Code specified live loads and potential for low carbon design

Introduction

Rising population coupled with urbanization has resulted in creating unprecedented problems for our cities. Unless appropriate urgent measures are taken immediately, these problems may result in catastrophic consequences. Globally, the built environment is responsible for consuming major raw materials and energy on the one hand and generation of solid waste and greenhouse gas emissions, on the other. Hence it has direct, complex, and long-lasting impacts on the biosphere. In order to achieve effective sustainability, a holistic cradle-to-grave life cycle assessment (LCA) methodology, which considers all phases of the building construction, should be adopted.

An important parameter affecting the economy of any building design is the load for which it is designed. Although the dead loads are calculated based on the size of members used, other loads such as live loads (also called as imposed loads), earthquake loads, wind loads, snow loads are prescribed in different national codes (for example in India, in IS 875(different parts) and IS 1893 codes).The minimum requirements pertaining to the structural safety of buildings against live loads (which is synonymous with ‘imposed loads’ and used interchangeably in the literature) are covered in IS: 875-2 (1987) [1]. A structural designer has to select an appropriate value of the imposed loads from this code, based on the use and occupancy classifications. The IS 875-Part 2 code also provides some live load reduction factors. Thus, the design engineer needs to select these loads and reduction factors based on engineering judgment and experience for the structure at hand, so that it results in safe and economical design.

The live load surveys done by several researchers in the past have shown that the actual floor live loads are well below the design values recommended in IS: 875-Part 2 (1987) or other international codes. Similar surveys done in other countries also have shown that the actual live loads are smaller than those prescribed in their national codes. It is believed that the live loads on floors and roofs prescribed in IS: 875- Part 2 (1987) have been codified based on the data available from foreign national codes and standards and other literature. The foreword to this code states that while formulating the clauses of this code, other national standards of countries like United States of America (USA), Canada, United Kingdom (UK), West Germany, Australia, New Zealand and International Organization for Standardization (ISO) standards were also examined. It needs to be noted that these other national standards have been thoroughly revised in recent years and in some cases they were also superseded by other codes of practice.

Several countries have committed to net-zero carbon emissions by 2050, with India committing by 2070. This requires several strategies and research efforts. To achieve these climate targets, it is necessary to reduce the embodied carbon of building structures by around 10% each year (Arnold et al., 2020) [2]. Lowering design live loads is the simplest action for reducing material consumption. This simple change will affect the sizing of all structural building components, requires no alternative design methods, no new construction technology, and may require only minimal coordination with other members of the design team (Hawkins et al., 2021) [3].

Live Load Surveys

Live load surveys conducted in India are much less considering the size and diversity of the country. Such surveys are used to derive representative loading parameters for several common occupancy types. Simulation is used to verify a number of probabilistic models describing the stochastic nature of the live load components. Area dependent sustained and transient load processes comprise the total load. The mode by which the maximum total load (or design load) is reached during the lifetime of a structure is not known with certainty. A probabilistic model that accounts for the relative likelihood of occurrence of each mode can be used to obtain loads corresponding to the mean of the lifetime maximum total load. Design loads can be obtained by multiplying imposed load values with the reduction factor (RF), which is a function of influence area .For small areas the effect of RF is neglected in design.

Live Loads in Residential Buildings

The first such Indian live load survey was conducted by Sridhar Rao and Purushothaman (1970) in some residential buildings of Mumbai [4]. The findings of Rao and Purushothaman (1970) in residential buildings of Mumbai city were published by Kumar and Rao (1994) [5]. The survey results of Mumbai showed maximum loads in a sample of 10 residences and not sustained imposed loads as reported in the other surveys. In addition, Kumar (2002a, 2002b) also conducted an extensive live load survey in some residential and office buildings of Kanpur city [6,7]. His work involved deriving lifetime maximum live loads, instead of measuring instantaneous live loads. All these surveys showed that the actual live loads occurring in buildings are much lower than those prescribed in the codes.

