loads need to be the same as the original building’s imposed loadings in order to provide an accurate comparison but this cannot be known due to limited data on the actual building’s imposed loadings, preventing in making a fully accurate comparison. However, the live load rating was predetermined to be the accessibility load found on the green roof from literature and therefore, this load was extended to be modelled on each floor as a consistent and fixed value. The green roof design loading of 3.5 kPa will be modelled on the plate as a distributed pressure to realistically simulate the green roof loading impact on the roof slab. Loads will be transferred directly to the columns using a container that will hold the green roof, which will then be placed directly onto the columns of the modelled building (Appendix E2).

The container was kept simple and will be made out of a combination of steel beams and a

concrete slab, doing so in the consideration of time constraints. The concrete slab will be used as a deck to hold the green roof while the steel beams will be placed under the slab to support

it. Steel beams will be positioned under the roof slab in both parallel and perpendicular

directions to the slab’s length, reducing the level of potential deflection in the roof slab. The

green roof concrete slab will be placed directly on top of the steel beams with an adequate offset, where connections between all steel members are assumed to be pin connections while

the connection between the green roof concrete slab to steel beams are shear connections.

Small steel columns will then situated under the steel beams but directly above the building’s

concrete columns to ensure the green roof does not make contact with the roof slab.

Analysis and Results

Limitations

The results are limited to only the comparison between the member loads on critical columns as the model did not closely resemble the Electrical and Electronic Building (Building Number: 193). Deflection profiles that contribute to the load distribution in members would cause the estimated loads to change. Therefore, the results do not reflect the structural feasibility of the building completely unless further detailed analysis is conducted. However, the results can instead show the structural adequacy of the columns and provide an analysis of the loads experienced by the column-based green roof system.

The degree of deterioration in the columns is also unknown due to variable deterioration rates and unforeseen events causing damage to the columns. Therefore, there is a limit to the accuracy of comparing the critical columns’ capacities to the extra load capacity required, in accommodating the green roof load, due to the uncertainty of the actual columns’ capacities.

The actual degree of extra capacity that can be loaded onto columns should be determined by the structural engineer and the building’s loading conditions. These factors lead to the over-design of columns, otherwise, these columns may be designed for the purposes of resisting lateral forces, resulting in a stronger column overall. Consideration of whether the building is lateral force dominant in its design can provide valuable information on the adequacy of the building for green roof retrofit.



The large size of data on the moments, shears and deflection of the beam and slab were not used in this analysis due to time constraints. Due to the differences between the sectional and material properties, the results from this data would be unreliable in forming a basis for analysis. The data on the columns are the common components between the two green roof models, and are therefore suitable for comparison. The data shows small differences in deflection and moment values for the columns between the two systems and hence, were not considered.

Further considerations should be made for beams and slabs to ensure no significant localised

effects occur due to differences in loading, in the two scenarios, creating a potential weak

point in the structure. The loading on the slab and beam at the roof level of the conventional green roof system can potentially be significant, but is beyond the scope of this study. Other

differences could be explained due to the buckling and bending of the columns, and its

influence on the beams and slabs, as a result of the green roof design loads imposed on both

systems, however, the model demonstrated no buckling. For the column-based green roof model, as the load is directly distributed into the columns, the amount of interaction with other

members is assumed to be negligible due to the light weight of the green roof, however, further

analysis is required for heavier green roof design loads. Other considerations such as resultant

forces, lateral effects, container bending moments and P-delta effects should also be considered for more accurate results.

Analysis of force on critical members

Table 2: Critical Member analysis

Normal Building

(kN)

Conventional (kN)

Column-based (kN)

Reduced Live load - Column-

based (kN)

Green roof (kN)

Container (kN)

Member 6 exterior

5050.95 5254.04 5632.50 5374.00 203.09 378.46

Member 13

interior

953.35 1000.69 1169.93 1010.13 47.34 169.24

For comparison purposes, members 6 and 13 (Appendix D4) were selected for analysis as they had the largest difference in load between the exterior and interior columns, therefore, the most critical column case comparison for the two models. The results show that the container only weighed approximately to two to four times as much as the green roof itself. This indicates that the container may be too heavy and a lighter alternative should be adopted instead, such as a lighter and stronger decking, although this is beyond the scope of this study.

