Design Flashcards
In the Eurocodes, what are the types of ultimate and serviceability limit state?
There are three ultimate limit states that check resistance:
STR: Internal failure or excessive deformation of the structure or structural members, including footings, piles, basement walls, etc., where the strength of construction materials of the structure governs;
GEO: Failure or excessive deformation of the ground where the strengths of soil or rock are significant in providing resistance;
FAT: Fatigue failure of the structure or structural members.
There are three more ultimate limit states that check stability:
EQU: Loss of static equilibrium of the structure or any part of it considered as a rigid body, where:
- minor variations in the value or the spatial distribution of permanent actions from a single source are significant, and
- the strengths of construction materials or ground are generally not governing ;
UPL: loss of equilibrium of the structure or the ground due to uplift by water pressure (buoyancy) or other vertical actions;
HYD: hydraulic heave, internal erosion and piping in the ground caused by hydraulic gradients.
There are numerous serviceability limit states that relate to human comfort and durability. These include but are not limited to deformation (deflection, rotation, curvature), settlement, vibration, cracking, thermal transmission, acoustic transmission and temperature gradients.
How are design actions calculated in Eurocode? What impacts the partial factor used?
The total effect of actions = permanent actions + prestress actions + leading variable actions + accompanying variable actions. There are also accidental and seismic actions, although these are only considered in certain design scenarios.
Actions are modified by partial factors dependant on: the type; the limit state being checked; and whether the actions is favourable or unfavourable.
Accompanying variable actions can have additional load reduction factors based on whether they are combination, frequent or quasi-permanent actions.
See BS EN 1990-1 for more information. The above reflects equation 6.10. Note that for the structural and geotechnical ultimate limit states, the more onerous of either equation 6.10a (as per 6.10 but the leading variable action also has a load reduction factor) or 6.10b (as per 6.10 but unfavourable permanent actions have a load reduction factor) can be used instead.
What are the different RIBA phases?
0 – Strategic Definition 1 – Preparation and Brief 2 – Concept Design 3 – Developed Design 4 – Technical Design 5 – Construction 6 – Handover and Close Out 7 – In use
What are the three main stress limits for a material?
1) Ultimate/tensile limit – loads above this limit will cause a material to fail immediately.
2) Yield limit – loads above this value a cause a material to deform plastically.
3) Endurance/fatigue limit – loads above this value will cause a material to fail due to fatigue at a point dependant on load frequency and size.
What are the various parts of the Build Regulations?
- Approved Document A - Structure (A1 Loading, A2 Ground Movement, A3 Disproportionate Collapse)
- Approved Document B - Fire safety (volumes 1 and 2)
- Approved Document C - Site preparation and resistance to contaminates and moisture
- Approved Document D - Toxic substances
- Approved Document E - Resistance to sound
- Approved Document F - Ventilation
- Approved Document G - Sanitation, hot water safety and water efficiency
- Approved Document H - Drainage and waste disposal
- Approved Document J - Combustion appliances and fuel storage systems
- Approved Document K - Protection from falling, collision and impact
- Approved Document L - Conservation of fuel and power (L1A New Dwellings. L1B Existing Dwellings, L2A New Buildings, L2B Existing Buildings)
- Approved Document M - Access to and use of buildings
- Approved Document N - Glazing
- Approved Document P - Electrical safety - dwellings
- Approved Document Q - Security in dwellings
- Approved Document R - High speed electronic communications networks
What types of material design stresses are there?
The major types of stress are:
- Nominal (σ = P/A) when axially-loaded forces are pushing towards (compression) or pulling away (tension) from the centre of an object.
- Shear (τ = V/bd) or (τ = QV/Ib) when forces applied to an object are parallel to the object’s cross-section. The first equation assumes that the shear stress is acting uniformly throughout the section. However, shear stresses are actually distributed parabolically with zero at the surface (extreme fibers) and maximum at the neutral axis.
- Bending (σ[b] = My/I) when forces are applied along the length of an object.
- Torsion (T = JG[phi]/[iota]) when torque is applied to an object.
- Fatigue (numerous methods) when an object is dynamically loaded.
Where:
• σ = nominal stress
• σ[b] = bending stress
• τ = shear stress
• T = torsional stress
• P = axial force
• A = cross sectional area
• V = shear force
• b = cross section breadth
• d = cross section depth
• Q = calculated statical moment. Google it for an explanation!
• I = moment of inertia around the neutral axis
• y = vertical distance from the neutral axis
• M = applied moment
• J = torsion constant of section
• [iota] = length of object over which torsion is applied
• G = shear modulus
• [phi] = angle of twist in radians
What is limit state design? What are the differences with permissible stress design?
