Chapter 2: Specifications, Loads And Methods Of Design Flashcards
adopted by nearly all building codes;
Standard setting body of steels
AISC ( American Institute of Steel Construction)
adopted by nearly all state highway and transportation departments
AASHTO (American Association of State Highways and Transportations Officials)
they are written, not for the purpose of restricting engineers, but for the purpose of protecting the public
Specifications
The intent of Specifications is that the loading used for design be the one that causes the largest stresses
TRUE
This was developed because of the need for a modern building code that emphasizes performance
International Building Code (IBC)
IBC is intended to provide a model set of regulations to safeguard the public in all communities.
TRUE
an up-to-date structural code addressing the design and installation of structural systems through requirements emphasizing performance through various model codes/regulations, generally from the United States, to safeguard the public health and safety nationwide
National Structural Code of the Philippines (NSCP)
establishes minimum requirements for structural systems using prescriptive and performance-based provisions
NSCP
What is the most difficult and most important task of a structural engineer?
Accurate estimation of Loads
what are the three classification of loads according to their character and duration of application.
- Dead Loads
- Live Loads
- Environmental Loads
-loads of constant magnitude that remain in one position
-consist of the structural frame’s own weight and other loads that are permanently attached to the frame
Dead Loads
Example of dead loads
for a steel-frame building, the frame, walls, floors, roof, plumbing, and fixtures
loads that may change in position and magnitude;
caused when a structure is occupied, used, and maintained
Live Loads
Classification of Live loads;
Moving and movable loads
live loads that move under their own power, such as trucks, people, and cranes
Moving Loads
live loads that may be moved, such as furniture and warehouse materials
Movable loads
Other types of live loads:
Floor loads
Traffic Loads for bridges
Impact Loads
Longitudinal Loads
bridges are subjected to series of concentrated loads of varying
magnitude caused by groups of truck or train wheels
Traffic Loads for bridges
are caused by the vibration of moving or movable loads
Impact Loads
stopping a train on a railroad bridge or a truck on a highway bridge causes longitudinal forces to be applied
Longitudinal loads
exertion of lateral earth pressures on walls or upward pressures on foundations
Soil Pressure
water pressure on dams, inertia forces of large bodies of water during earthquakes, and uplift pressures on tanks and basement
structures
Hydrostatic pressures
Cause by explosions, sonic booms, military weapons
Blast loads
This is due to the changes in temperature, causing structural deformations and resulting structural forces
Thermal forces
Those on curved bridges and caused by trucks and trains or similar effects on roller coasters
Centrifugal forces
This load is caused by the environment in which a particular structure is located.
Environmental loads
This is under rain loads. This is the result if water on a flat roof accumulates faster that it runs off.
Ponding
wind forces act as pressures on vertical windward surfaces, pressures or suction on sloping windward surfaces (depending on the slope), and suction on flat surfaces and on leeward vertical and sloping surfaces (due to the creation of negative pressures or vacuums)
Wind Loads
there is an acceleration of the ground surface motion; vertical and horizontal components – the vertical component is assumed to be negligible
Earthquake loads
It is needed for the expected effects of an earthquake should include a study of the structure’s response to the ground motion caused by the earthquake
Structural analysis
the movement or displacement of one story of a building with respect to the floor above or below
Drift
two acceptable methods for designing structural steel members and their connections
LRFD (Load and Resistance Factor Design)
ASD (Allowable Strength Design)
This provides boundaries of structural usefulness
Limit state
used to describe a condition at which a structure or part of a structure ceases to perform its intended function
Limit state
define load-carrying capacity, including excessive yielding, fracture, buckling, fatigue, and gross rigid body motion
Strength limit states
Define performance, including deflection, cracking, slipping, vibration, and deterioration
Serviceability limit states
Its calculated theoretical strength, with no safety factors (Ω) or resistance factors (𝜙) applied
The nominal strength of a member
resistance factor, usually less than 1.0, is multiplied by the nominal strength of a member
LRFD method
nominal strength is divided by a safety factor, usually greater than 1.0
ASD method
possible service load groups are formed, and each service load is multiplied by a load factor, normally larger than 1.0, to account for the uncertainty of that particular load
LRFD Load Combinations
the resulting linear combination of service loads in a group, each multiplied by its respective load factor
Factored Load
used to compute the moments, shears, and other forces in the structure
Largest values determined
(reduction factor 𝜙)(nominal strength of a member) ≥ computed
factored force in member, 𝑅𝑢
or 𝜙𝑅𝑛 ≥ 𝑅𝑢
LRFD
the service loads are generally not multiplied by load factors or safety factors
-loads are summed up, as is, for various combinations, and the largest values obtained are used to compute the forces in the members
ASD Load Combinations
nominal strength of a member safety factor Ω
largest computed force in ≥ member, 𝑅𝑢
Or 𝑅𝑛 ≥ 𝑅𝑎
ASD
both LRFD and ASD have goals to obtain a numerical margin between resistance and load that will result in an acceptably small chance of unacceptable structural response
in general, Ω = 1.5 𝜙
where Ω = safety factor, a number usually greater than 1.0 used in ASD 𝜙 = resistance factor, a number usually less than 1.0 used in LRFD
FACTOR of SAFETY