The dimensions modelled for the exterior columns were 508m x 711m, which equates to a

conservative capacity of approximately 6000 kN, calculated through the following equations:

�� ℎ �� = �� = �� (Clause 10.6.2.2 AS3600, 2009) … eq 1

�� ℎ = � �� … eq 2

Where � = 0.863 is an assumed conservative reduction factor value that accounts for the

loss of durability due to weathering, causing a loss of 86.3% within the concrete’s strength

(� is determined as the value based on the result of a test conducted by Ismail et al. (2010),

concrete capacity after 5 years of heavy corrosion)



• Initial capacity: 32.1 kN/m2

• Final Capacity: 23.35 kN/m2

• Df is determined as 27.7 kN/m2 / 32.1 kN/m2

(27.7 is the midpoint between the final and initial capacity)

For the purposes of this study, the � factor accounts for durability issues and will be used

as a reference capacity for further calculations, forming a basis of comparison between the

loads and strengths of the columns. Further reductions of strength can be found with

severely weakened columns and may not represent the actual reduction factor required for

the specified critical column used for modelling purposes. Further assumptions of short and braced columns will be also used for this factor.

The capacity calculation above has been made conservative (to appropriately compare the

effects on critical members between the two models without extending critical capacity) with consideration that the actual tributary area is larger and contributions from steel

reinforcements are to be omitted. Therefore, it is reasonable to state that the critical columns

are able to resist the green roof design loads under extremely conservative calculations.

However, a more thorough process should be conducted to accurately model an existing building and compare it with the critical capacity to determine the amount of extra loading

that can be achieved. This assumes that the material and sectional assumptions do not

severely affect the results.

Each floor has an area of approximately 787m2,resulting in an extra load on each floor of

approximately 787 kN, equating to a total assumed load of 3935 kN for five floors with an

unfactored 1 kPa live load rating allowance. Assuming a tributary area of 288 m2 for member

6 on each level, the total force imposed on the critical column is 288 kN. These calculations,

with a reduction of 1 kPa to the live load, have also been reassured through Space GASS

analysis, which provides an output of 258 kN within the critical column. Therefore, this

calculation method can be reassuredly used as a method for preliminary assessment of loads on potential existing buildings’ columns, but only when the pressures imposed on the

building’s slabs are known.

Discussion

This method can be used to compare the capacity of the actual building’s column(s) at the critical points, where the maximum difference in load distribution exists. The capacity to extra load ratio of the column-based green roof system is in the range of 6% to 13%. The actual ratio, in reality, would be lower since the columns of the proposed building have much larger capacities. A further allowance of 1 kPa yields a ratio of 3.38% to 5.4%, however, additional dead loads, such as partitions, were not considered. This provides a guide towards the range of capacity to extra load ratio values that would be needed to determine if a building is structurally capable of a green roof retrofit.

The extra loading required on the columns depends on the tributary area of the column, which can be categorised into:

• The number of container legs; • Number of building columns; and • Green roof load rating.

Peck & Kuhn (2003) found that the heavier elements should be placed closer in proximity to the columns, therefore, the placement of load elements should depend on the spacing of the columns and the tributary area. A building with closely spaced columns and/or larger columns



would be stronger, resulting in a more even load distribution, allowing the building to accommodate more mass; further optimizations can yield a lighter container and less loading on the columns. These factors can assist in the process of analysing a building's suitability for retrofit.

Preliminary feasibility

The results demonstrate the potential for a method that can be used to determine the amount of load taken by columns and can be used by owners to determine whether their building is more suitable for the conventional or column-based green roof system. The preliminary calculations depend on the following factors:

• The green roof load; • The tributary area of columns; • The number of columns; • The spacing of columns; • The capacity of columns; and • Age of building.

