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Engineers Notes

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Posted by on in Engineers Notes

The term "factor of safety" is widely misunderstood and often misused - so that is why structural engineers do not use it!

What we have instead is a system to ensure that the structure we design will withstand all the applied forces throughout the life of the structure, while remaining serviceable and safe. It is also necessary for the structure to be economical and environmentally sound, because to provide more structural capacity than necessary is wasteful and can in some cases be unsafe.

This system is called Limit State Design, where a factor is applied to the design actions (or loads) on the structure according to their type, and then combined for comparison against the design resistance (strength or stability) of the structure.

These factors are smaller than you may think - for Eurocode design, the factor on permanent actions (generally the weight of the building itself; in British Standard design it is called Dead Load) is 1.35 and for variable actions (Live Load or Imposed Load) it is 1.5.

A combination of these actions used for design would then result in the actual forces being increased by about 40-45%, which does not leave much room for guesswork or error! Other partial factors will apply for other types of action, for example wind pressure or accidental forces, or if resisting an overturning failure. These factors can be combined in several ways and it is notionally possible to achieve an overall factor of just over 20% on the characteristic (i.e. unfactored) actions, which the competent designer would not wish to do!

Within the standards for construction materials, there are also factors related to the probability of the material being able to meet the design values, so for a consistent material like steel the material factor is quite small (1.05) but for less consistent materials such as timber, concrete or soils the factors would be correspondingly greater in order to ensure that the actual material will reliably meet the design requirement.

While nostalgia for the seemingly simpler world of British Standard design remains strong, the use of Eurocodes for design is now established and gaining ground in the UK. However, care in their use is vital and even simple beam design has passed beyond the reach of the amateur.

For professional structural design, contact Structural Engineers Cambridge.


The next phase in the collaboration between Structural Engineers Cambridge Limited and T & F Designs is under way, as Graham, Riccardo, Tracey and Debbie move in to the White Horse in Pampisford near Cambridge.

With twice the amount of Structural Engineering expertise under one roof, the White Horse is the "go to place" for the best advice for all types of construction projects, from domestic alterations, extensions and loft conversions right up to larger commercial and development projects, as well as diagnostic investigations and reports.

Now the boxes are unpacked and stowed away, it's time to get to work! Contact us on (01223) 833555 for all your Structural Engineering needs!

Posted by on in Engineers Notes


There’s a lot written on the web and elsewhere about the magical powers of lime, but there still seems to be a lot of confusion amongst bricklayers who (like me) were brought up in the Cement Age, when lime was thought to be a thing of the past, like slide rules and smallpox.


Scenario: a brick wall, built before 1920 and looking a bit tatty. Some of it might have already been re-pointed, using a cement mortar – this brittle grey hard stuff sticks out like a sore thumb and already the surrounding bricks are starting to spall and crack, and damp is penetrating to the inside of the building. What is going on?


Basically, Portland cement is chemically different from lime and its strength, rapid hardening and resistance to moisture can be damaging to traditional masonry.


Using traditional lime mortar takes time, and this is expensive – it can be tempting to add “a bit of cement” to speed things up, but this will shorten the life of the wall and do more harm than good – don’t do it!


Often, there is confusion as to what lime mortar actually is – sometimes a cement mix with a greater than normal proportion of bagged lime is called a lime mortar, when in fact it is not. In this case, the lime is added as a plasticiser to improve the workability and adhesion of the mix but the presence of cement makes it a cement mortar!


Bagged lime is similar to slaked lime, being Calcium Hydroxide, but the addition of water to quicklime (Calcium Oxide) is carefully controlled to produce a dry powder instead of a putty. It is possible to make lime putty ready for use from bagged lime, but the process of maturation over several weeks improves the particle structure of the lime and its performance in use. Hence, nowadays it is generally more convenient to buy ready-matured lime putty in plastic tubs.


Traditional lime mortar is simply slaked lime and sharp sand (no cement!); to make a good match between the original work and the repairs it is important to match the colour and texture of the locally-quarried material.


Traces of clay in the lime may add silica and alumina to make it weakly or feebly hydraulic – this is defined as the ability to set under water and is produced by a separate chemical reaction to the usual absorption of carbon dioxide from the atmosphere.


Natural hydraulic lime (NHL) is quarried from deposits (often in France) where the impurities in the limestone give the appropriate blend of strength and setting time. This is graded from NHL2 to NHL7; the weaker grades being more flexible and appropriate for re-pointing and rendering and the stronger grades used for civil engineering work such as the great harbours, locks and bridges of the 18th and 19th centuries that are still in use today.


NHL can be used, with caution, for repair work where the circumstances allow a strict authenticity to be exchanged for a reduction in setting time. However, for most bedding and pointing repairs, the mix must not be too strong. In all cases, the mortar must be weaker than the masonry units (bricks, blocks or stone) that the wall is made from. This may mean that when building a cavity wall with a block inner leaf and brick outer leaf, a different mix is required for each material.


 An analogy is that the mortar should act a bit like cartilage in a skeleton; it provides a cushion to distribute stress evenly across the masonry unit and allows for a degree of movement. Unlike cement mortar (and my knees), lime has the ability to “self-heal” by forming new bonds between particles.


To sum up: for pre-1920 masonry; cement is naughty. Even for modern brickwork, mortar should not be over-strong and the addition of lime improves workability and tolerance to movement. Maybe I should come up with a nice mnemonic, but to rhyme with lime is fine, not so easy with OPC!

From 1st January 2015, Structural Engineers Cambridge Limited will be the new company identity, taking over from Charles Tallack Engineering Consultancy.

This exciting development will facilitate the growth of the business to meet the huge demand for structural engineering expertise and helpful cost-effective advice, which has become the key to our success over the last eleven years.

Structural Engineers Cambridge Limited will continue to serve property owners, architect, builders and property professionals in and around Cambridgeshire and is under the same management and ownership as before.

This development is an exciting opportunity for us, and we look forward to sharing the news with you over the weeks to come. If you wish for more information, please email or call us on (01223) 833555.

Posted by on in Engineers Notes


A "knock-through" is the term for removing an internal wall in a building to connect two room spaces or create an open-plan area. It is one of the humblest aspects of the Structural Engineer's work, but none the less important for that.

Engineers Notes

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