Chapter 2
-- Specifications. Loads and Methods of Design
Specifications and Building Codes: are good guides to the designer, they secure
structural safety, and protect the public. Municipalities and state governments develop the
codes that engineers have to go by while designing. Most government agencies get their
codes from organizations that develop specifications for guiding the designer. AISC and
AASHTO are 2 examples.
Loads: To be able to design a safe, efficient and economical structure, we have to have an
accurate idea of the types of loads the structure will be exposed to during its life time, and
what combinations of these loads can occur at the same time.
TyPes of Loads:
l) Dead Loads: have a constant magnitude and a fixed position.
That includes the structures own weight and anyt:hiRg'fixed to
it. However, to estimate the structures weight we have to mow
what members are being used. Therefore, we assume the
members then check our results. The more experience the
designer has, the lower the number of member estimates he has
to do.
2) Live loads: change in magnitude and position. If it is not a dead
load then it is a live load. Live loads are of2 types: a) moving
loads that move by their own power (cars and trucks). b)
movable loads (furniture).
Few examples of live loads are:
a) floor loads: measured in Ib/ft;t'2.Different types of
structures have different floor load requirements. For
example: 40 Ib/ftl\2 for appartements and 100 Ib/ftl\2 for
office lobbies.
b) Snow and ice: one inch of snow is equivalent to 0.5 Ib/ftI\2.
Normal values range from lO to 40 Ib/ftl\2 for flat to
slopped roofs up to 45° angle.
c) Rain especially on flat roofs because ponding develops
causing deflections.
d) Traffic loads for bridges.
e) Impact loads: such as falling objects or sudden car braking.
£) Lateral loads: such as wind, which changes with height,
geographic location, surrounding structures. .. Wind loads
should be designed foy if height of structure divided by the
least lateral dimension is greater than 2. Wind acts like
pressure and on a vertical surface can be estimated to be
P(lb/ftI\2) = 0.002558CsV1\2 where Cs is a shape coefficient
and V is wind velocity in miles/hour.
- Earthquakes are another example of impact loads. They
create seismic forces. This horizontal acceleration of the
grotUldneeds to be considered in design. The effect of
earthquakes on buildings depends on the mass distribution
of the buildings above the level being considered, and the
ability of the soil to withstand the lateral motion.
g) longitudinal loads: such as sudden stopping of trains or
trucks on bridges.
h) Other live loads: soil pressure on walls or foundations,
water on dams, explosions, thermal forces due to
temperature changes.. ..
Selection of Design Loads: besides all the specifications and building codes available, an
engineers experience and insight in the future helps him select design loads accurately.
Elastic Design: or allowable stress design or working stress design. In these cases the
loads are estimated and the members designed according to their allowable stresses (a
fraction of the minimum yield stress of steel). Method described in appendix A.
Plastic design: method estimates the loads and multiply them by a safety factor and
members are designed based on collapse strength. Therefore the steel is used to its
maximum limit making the approach more economical.
Load and Resistance Factor Design (LRFD): method is based on a limit state philosophy.
Limit state: a structure or a part of the structure does not do its function. It is devided into
two categories: 1) Strength limit state based on the structures load carrying capacity,
buckling strength, plastic strength, fatigue and fracture. 2) Serviceability limit state based
on how the structure act tUldernormal loads, such as deflections, cracking, vibrations,
and slipping.
In the LRFD method the working or service loads are multiplied by a safety factor,
usually greater than 1, and the structure is designed to have an ultimate strength sufficient
to support the factored load.
Ultimate strength = nominal strength or theoretical strength x 0
Where 0 is less than one to account for possible tUlcertaintiesin design or
materials.
Load Factors: increase the values of the loads to accotUltfor tUlcertainties.Since dead
loads are estimated more accurately than live loads, their factors are smaller than those of
live loads. The LRFD manual provides many load formulas for various load
combinations. For example: the usual load combinations used are given by equations
A4-1 and A4-2.