Green roof loads can vary from a minimum of 0.59kPa (Table 1) to a maximum value that can be determined by the capacity to extra load required ratio. The tributary area, the number of columns and spacing of columns will determine the total amount of load that is transferred into the columns. Therefore, a smaller tributary area will result in less load being distributed to the column. However, as these properties will influence the capacity of the column, they may not provide a good indication of its ability to resist the load. The age of the building would be a better indication of the current strength of the column and whether it can acquire an extra 1 KPa capacity due to differences in the design guidelines and standards. The load on the columns can be determined by estimating the green roof loading to be applied and determining the total forces acting on the installation area of the green roof. The total force can then be distributed evenly to the columns to determine the range of loading the columns will encounter, that is, a total force over the number of columns. Customisation of the green roof can be used to optimise the total force, although this is outside the scope of this study.

Other variables, such as column specifications and the age of the building, can be used to further check the feasibility of the building for green roof adaptation. The size of the columns can be used to determine a conservative capacity value using Equation 2 presented in the analysis. Although this value gives a recommended conservative factor, further considerations need to be made for the condition of columns. This assumes that the columns are experiencing a very low magnitude of moment and a high magnitude of force. The conservative capacity value can be used to compare with the extra load required for the green roof. A capacity to load ratio can be found to determine the percentage required and can be compared to a guide value based on a probability analysis and presents the opportunity for further studies to provide a process to identify the feasibility of a building for retrofit of column-based green roof systems via visual inspection and simple calculations.

This process make it more feasible for owners to determine if their building is suitable for green roof implementation, using more thorough structural checks to determine the structural feasibility of the building.

Building scope and application

The results can be extended into other similar buildings of similar age, with a maximum extra capacity requirement of 5.8%, in this example. Buildings of the same age are most likely able to carry this mass, however, heavily damaged or weakened columns may not be able to carry



such mass and strengthening of the column would be required, regardless of the green roof type. As this model and the capacity calculations are conservative, it is likely existing buildings will be able to support more load than estimated in this study.

The proposed building is symmetrical with columns are closely spaced making it easy to model the container. Therefore, buildings with similar column symmetrical arrangements could be feasible. However, buildings with asymmetrical placement of columns may overcomplicate the arrangement of the beams needed to support the container, or may not provide adequate spacing and hence, limiting the size of the container.

The results could be applied to residential buildings that have many closely spaced columns

and walls present, allowing the thickness of the decking for the container to be reduced due to smaller spans. Residential buildings above a certain height, where columns are designed

stronger than usual for lateral action, are generally more suitable for the green roof loading

to be directly transferred to the columns.

Advantages

There are many benefits of green roofs and the potential of expanding the current range of building retrofitting can make a big difference to society in terms of added value. Structurally, a column-based green roof system provides the following benefits:

Scenario 1: Critical slab and column Benefits: Requires only to strengthen the column capacity to carry the green roof.

Scenario 2: Critical slab Benefits: Does not require costly and intrusive strengthening of slab.

Scenario 3: Current limitations in range of buildings suitable for retrofitting Benefits: The range of suitable buildings is increased and the value per cost added is greater than strengthening in order to accommodate the green roof weight.

The Growing Green Guide (City of Melbourne, 2014) outlines the costs of currently existing options in the market for green roof installation and structural strengthening . The costs of a

column-based green roof system should be closer to the cost of the green roof installation

than strengthening. The additional costs involved are: the material costs, extra installation

costs and design costs. The Growing Green Guide gives a general cost of green roof at about 33% to 66% to the cost of strengthening.

