CROSS Safety Report
Design responsibility for steel to concrete connections
Overview
The reporter has provided an illustrative example to support their view that the design responsibility for steel to concrete connections should be shared between the responsible designers of both materials.
Key Learning Outcomes
For designers:
- A Lead Designer is required when there are numerous interfaces between designers, to ensure the designs are properly integrated and all assumptions closed out
- The need to form connections is often the whole driver behind a design solution
- In areas where there is congestion a 3D fabrication drawing may be required to coordinate reinforcement and fixing locations
Full Report
Find out more about the Full Report
The Full Report below has been submitted to CROSS and describes the reporter’s experience. The text has been edited for clarity and to ensure anonymity and confidentiality by removing any identifiable details. If you would like to know more about our secure reporting process or submit a report yourself, please visit the reporting to CROSS-UK page.
The reporter believes design responsibility for steel to concrete connections should be shared between the responsible designers of both materials. In the event the steel fabricator is specifying connections, they add it should be the responsibility of the designer of the reinforced concrete frame to review and approve the fabricator’s design and calculations to ensure the assumptions made about the concrete frame, and loads from the items to be supported, are consistent with the overall design. They also believe the designer should check the position of anchors do not clash with reinforcement.
To illustrate their point, the reporter describes the circumstances of a multi storey reinforced concrete frame building they were involved with as part of a new team. The building had a series of steel balcony structures which were fixed to the reinforced concrete (RC) frame at each level using post-installed resin anchors. The façade consisted of a masonry brick outer leaf and a lightweight structural framing system (SFS) inner leaf.
During construction the main contractor went out of business causing the project to temporarily shut down. At the time of the pause, the RC frame had been constructed along with the steel balconies connecting into the RC frame at each level. The construction of the masonry façade had not commenced.
for many of the connections, the anchors were set out to directly clash with the main top and bottom slab reinforcement
The project re-started with a new main contractor but without the original engineering consultancy or the steelwork fabricator, who were no longer on the project. The reporter’s consultancy was appointed to provide structural engineering services on the assumption that the design was complete. The reporter, however, reviewed the construction drawings including the connections between frame and balconies. They found that, for many of the connections, the anchors were set out to directly clash with the main top and bottom slab reinforcement.
Another issue was that holes in the steel end plates were oversized with respect to the anchor diameter and Table 6.1 in Eurocode 1992-4. The fabricator may have elected to oversize the holes, the reporter suggests, to allow for tolerance during installation. Such connections are predominately working in shear and by oversizing the clearance holes the designer cannot guarantee an equal distribution of the applied shear force to each of the anchors. A possible solution to this issue is to fill or seal the oversized anchor clearance hole by using special washers but this had not been done. It was found on site that there were connections where the end plate was not directly bearing on all the anchors.
A further unsatisfactory detail was that, for some of the connections, increased tolerance had been allowed for between the concrete face and the steel end plate. This is acceptable providing the designer checks the anchors for the increased lever arm to Eurocode 1992-4. However, the reporter found anchors which would have failed in shear (Cl 6.2.2.3 (3)) because of the length of the lever arm.
connections were found to be failing when checked to Eurocode 1992-4 and strengthening works were required
A steel beam connected to the face of the first floor RC slab was intended to support three storeys of masonry above but the review found the connections had insufficient capacity. Fortunately, the masonry façade had not yet been constructed. To resolve this issue, additional proprietary masonry support brackets were installed at levels two and three to share the load. In addition, other connections were found to be failing when checked to Eurocode 1992-4 and strengthening works were required to ensure the connections had sufficient capacity. The reporter also noted a lack of redundancy with some connections.
The reporter, who was not involved in the original design, believes these connections were not actually designed and were only specified and detailed by the steel fabricator in a way that facilitated the easiest form of installation. They conclude their report with a reminder that all design should be carried out by competent professionals and should comply with the relevant codes and standards.
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Feedback
The Steel Construction Institute has published some excellent guidance relating to this, for example P102 Connections Between Steel and Other Materials and P416 The Design of Cast-in Plates.
There is actually plenty of documentation that lays out the design responsibility of steel to concrete interfaces, and more is being produced. The problem is that many engineers are unaware of it. Even on a simple steel column to concrete foundation the guidance is that the steel fabricator should propose a detail and take responsibility for the base plate, weld and steel bolt strengths but the foundation engineer should take responsibility for the concrete and any reinforcement surrounding the bolt. This is entirely logical, but I would say a majority of engineers do not know this and many simply assume that the steel fabricator has done all of the concrete checks for them.
Expert Panel Comments
Expert Panels comment on the reports we receive. They use their experience to help you understand what can be learned from the reports. If you would like to know more, please visit the CROSS-UK Expert Panels page.
This report demonstrates the need for a competent and capable lead designer who can integrate all aspects of the components forming the structural system, ensuring the designs are coordinated and with compatible assumptions. With increasingly fragmented design responsibility, in particular subcontracted design, the importance of an overview cannot be over emphasised. The involvement of the Principal Contractor in this process is also vital, in order to properly understand the constructability aspects and the need for appropriate detailing at the design stage.
The reporter also highlights the importance of the consideration of tolerances. The idealistic, perfectly detailed and coordinated CAD drawing rarely accounts for construction tolerances. Yet, lack of fit is one of the primary reasons for ad hoc alterations on site and subsequent latent defects.
The trend in recent years to minimise independent site inspection or supervision by designers has arguably led to an increase in the number of such problems not being recognised. However, with the introduction of the new Building Safety Regulator and their new powers, it is hoped that designers and contractors will become more focused on identifying safety critical matters and taking appropriate action.
the steel/concrete interface will be a key connection on which significant safety depends, this is no place to have unclear responsibility for design
While this report draws attention to the design of fixings, it is also important to design the reinforcement in the concrete element so that a failure beyond the fixing cannot occur. EC2-4 gives guidance as to when such checks are necessary and that is the case for all but the smallest of loads.
The geometric fit-up of the fixings and the reinforcing bar in the concrete has to be coordinated. This is not helped by the way reinforcing drawings are made, individual bar locations are not detailed so in general reinforcement installers cannot be held to account for precise individual bar locations. However, in areas where there is congestion something akin to a steel fabrication drawing may be required.
Frequently, the steel/concrete interface will be a key connection (e.g. on a balcony) on which significant safety depends, so this is no place to have unclear responsibility for design.
The Construction Fixings Association provides guidance in relation to EC2-4 and BS 8539:2012.
The Health and Safety Authority in Ireland has produced a Code of Practice for the Design and Installation of Anchors. This contains a lot of useful information including a section on responsibilities for each of the key duty holders. While not a requirement in the UK, it does provide useful guidance.
Finally, the report illustrates the difficulties caused by 'handovers' and the need for those who take over to satisfy themselves in relation to the design and construction they are inheriting. All too often when contractors start to have financial difficulties corners are cut in an attempt to rescue the business. Designers need to be aware and particularly vigilant in these circumstances.