Based on the survey results obtained, Kumar (2002b) proposed following formulae for the mean of total live load, Land reduction factor, RF for residences [7].

For tenant-occupied:

E [L_t] = -0.059 + 7.934/ √A kN/m² (1)

For owner-occupied:

E [L_t]] = -0.065 + 7.972/√A kN/m² (2)

Where, A is the influence area in m² The values based on equations (1) and (2) almost coincide with each other. The following reduction factor, RF , for A> 20 m² was also proposed.

RF = -0.034 + 4.626/ √A (3)

Using the above equation, for an influence area A = 20 m ² (tributary loaded area of 10 m ² for beams), the design load for residences would be 1.72kN/m ² instead of 2.0kN/m ² currently followed in the Indian code. Further, the reduction factor based on equation (3) gives lower values of live loads than the values recommended in the Indian code.

Live Loads in Office Buildings

Ranganathan (1985,1987) carried out live load survey in three multi-storied office buildings of 20 to 40 years of age at Mumbai [8,9]. The analysis of combined data of all the buildings suggested mean life-time maximum live load of 2.48kN/m² . Srinivasa Rao and Krishnamurthy (1993) conducted load survey on live loads acting on office buildings in Chennai and observed that the 95 percent probable equivalent uniformly distributed load (EUDL) for five surveyed office buildings was 2.35kN/m ² without separate provision for store rooms [10]. Both the above-mentioned surveys of office buildings showed that the actual live load is much smaller than the 4.0kN/m ² as specified in IS:875- Part 2.

Similar load surveys have been done in the USA (McGuire and Cornell, 1974) and countries like UK (Mitchell, and Woodgate, 1971) and Sweden (Sentler, 1976) [11,12,13]. Based on a live load survey conducted in the USA, Culver (1975 and 1976) observed that the values of live loads in office buildings are least affected by geographical location, height or age of the building, and the type of occupancies (government or private) [14,15]. However, all these load surveys involved observation of load intensity at an instant of time, i.e. only at the time of the observation. Moreover, floor loads in a particular structure may vary randomly with time, and it is important to know the magnitude of these lifetime loads. However, very little information of this kind is available. Hence the codes often specify only extreme values (which occur rarely and may be regarded as lifetime maximum loads). Thus, the present live load specified in IS:875 (Part 2):1987, may be considered to represent characteristic live loads which have 95% probability of not being exceeded over a 50-year period, based on Weibull type distribution (Walpole and Myers 1978) [16].

Kumar (2002 _c ) proposed the following formulae for the mean of total load, L_t, and RF in office buildings [17].

Using gamma distribution:

E [L_t ] = 0.703 + 9.498/√A kN/m² (4)

Using log-normal distribution:

E [L_t] = 0.623 + 8.754/√A kN/m² (5)

The following equation was also proposed for the reduction factor, RF, for A > 20 m ²

RF = 0.25 + 3.36/√A (6)

Similar to the residential buildings, for an influence area of A = 20 m ² (tributary loaded area of 10 m ² for beams), the design live load for offices based on the survey results of the Kanpur city calculated using equation (4) is 2.83 kN/m ² instead of the current 4.0 kN/m ² specified in the Indian code. Further, the reduction factor proposed here gives lower values of live loads than the values recommended in the Indian code.

From the above discussions, it is clear that the design loads proposed in the Indian code are far in excess of the live loads obtained from the limited Indian load surveys. The reduction formula proposed by the Indian code differs considerably from the formulae based on actual live load surveys. The statistical analysis suggests that different types of occupancy require different rates of reduction, a fact normally ignored in the codes and standards (Jadon et al., 2022) [18].

