Compliance: The Problems of Ground Beams and the NHBC Standards
Ground beams supported by piles are a tried and tested foundation solution for the foundations of buildings on poor ground. Ground beams are linear in nature and were traditionally designed as if they run from Point A to Point B, Point B to Point C and so on.
However, we noticed that although there has not been a change in NHBC standards there has been a change in the schedule of conditions. Although not published in the standards the schedule on condition requires that when you want to cast ground beams in the ground they should be designed as continuous beams where they are poured as such.
Common practice was to design the ground beams as simply supported between the piles so that the cages could be fabricated in short sections and placed by hand between piles. Splice bars would then be installed between the piles to join them together over the heads of the piles.
The design of continuous ground beams is simple, but the implementation can be difficult, particularly where the reinforcement cage for the beams is too heavy to lift by hand.
It is hard to format a single continuous cage in pieces and splice it together in the trench. The breaks in the reinforcement (laps) need to be in different places in the top and bottom of the beam but the links need to be continuous along the length of the beam.
What are the alternatives? Build the beams in the trench? This raises a different set of health and safety concerns, particularly for the groundworkers who will be bent over forming the cages so that their heads are below the top of the trench.
The only realistic option for longer beam is to form the cages in one piece and lift them into position with a crane. Whilst this does resolve the issue of working in the trench there are other health and safety factors which need to be resolved by the installer of the beams.
However, continuous beams are not all bad. By asking the beam to bend between piles and over the top of the piles the bending moments, and therefore deflections, induced in the beams are reduced. Therefore, we can use shallower ground beams to carry the same load, provide material savings in terms of reinforcement and concrete placed, reduce material removed from site.
These material savings also provide a significant reduction in the carbon footprint of the building too. Given the push to make buildings more sustainable and work towards net zero carbon emissions then it’s measures such as these that are going to need to be taken to make the best use of the resources we have.
Balancing these issues with “buildability” is an art which we continue to develop and refine to try and resolve the conflicting needs of various project stakeholders. It’s an interesting and ever-changing challenge which we enjoy facing.
It seems that this particular change in conditions has been imposed by stealth as it is not in published the standards and we only found it in a Schedule of Conditions. We understand why this condition is imposed and there are sound engineering reasons for the beams to be designed and detailed continuously. However, it would benefit not only us (the designer) but the industry at large (contractors) if this change was incorporated into the standards and an appropriate guidance note released to assist with the implementation of this condition.
We have our own internal guidance for piled foundations which includes consulting with the checking structural engineer from the NHBC to work with them and find the best, safest and most cost-effective solution for our clients within the guidance and with the approval of the NHBC.
If you are struggling with the challenge of ground beams or any other structural engineering element, please call us on 01993 225 085 for an initial discussion about your project.
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Author: Charlotte Martin