Optimisation of Container

In this study, the design of the container for the purposes of analysis was kept simple using steel beams and a concrete slab. However, this combination of materials resulted in the container weighing two to four times the weight of the green roof by itself. Different materials, that are lighter and stronger, could be used for the deck to optimise the container. The shape and arrangement of the container’s supporting beams could also be further optimised to reduce the number of beams needed. It should also be noted that an even distributed load was used as a representation of the green roof’s mass, which may not be the case in reality. For areas with uneven distributed loads and/or heavy green roof elements present, these heavier loads should be placed closer to the columns to reduce the maximum moment on the container’s deck and beams.



Further Research

As previously mentioned, the study was limited in its scope due to time constraints. The results suggest that for buildings with a structure that cannot resist the extra weight of a green roof being installed, it may be feasible to instead transfer the loads directly to the columns. However, further research would be required before this practise can be considered for use.

• The load of the green roof was assumed to be even throughout the area of the roof, however, this would not be case in a practical sense. More insight is required into the weights of the individual components of a green roof, such that, green roofs can be designed with a building’s load capacity considered.

• Further research into the range of ratio values between loading and capacity may can suggest if a building is structurally capable to handle the load of a green roof. This would help streamline the process of structural analysis before installing a green roof.

• The age and condition of members of building is also an important factor to consider. The results of this study can be extended to other buildings with a similar age, however, older buildings would either be designed more conservatively and be able to resist more loading, or their ability to resist load has deteriorated over a long period of time.

• The modelling was based on a building that was mostly symmetrical with evenly spaced columns and members. However, this may not be the case for all buildings. Further research could be conducted to determine the effect the member positions and spacings will have, also considering the degree of tolerable asymmetry.

• It would be more complicated to implement a column-based system in practise and would also require more resources. A financial study would therefore be necessary in order to assess the commercial feasibility of such a system.

Conclusion

Current literature only primarily covers the general benefits of green roofs, however, there is a lack of research about how the weight of a green roof will impose on the structure on a building. Generally, green roofs are installed with the load transferred to the slab, however in some cases, the slab of the roof was not originally designed to handle the extra weight of a green roof. For these buildings, the load of the green roof could instead be transferred directly to the column.

For comparison purposes, a model of a building was developed based on the Electrical and Electronic Building (Building Number: 193) at the University of Melbourne, Parkville campus. The structure of the model building was created using Space GASS, where different loads could be imposed onto the structure. In order to transfer the load of the green roof directly onto the columns, a theoretical container was developed in holding green roof, where the model was then loaded onto the building model.

Members 6 and 13 (Appendix D4) were found to be the most critical columns and were

chosen for analysis. It was found that the capacity to extra load ratio for the column-based

green roof system was in the range of 6% to 13%. The results provide preliminary evidence

that a column-based green roof system is advantageous over the conventional green roof

system under idealised circumstances, although further research is required before such a system can be used in practise.

Reference List Almusallam, A., Al-Gahtani, A., Aziz, A., & Dakhil, F. (1996). Effect of reinforcement

corrosion on flexural behavior of concrete slabs. Journal of materials in civil

engineering, 8(3), 123-127.



Castleton, H., Stovin, V., Beck, S., & Davison, J. (2010). Green roofs: building energy

savings and the potential for retrofit. Energy and Buildings, 42, 1582-1591.

Chan, A., & Chow, T. (2013). Energy and economic performance of green roof system under

future climatic conditions in Hong Kong. Elsevier B.V.

City of Melbourne. (2014). Growing Green Guide: A Guide to Green Roofs, Walls and

Facades in Melbourne and Victoria, Australia. Melbourne: Creative Commons.

Cutlip, J. (2006). Green roofs: a sustainable technology. UC Davis Extension.

Environmental Protection Authority . (2013). Reducing Urban Heat Islands: Compendium of

Strategies. Washington DC, USA: Environmental Protection Authority .

GHD Consulting. (2015). Rooftop Validation Template Version 1 and Category Scoring

Matrix. City of Melbourne - Rooftop Adaptation Project.

Jenrick, K. (2005). Green roofs: a horticultural perspective. Royal Botanical Gardens, Kew,

UK.

Köhler, M., & Keeley, M. (2005). The green roof tradition in Germany: The example of Berlin.