Comparison Live Loads in Different Codes

A comparison of minimum recommended values of live loads for office buildings prescribed in some selected codes is given in Table 1 Reference should be made to the respective codes for the prescribed minimum live loads for other types of occupancy-for example, Table 1 of IS 875-Part 2:1987. Jadon et al., 2022 [18] and https://www.meicon.net/global-loading-codes also provide comparison of minimum live loads of other types of occupancy of some selected codes. It has to be noted that the loads specified in the IS 875-Part 2:1987 code does not include extra loads occurring during the construction. Hence, Note 7 below Table 1 of IS 875-Part 2:1987, suggests that close supervision during the construction is essential to ensure that overloading of the building due to stacking of building materials or use of equipment (for example, cranes and trucks) during construction or loads which may be induced by floor-to-floor propping in multistoried buildings does not occur.

Table 1 Comparison of minimum recommended live loads for office buildings in different Codes

The average office live loads of all countries for minimum area load, area load (heavily loaded space), minimum point load, and point load in heavily loaded rooms are 2.38kN/m ², 3.51kN/m ², 1.81kN and 5.36kN respectively and the median live loads are 2.40kN/m ², 3.00kN/m ², 1.50kN, and 4.50kN respectively ( www.meicon.net/real-floor-loading). An extensive review of real floor loading studies, where the real loading in offices was measured manually over 2.5 million square meters of floor area, was presented by A.E. Peters ( www.meicon.net/real-floor-loading). Based on an area-weighted calculation, the mean load was found to be 0.60kN/m ², with a standard deviation of 0.34kN/m ². It was also found that 99.97% of the measured floor area had a load below the code specified 2.5kN/m ². These studies also highlight a tendency for higher variability over smaller sampling areas. It has been observed that structural failures occur rarely by the under-estimation of live loads, but are due to other reasons such as design errors, improper detailing, construction error, poor maintenance, or extreme scenarios such as blasts, impacts or earthquakes (Wardhana and Hadipriono, 2003 and IStructE, 2021) [23,24]. In rare cases, overdesign may inadvertently act as a buffer against these failures, but in the vast majority of structures it remains severely under-utilized. It has to be noted that there is no evidence to suggest that countries with lower live load requirements experience more structural failures, which justifies that there is a need for reducing the values of live loads in the Indian and other codes which have specified a live load of 2.40kN/m ² or more.

Reduction Factor For Live Loads

From Table 1, it is seen that all the codes give conservative values of live loads. Considering a space of 1m x 1m, and 4 people of 75 kg weight standing in that space (as shown in Fig.1), the total load will be 300kg or 3.0kN/m ² (It has to be noted that even for residential buildings IS 875-Part 2:1987 stipulates a live load of 2.0kN/m ²). Such a uniformly distributed load acting on all floors and in all rooms of the building at the same time is not possible. Hence, some reduction factor has been suggested in different codes, as given in Table 2 . In addition, clause 3.2.2 of IS 875-Part 2:1987 allows a reduction of 5 percent for each 50 m2 area, subject to a maximum reduction of 25 percent, when a single span of beam, girder or truss supports greater than 50 m ² of floor at one general level.

Fig. 1 Live load specified for office in codes of 3.0-5.0kN/m ², represents more than four people in a square meter

It is very important to understand the reduction factor in this table correctly. It means that if the column under consideration has only one slab above then no live load reduction is applicable. If there are 2 floors/slabs above, then 10% of live load can be reduced in the column in the lower level. If there are more than 10 floors above, then 50% of live load can be reduced

Table 2 Reduction factor for live loads in different codes

Loads Due to Light Partitions

In office and other buildings where actual loads due to light partitions cannot be assessed at the time of designing, the Indian code suggests an additional uniformly distributed load of not less than 339 percent of weight per metre run of finished partitions, subject to a minimum of 1kN/m ², provided total weight of partition walls does not exceed 1.5kN/m ² and the total weight per metre length is not greater than 4.0kN. The ASCE/SEI 7-22 code suggests that in buildings where partition locations are subjected to change, a partition load of not less than 0.72kN/m ² should be used [19]. Whereas, the Eurocode EN 1991-1-1:2002 specifies self weight of movable partitions as 0.5, 0.8 and 1.2kN/m ² for partitions with a self weight of less than or equal to 1.0 kN/m, between 1.0 to 2.0kN/m and greater than 2.0 but less than 3.0kN/m per wall length, respectively [25]. However, a minimum value of 0.5kN/m ² only is suggested by AS/NZS 1170.1: 2002 for most partitions fabricated from studs supporting glass, plywood and plaster board [21]. Comparing these values suggested in other country codes, the value recommended by the Indian code of 1.0 kN/m ² is high and can be reduced.

