Design Loads Flashcards
two main sources of loads
gravity and environmental
what determines the loads the structure will potentially be exposed to?
intended use and location
minimum load
is representative of the loading that the structure will potentially experience
Describe heirachy of building controls
building act 2004
Building Regulations (building code)
Standards
pyramid order
What is the building code
It sets the minimum performance standards buildings must meet
e.g. structural stability, fire safety, durability, access…
NOTE: doesn’t include how to meet these requirements
Building Controls (3)
- building act 2004
- building regulations 1992
- Building code
What is the building act?
Is the primary legislation governing the building and construction industry. The Act sets out the rules for the construction, alteration, demolition and maintenance of new and existing buildings in New Zealand
Purpose of the building act
- people can use buildings safely and without endangering their health
- buildings have attributes that contribute appropriately to the health, physical independence and
wellbeing of the people who use them - people who use a building can escape from the building if it is on fire
- buildings are designed, constructed and able to be used in ways that promote sustainable development.
three levels of clauses in the building code
- objective
- functional requirement
- performance
ps1
ps2
ps4
design
design review
construction review
Purpose of design
capacity must equal or exceed demand
or
design strength must equal or exceed design actions
Ultimate limit state
is the upper bound.
We check fracture, buckling, overturning, yeild…
less than a 5% probability of the loads being exceeded in the 50yr design life.
Ultimate Limit State Design brings together the demand and the capacity, using load factors on the
demand, and strength reduction factors on the capacity
Serviceability Limit state
lower bound
We check excessive deflection, cracking, vibration…
the load factors for the serviceability cases are such that there is a 5% change of the magnitude of the action being exceeded in any year.
When should the imposed action be applied?
When its application makes the design actions (moments/forces) more critical
nominal strength
used to further reduce the probability of structural collapse.
There is a 95% probability that the
actual strength will exceed it
What is Load and Resistance Factored Design (LRFD)
Want to ensure that the ‘gap’ between the nominal load and the nominal strength is acceptable, and we call this a ‘Factor of Safety’.
To achieve this acceptable F.o.S we apply Load Factors to the nominal design loads, and Strength Reduction Factors to the nominal strength
design strength
is the nominal strength reduced by the strength reduction factor YSn
describe seismic design philosophy
Thinking about our choice of design loads, the potential structural configuration and materials, and our analytical techniques and design procedures that we will employ.
seismic design philosophy prinicples
- A minor earthquake should cause no damage
- A moderate earthquake may cause repairable damage
- A large earthquake can cause controlled extensive damage but the building shall not collapse, people shall still be able to escape the building, and there should be no loss of life.
earthquake excitation and structures response
Acceleration of the ground. Response of a structure to an earthquake only depends on natural frequency and damping ratio.
Why is it called a SDOF system
The number of degrees of freedom needed to define the displaced position of all the masses relative to
their original position defines how we refer to a system.
Natural frequency (period)
is a function of the stiffness of a structure and the mass of the structure
Ductility
is the structures ability to undergo large deformations without collapsing
Why is ductility desireable
- Ductile components of buildings can absorb energy from earthquake shaking.
- Ductile buildings are designed for lower seismic forces resulting
in less material and therefore less resources. - Designed to undergo significant deformation and therefore will not fail in a brittle sudden manor.
- Ductile buildings have built-in protection for a larger than predicted earthquakes