Green Roofs: Ecological Design and Construction.

Koppany, P. D. (2002). Green roofs in the city environment. Department of Architecture and Building Construction, Faculty of Building and Environmental Engineering,

Sz´echenyi Istv´an University of Applied Sciences, Hungary.

Liu, K. (2012). Retrofitting existing buildings with green roofs. SABMag, Canada.

Ngan, G. (2004). Green roof policy: tools for encouraging sustainable design. British

Columbia Society of Landscape Architects, United Kingdom.

Peck, S., & Kuhn, M. (2003). Design Guidelines for Green Roofs. Ontario Association of

Architects.

Philippi, P. M. (2002). Introduction to the German FLL-Guideline for the Planning, Execution

and Upkeep of Green Roof Sites. Forschungsgesellschaft Landschaftsentwicklung

Landschaftsbau e.V.

Sofi, M., Zhong, A., Lumantarna, E., & Cameron, R. (2014). Addition of Green: re-evaluation

of building structural elements. Melbourne: Engineers Australia Convention 2014.

Standards Australia. (1971). Minimum Design Loads on Structures Part I - Dead and Live

Loads. Standards Association of Australia.

Standards Australia. (2002). Minimum Design Loads on Structures Part 1 - Dead and Live

Load. Standards Association of Australia.

Standards Australia. (2009). Concrete Structures. Standards Association of Australia.

Tam, V. W., Zhang, X., Lee, W. W., & Shen, L. (2011). Applications of Extensive Green-roof

Systems in Contributing to Sustainable Development in Densely Populated Cities: a

Hong Kong Study. Australasian Journal of Construction Economics and Building.

Wilkinson, S. J., & Reed, R. (2009). Green roof retrofit potential in the central business

district. 27(5), pp.284-301.

Worden, E., Guidry, D., Ng, A. A., & Schore, A. (2004). Green roofs in urban landscapes.

Department of Environmental Horticulture, University of Florida, United States.



Acknowledgements

Dr Massoud Sofi, Dr Elisa Lumantarna, Dr Graham Moore and GHD Consulting.



APPENDICES

Appendix A - Architectural CAD Drawings: Building Number 193

Appendix A1: Level One



Appendix A2: Level Two



Appendix A3: Level Three



Appendix A4: Level Four



Appendix A5: Level Five



Appendix B - Sectional and material properties

Appendix B1: Material properties

Material Name Young's Modulus

(MPa)

Poisson's Ratio

Mass Density (T/m3)

Thermal Coefficient

(strain/deg C)

Concrete Strength

(MPa)

Library

1 AS-CONC2009-

50

34800 0.2 2.4 0.00001 50 AustConc

2 STEEL 200000 0.25 7.85 1.17E-05 0 Metric

Appendix B2: Critical column - sectional properties

Section Name Area mm2 Torsion Constant Iyy mm

4 Izz mm4 D(m) B(m)

1 Outer Columns 3.61e05 1.74e10 1.52e10 7.77e09 508 711

2 Inner Columns 9.3e04 1.22e09 7.21e08 7.21e08 305 305



Appendix C - Column-based green roof model: different views

Appendix C1: Corner view (from top)

Appendix C2: Corner view (from bottom)



Appendix C3: Front view

Appendix C4: Side view



Appendix D - Model critical member dimensions and locations

Appendix D1: Sectional view (from top) - Roof slab

Appendix D2: Sectional view (from top) - column-based green roof model



Appendix D3: Sectional view (from bottom) - column-based green roof model

Appendix D4: Sectional view (from top) - location of critical columns



Appendix D5: Building core locations



Appendix E - Load path distributions within each model

Appendix E1: Conventional green roof scenario

Appendix E2: Column-based green roof scenario



Appendix E3: Load distribution analysis in critical columns (both models)



Appendix G - GHD Consulting Matrix Model



Appendix G - Methodology Flow Chart

a study of critical member capacities and green roof loading

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