Potential For Carbon Reduction

From the above discussions, it is clear that lowering live loads will lead to weight reduction in structures and savings in foundations. In addition, lowering the weight will also be beneficial for earthquake resistance design. However, the savings in embodied carbon when the live loads are reduced are not clear from the above discussions. Hawkins, et al., 2021considered a hypothetical four-storey building with reinforced concrete flat slabs and a raft foundation as shown in Fig. 2(a), to show such savings in upfront embodied carbon [3]. They designed this building according to the provisions of Eurocode 2 and optimized the sections for minimum embodied carbon using the generative design software PANDA (Dunant, et al., 2021) [26].

Fig. 2 Variation of embodied carbon with respect to live load and span for a four-storey reinforced concrete building with flat slabs (Source: Hawkins, et al., 2021[3])

Gibbons and Orr, 2020 first designed the building for a live load of 3.0kN/m ², considering 9m spans and the variation of embodied carbon was found as shown in Fig. 2(b) [27]. From this figure it is seen that a linear relationship exists between live loading and embodied carbon, with each reduction in live load by 1.0kN/m ² results in the saving of 12.6kgCO2e/m ² (4% reduction over the base case). The majority of this saving was found to be due to the savings of concrete in the floor (6.2kgCO _2e/m ²) and the foundations (5.5kgCO _2e/m ²).

It was also found that by reducing the spans more carbon reduction is possible, as shown in Fig. 2(c), and this reduction is much greater than that obtained while reducing live loads. For example, reducing the span from 9m to 8m resulted in a saving in carbon emission of 41.7kgCO2e/m ² (13.1%), primarily due to a reduction in the slab thickness. In 2020, it was demonstrated that changes in material specification, floor system and foundation type could also result in more carbon savings (Roynon, 2020) [28].The carbon savings of 12.6kgCO2e/m ² due to the reduction 1.0kN/m ² live load may not look considerable. However, if all of the 5.3billion square meters of buildings constructed annually in the UK reduced their live load by 1.0kN/m ², the total embodied carbon saving would amount to 67MtCO ₂, which is greater than the total annual electricity consumption of the UK!

Summary and Conclusions

Previous load surveys conducted at different locations in the world show that actual live loads in buildings are consistently and significantly lower than those prescribed as design live loads in different national codes of practice. The design live loads proposed in the Indian code, IS 875-Part 2, are far in excess of the total maximum load obtained from the Indian load surveys. Based on survey results, different reduced live loads as well as reduction factors for different building occupancies have already been proposed. In addition, the live loads of roofs not accessible except for maintenance and repair work can also be reduced based on the recommendations of latest standards of other countries. Similar reduction in live load due to partitions is also possible.

Several countries have committed to net-zero carbon emissions by 2050, with India committing by 2070. This requires several strategies and research efforts. To achieve these climate targets, it is necessary to reduce the embodied carbon of building structures by 10% each year. In this connection, it is notable that adopting the code specified live loads has a direct impact on embodied carbon. Reducing design live loads is one of the quickest and simplest actions for reducing material consumption and can impact carbon emissions. This simple change will affect the sizing of all structural building components, requires no alternative design methods, no new construction technology, and may require only minimal coordination with other members of the design team. Any possible reduction in design live load may lead to reduced dead loads and earthquake loads, and thereby resulting in overall savings in the construction cost. As shown by Hawkins, et al., 2021 [3], even small reductions in carbon emissions in a building, will result in huge savings, if the whole country is considered. Hence, it is important to conduct more statistical load surveys in order to reduce minimum design live loads considerably in codes consistent with the safety requirements.

References

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