Limit state design focuses on strain, rather than stress. It utilises the plastic zone of a materials strength where appropriate. It also recognizes the uncertainty of different failure modes. For example, flexural capacity of a concrete beam is fairly predictable; therefore we count on 90% of the theoretical value. Shear in concrete is much less predictable; therefore we only count on 70% of the theoretical value. Limit state design produces more economical solutions, but as a result serviceability (deflection/cracking/vibration/gradient) has to be checked. It also requires significantly more calculation generally. This forms the basis of most BS EN Standards (‘Eurocodes’), which are mandated on Publicly procured projects.
Permissible stress design factors ultimate strengths to provide allowable working stresses, which are compared against unfactored loads. No plastic behaviour is generally accounted for and designs are often uneconomical as a result, although require less calculation. They can also be unsafe in some edge case scenarios. This method forms the basis of most BS Standards (‘British Standards’).
What is Young’s Modulus?
Youngs modulus is a measure of the ability of a material to withstand changes in length when under tension or compression.
• Young’s modulus = Stress / Strain. It is generally measured in Newtons per square millimetre or gigapascals (GPa)
• Stress = Force / Area. It is generally measured in Newtons per square millimetre or megapascals (MPa). Stress can be tensile, compressive, or shear.
• Stain = Extension / Original Length. It is dimensionless.
What are the different categories of check and what determines which category to be used? Would it have been appropriate to consider a different category for BSCU platform design?
0 - Restricted to standard solutions only, to ensure the conditions do not conflict with the scope or limitations of the chosen standard solution. Because this is a site issue, the check may be carried out by another member of the site or design team.
1 - Simple designs (including falsework and formwork, needling and propping to brickwork openings in single storey construction). This check may be carried out by another member of the design team.
2 - On more complex or involved designs. Designs for excavations for foundations for structural steelwork connections, for structural steelwork connections for reinforced concrete. This check should be carried out by an individual not involved in the design and not consulted by the designer.
3 - For complex or innovative designs, which result in complex sequences of moving and/or construction of either the temporary works or permanent works. This check should be carried out by another organisation.
Hierarchy of Risk management. Give an example of when you applied it at BSCU?
- Avoiding the risk.
- Evaluating the unavoidable risks.
- Combating the risks at source.
- Adapting the work to the person.
- Adapting to technical progress.
- Substitution.
- Development of an overall prevention policy.
- Collective protection measures.
- Instructions to workers.
The acronym ERICPD is often used as a simplification of the above. ERICPD stands for eliminate, reduce, isolate, control, protect and discipline. ERIC PD is also known as the general hierarchy of risk controls.
Regarding the access platform, can you talk more about you assumed the loading to convert it to a UDL? How did you assess the weight of the operatives? Where would they be standing to give the worst case? What were the boundary conditions you assumed in your analysis? How realistic was this?
6 operatives (100kg each).
Mass of the skip and material (worst case scenario as a point load) with the 2no operatives either side.
Bending moment diagram
Shear force diagram
Worst case scenario was in the middle of the deck
Pinned connections: A pinned support can resist both vertical and horizontal forces but not a moment. They will allow the structural member to rotate, but not to translate in any direction. Many connections are assumed to be pinned connections even though they might resist a small amount of moment in reality.
What information is recorded on the Temporary Works Regsiter?
Temporary works register contains: • Design brief number (for each item) and date issued • Short description of temporary works • Date required • Category of temporary works • Designer • Design Checker • Date design complete • Date design checked/approved • Erection complete and checked or “Permit to Load” “Permit to Dismantle”
How did you ensure your own competence to complete design checks? What other Cats are there and when would they be chosen?
If it was new style of design that I was not used to, I ensured I was aware of what standards I was required to design to and undertook the necessary research to ensure I was aware of what design checks were needed. I would often check this with a senior designer (not my checker) to confirm the correct understanding. Once happy, I would proceed with the design and calculations. Submit the drawings, calculations and designer’s risk assessment to the checker and answer any questions.
CAT0, CAT1, CAT2 and CAT3.
BS5975: What is the latest version, what has been updated?
BS5975:2019 replaced BS5975:2008+A1:2011. See comparison document here!
Both versions have 2 main sections:
- Procedural control of temporary works
- Falsework Design
Old version had legislation section but this is now removed.
Procedural control section: - Update primarily to take account of CDM regs – specifically interface between design of temp and perm works. Specifies the responsibility of tempo works designer to consider perm work designer and visa versa. Also for both the raise any areas of missing responsibility to PD.
- Addresses issue of sub-contractor TWS acting as the TWC for their organisation – PC’s TWC retains overall responsibility for TW on site (even for contractors appointed by client) but subcontractor can have their own procedures and appoint their own TWC.
- Responsibility is traceable from the PC’s TWC to the PC’S temporary works DI to the PC’s board of directors.
Design Section:
- Unchanged in this revision bar one safety consideration regarding fatigue.
BSCU Access Platform - Field Change notice: Did it then need to be re-checked? What CAT?
Yes. I made the re-did the calculations and marked up a drawings showing which standard had been removed. Then submitted this to the same checker and once happy with it, the TWC signed it too.
