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 wind loads.
-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 your site.
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.