The question is: "Do you need an engineer for
your project?" Before you decide on a certain
plan, contact you local building officials and ask
if they require engineered plans. Often they need
to look at the plans before they can make an assessment.
Complicated designs with many point loads or stick
framed roofs are likely candidates for structural
engineering. Sometimes you will just need to "size"
a couple of beams.
Beams, roofs and floors must be structured to carry
a load. Loads are generally broken into the following
categories: live loads, dead loads, snow loads and
-Is a load imposed on a structure from
occupancy and use of the building. EX. People or equipment
inside of a building.
-The total weight of buildings structural
components, fixtures and permanently attached equipment.
EX. The shingles on a roof are part of the dead load
imposed on the roof.
- The load calculated into the structure
to resist the horizontal force of wind.
-The live load calculated into the structure
that accounts for the weight of snow. Usually dictated
by local building codes.
Roof trusses are the structural member that forms
the roof. Trusses are generally installed 24"
o.c. (on center). Trusses allow an engineer to create
a member that is stronger than typical stick framing
while using less lumber or smaller dimensioned lumber,
which is less expensive. Using trusses is generally
cheaper than stick framing the roof.
Click on the image below
to view typical roof truss
A crane is required to lift the trusses on top of
the exterior bearing walls. Roofs must meet certain
load requirements. The truss company will certify
that the trusses have been engineered to a hold a
specified load. The city building dept. can give you
information on required snow and wind loads for your
area. Make sure that the truss company has this information
before they begin building your trusses. During framing
inspection, the inspector will usually ask for the
truss engineering calculations. These calculations
will need to be stamped by a licensed engineer (P.E.).
Typically the truss manufacturer will have their staff
engineer stamp the drawings.
Under normal residential loading 2x10, 2x12 and "I"
joists have standard allowable spans. Check with your
local building officials or I joist manufacturers
for specific information on allowable spans. The spans
will change based on the distance that that the joists
are spaced. Tyipcally the joists are spaced at 16"
o.c. However, sometimes they will be spaced at 12"
or 19.3" o.c. Roof rafters or trusses are typically
are spaced 24" o.c.
Engineers calculate loads and size posts and beams
accordingly. If you look at older homes you will often
notice that the garage door header has sagged because
it was not properly sized..
Geotechnical engineers investigate soil conditions
to determine if the soil can support the planned structure.
Even if your house is structurally sound, poor soil
can render it worthless. Soils hold a varying amount
of weight depending on their mechanical structure.
If the weight of your house ( psf-lbs. per square
foot) exceeds the bearing capacity of the soil, your
house will sink into the earth.
It is difficult to know
if you need a geotechnical engineer for your project.
It is safe to say that if you are building on bedrock,
shale, large stones or gravel you probably won't need
an engineer. However, if you soil is made of fine
silt, sand, clay or a mixture to these, it is probably
a good idea to have an engineer visit your site. Often
an engineer familiar with the area can quickly determine
soil conditions. If he is unsure he may recommend
a soils test. If a soils test reveals that your proposed
site has collapsible soil (low bearing weight soils)
or expandable soil. The engineer can usually design
a foundation that will prevent your house from sinking
or heaving. Designing a house for poor soil conditions
can add several thousands of dollars to the cost.
Often the local building official will require a geotechnical
investigation in areas known to have poor soil. If
you're are unsure it is best to have an engineer visit
Check with the city building officials about structural
and soils engineering requirements prior to selecting
a house design. Engineering requirements may affect
the plan that you choose and the cost to build the
house. Engineering specs form part of the construction
documentation. Make sure that engineering specifications
are attached to plans before they are sent out for
bids. An engineered house will give you peace of mind
and save you potential catastrophes in the future.
Evidence that a house has been engineered will positively
affect resale value.