Capital Project Word of the Week - Civil/Structural Engineering

Thank you for visiting our Capital Project Word of the Week website feature. Our theme this month is Civil/Structural Engineering. We hope this provides you with some meaningful information about our industry and the services we provide. Each week we will be adding a new Word of the Week to the page. Come back frequently to see our new additions!

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Week of April 26, 2021

P-Delta Effect

[ p ] [ del-tuh ] [ ih-fekt ]

Based around deflection, the degree to which a structural element is displaced under a load, the P-Delta (P-Δ) Effect is a second-order effect on a structure that must be considered when designing a system.  This effect occurs in every structure where elements are subject to lateral load.

While the first-order effect on a structure is focused on the initial deflections and reactions in a structure caused by its vertical and lateral forces, the second-order effects (P-Delta) are the resulting deflections and reactions in a structure after analyzing it in its deflected condition.

A simplified example of this is illustrated below:

The initial loading on this column happens and is shown in the non-deflected shape.  Since there is a horizontal component to the loading (H), the column will deflect some distance to the right.  After the column deflects, it will still have the vertical load (P) applied to it.  Since the load, P, is now offset from the centerline of the column base, it results in an additional Moment acting about the base that needs to be considered.

The new Moment is calculated by multiplying P x Δ, which is where the term P-Delta Effect comes from. The taller and slenderer a structure is, the more critical this type of analysis becomes.

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Week of April 19, 2021

Allowable Strength Design (ASD) and Load Resistance Factor Design (LRFD)

[ uh-lou-uh-buhl ] [ strength ] [ dih-zahyn ] &  [ lohd ] [ ri-zis-tuhns ] [ fak-ter ] [ dih-zahyn ]

Allowable Strength Design (ASD) and Load Resistance Factor Design (LRFD) are two different design approaches used in structural engineering.

• ASD is performing calculations with un-factored loadings and a reduced nominal (allowable) strength.
• LRFD is performing calculations with factored loadings and an only slightly reduced nominal (allowable) strength.  

For example, imagine you have to design a single piece of steel to bridge over a creek, one that is strong enough for a 300 lb person to walk across.  Below are the two different ways you could design that beam.

• For this design, using ASD, we use our un-factored design load of 300lbs for the calculation, but we then reduce the nominal strength of the beam by a safety factor (1.67) recommended in the AISC Steel Construction Manual, therefore using ASD we need to pick a beam that has a strength of at least 501lbs since 501 lbs / 1.67 = 300 lbs.

• For this design, using LRFD, we factor our design load by 1.5 (depending on the type of load, different factors can be used and these usually range from 1.0 to 1.6, 1.5 was picked for this example using engineering judgement and for simplicity) so the design load will be 300lbs x 1.5 = 450lbs.  We also reduce the nominal strength of our beam by 0.9, therefore using LRFD we need to pick a beam that has a strength of at least 500lbs since 500 lbs x 0.9 = 450 lbs.

As you can see, both methods produce similar results and require a beam capable of withstanding around 500lbs.  For safety reasons, you would not want to put a beam out there that can hold up only 300 lbs before it fails, that could be dangerous for the person walking across, especially if they had a big lunch that day!  We always want a reasonable amount safety built into our calculations in order to produce safe and economical designs.

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Week of April 12, 2021

Void Box

[ void ] [ boks ]

A void box, usually made from an easily degradable material, is used to create a space between the bottom of foundations and expansive soils, or the bottom of foundations and existing underground infrastructure. Void boxes are commonly used throughout residential, commercial, and industrial settings.  

Void boxes are strong enough to withstand the initial placing of concrete but, over time, once concrete is hardened, it will deteriorate or crush when pressure is applied to them. The image below shows how void boxes can be used to resolve the most common issues when placing foundations. If the underlying soil expand upwards, the void box will crush, or will have already deteriorated.  If the foundation settles downward, the void box will do the same as previously mentioned, either crush, or will have already deteriorated.

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Week of April 5, 2021

Moment

[ moh-muhnt ]

Moment is the measure of force that causes an item to want to rotate about a specific point or axis and is a term regularly used in the field of structural engineering.  The magnitude of the moment is affected by two variables, “Force” and “Moment Arm”.  To calculate the magnitude of a moment, you multiply a “Force” times its “Moment Arm”. In industrial designs, these types of forces are seen in almost every design. Moments need to be accounted for to assure we have designed and erected a safe structural system.

Moment = Force x Distance or M = (F)(d)

Example calculation: Pushing on a 2ft long wrench with 10lbs of force to turn a nut.

The Moment calculation for this example is shown below. Forthis example, you would say you are applying 20 foot-pounds of moment on thenut.

• Moment = Force x Moment     Arm
• Moment = 10lbs x 2ft
• Moment = 20 ft-lbs

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