CROSS Safety Report
Overloading a portal frame with suspended services
A reporter designing a single storey portal frame used a standard roof service loading of 0.25kN/m2. After the project was completed, a tenant for the unit took over fit out and added a considerable amount of sprinkler pipework into the roof. This created local overloading.
Key Learning Outcomes
- The weight of suspended pipes full of liquid can be significant and may exceed nominal allowances for suspended roof services
- Clip type fixings on the flanges of purlins may become overloaded and cause damage
- Heavy point loads from secondary steelwork may compromise the integrity of main portal frame steelwork
- Make sure, whenever possible, that the client/owner is aware of load allowances and passes this information on to tenants
For contractors and sub contractors:
- Check with the designer of the primary frames that applied loads from heavy pipework are acceptable
- Be aware of what nominal allowances such as 0.25kN/m2 means in practice
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This report concerns the overloading of a roof due to a concentration of services that were not allowed for in the design. A reporter designed a single storey portal frame using an allowance for roof service loading of 0.25kN/m2. After the project was completed, a tenant for the unit took over the fit out and added a considerable amount of sprinkler pipework into the roof supported from some newly added secondary steelwork. The reporter was consulted over the effect of the localised point loads applied to the main structure by the installed services and additional steelwork.
During their assessment, the reporter observed a considerable amount of other pipework in other areas of the roof that was supported directly from the cold rolled purlins, generally via a clip type fixing to the flange. In some locations, the total pipe loads were in excess of 100kg per metre run of pipe, with a significant number of pipes supported from a pair of cold rolled purlins. The reporter goes on to say that the loads, and their local effects, exceeded that generated from the 0.25kN/m2 allowance and could have led to a serviceability failure of the supporting steelwork. This could have compromised the integrity of the roof, and at the very least led to leaks. However, more seriously, it could also have led to a partial collapse of the roofing system.
the loads, and their local effects, exceeded that generated from the 0.25kN/m2 allowance and could have led to a serviceability failure of the supporting steelwork
The attachment of the mechanical services to the purlins, with very high point loadings due to the weight of the pipework being filled by water, was also not acceptable, even though the designer’s risk assessment had raised it as a matter to be considered. The reporter goes on to say that clip fixings with large loads can lead to excessive local deformation of the purlin flange and cause the mechanical services to become detached. This would lead to a progressive collapse of the supported services. Fortunately, the sprinkler pipework had not been commissioned and was not full of water, and so no adverse effects were observed to have yet resulted. The reporter adds that a comprehensive programme of remedial works was specified. These included rerouting some services where portal frame stability was impacted.
In the opinion of the reporter, a number of factors contributed to the overloading problem:
- A lack of communication between the fit out contractor and the main contractor. The fit out contractor had been employed directly by the tenant. The main contractor was not aware that the fit out was being done and had not communicated any specific risks identified by the design team to the fit out contractor
- A lack of understanding by the fit-out contractor of the need to comply with mechanical services load allowances. It became clear through communication with the fit out contractor that they likely did not understand what a service load allowance of 0.25kN/m2 meant in practical terms
- A lack of understanding of the capacity of the clip fixings. When challenged about the use of clip fixings to the purlins, the contractor said that they were adequate as the applied loads were less than the specified clip load capacity. The reporter advised that this may well have been true for the clip itself, but not so for the cold rolled purlin toe to which the clips were fixed
Following this experience, the reporter's firm reassessed what information they would provide as part of designer’s risk assessments. They decided that they would in future, issue a document giving practical examples of mechanical services to a load of 0.25kN/m2, supported off secondary steelwork.
the reporter notes that modern industrial buildings tend to have many more services than in the past
As a wider discussion point, the reporter notes that modern industrial buildings tend to have many more services than in the past. In this case, the main culprit was sprinkler pipework but the reporter's firm has seen industrial premises with sprinklers, heating, cooling, data and electrical services, all of which are deemed to be within the 0.25kN/m2 services allowance.
The reporter asks if there is an argument that this allowance is no longer fit for purpose. They wonder if the allowance should be circa 0.4kN/m2 for buildings without photovoltaic (PV) panels, and 0.55kN/m2 for buildings with PV panels. The reporter however notes that increasing service loads is likely to increase the amount of steelwork being used and therefore will have a negative impact on projects, both commercially and environmentally.
Expert Panel Comments
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This report underpins the role of designers who, before putting pen to paper and making calculations, need to open a dialogue with the client and/or architect about the function of a structure and what loads are applicable. As services are often an unknown at the early stages of a project, a prudent designer might suggest a working allowance, adding that it should be verified at a later stage.
It can be illuminating to ask students or indeed members of a construction team, what 0.25kN/m2 looks like. Certainly, the fit out contractor did not appreciate what it meant in comparison with the loads that were going to be applied.
It is essential to have good communication with the original client about the future proofing of the structure
It is always a risk that a carefully thought through engineering design is compromised by a late and ill-considered addition. There are many examples of where this has led to failure and collapse. The message from CROSS is that all changes must be approved by the designer; for example, in the case of this report, the designer of the main structure. It is essential to have good communication with the original client about the future proofing of the structure. For example, by suggesting that provision is made for additional roof mounted loads; externally for PV installations smoke vents and the like, and internally for sprinklers, ducts and other services.
The structural and services designers need to clearly describe the available capacity to the principal designer and provide clear visual information for inclusion in the Health & Safety file. Such information will be of value when changes are being proposed to the loads on a roof.
Such loads are substantial and would, in almost any circumstances, require special consideration
The reporter makes the point that services may be heavier than they used to be. Interestingly, old Institution of Structural Engineers manuals, Manual for the design of reinforced concrete (1985) and Manual for steelwork building structures (1989), as well as SCI P359 Composite design of steel frame buildings (2011) give design allowances of 0.32 to 0.45 kN/m2. The current Institution of Structural Engineers Manual for the design of steelwork building structures to EC3 (2010) gives between 0.1 to 0.5 kN/m2 for ceiling and service load, depending upon building type. The British Council for Offices recommends 0.24 to 0.49 kN/m2. These put into perspective the very high point loads mentioned by the reporter. Such loads are substantial and would, in almost any circumstances, require special consideration. Sprinkler pipes full of water are heavy!
The reporter says that high local loads distorted the lip on the cold rolled section which is potentially a serious weakening of the section buckling capacity and could compromise safety.
a matter of individual designer judgement
There is a difference in approach between those who would increase allowances to protect safety and those who want to reduce structures to the bare minimum to protect the environment. It is a matter of individual designer judgement, though the client should always be made aware of the chosen position and be in agreement.
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