Design Life vs Structural Age
Design Life does NOT represent the age of a building or structure.
Often misunderstood and unfortunately used interchangably.
Design life = assumed period of time for which a structure or part will behave adequately (i.e. within parameters of safety and reliability) without requiring extensive or major repairs.
Building age = physical time that has passed between construction to current date.
Key assumptions to BS EN 1990 for design life - adequate design, supervision and execution standards are followed, as well as maintenance and usage in accordance with design assumptions.
Typical maintenance examples - regular inspections, patching up cosmetic cracks as these appear (to prevent water/moisture infiltration), repainting or cleaning exposed elements etc.
Once a structure reaches it's original design life this does NOT mean it is no longer safe.
It MAY however require more substantial repairs or a detailed structural appraisal (dilapidation survey, structural repair schedule, strengthening works potentially) to ensure it is robust.
Takeaways for Clients:
- Clearly define use cases for each level and advise engineers of requirements;
- Consider allocating additional budget for initial surveys and repair works which may be required once more structural elements are exposed; as well as potential unexpected temporary works (i.e. propping);
Takeaway for Designers:
- Consider detailed investigations and surveys prior to carrying out design works;
- Check original load assumptions and historic documentation (if available) - alternatively determine historic loads from investigations (permanent loads) and historic design codes (imposed loads);
Takeaway for Contractors:
- Allow sufficient time in the programme for investigative work and reactive repair works;
- Consider potential unscheduled hot works (for steelwork structures) for repair works with all necessary precautions;
- Consider complex temporary works may be required if historic details are missing/inaccurate.
Consider a structure - car analogy:
- requires maintenance (oil changes -> inspections/painting);
- must be used in accordance with original assumptions (driving a normal car like a race car will degrade all components at a higher rate, in the same way overloading elements beyond original assumptions may lead to deterioration/cracks etc);
- once past a certain age it may require substantial work to keep it running (engine rebuilds -> strengthening works).
Basement Waterproofing
Basement waterproofing basics:
- Client has to define requirements (Waterproofing Grade);
- Engineers and subcontractors provide solutions;
- Different types of waterproofing exist - including barriers (i.e. membrane types), structurally integral barriers (i.e. waterproof concrete) or drained cavity systems;
- Different types may be combined to achieve requirements;
See BS 8102-2022
Takeaways for Clients:
- Define your requirements - what is in the basement? How sensitive is this to moisture/water ingress?
- Can a higher requirement be localised through compartmentation or does the entire level have to meet a specific target?
Takeaway for Designers:
- Consider the ground conditions, monitored ground water level and determine your design ground water level;
- What happens if a pipe bursts in the vecinity? Can the system cope with a higher pressure temporarily?
- Consider workmanship mishaps during construction, can the system be rectified?
- Consider using thicker sections for basement/liner walls to prevent reinforcement congestion and ease workmanship requirements;
- Consider pouring sequence to prevent early age cracking and consider joint locations;
Takeaway for Contractors:
- Has the Client confirmed requirements?
- Have the Designers considered build sequence and timing of works?
- Does the ground water level on site match the site investigation report?
- If dewatering is required due to elevated ground water level, until this is turned off the wall is not exposed to water, and thus no apparent defects will be noticeable, consider alternative QA testing;