Question

Are occupant-induced floor vibrations more perceptible in a steel-framed building than a building framed in other materials (concrete, wood)?

Response

No. Vibration has been studied and AISC provides accurate methods to help designers properly design a steel-framed floor to minimize perceptible vibrations.

According to ANSI/AISC 360-22 Specification for Structural Steel Buildings, Section L4: The effect of vibration on the comfort of the occupants and the function of the structure shall be considered. The sources of vibration to be considered include occupant loading, vibrating machinery, and others identified for the structure.

Where sound and noise control are tied to floor and wall assemblies and not the structural steel frame itself, the structural steel plays a much bigger part in controlling floor vibrations. As a result, AISC’s Design Specification for Structural Steel--used by structural engineers when designing structural steel buildings–includes a requirement that the effects of floor vibration be considered in the design.

AISC has developed resources to help designers evaluate their structures for vibration. When a designer checks a steel structure for vibration, they’ll be referring to AISC Design Guide 11. This design guide provides state-of-the-art design methods to ensure that vibration has been properly accounted for.

A database of 105 floor bays, 76 with complaints of lively vibration and 29 without complaints, has been studied by the authors of AISC Design Guide 11. The AISC Design Guide 11 evaluation criterion correctly predicted unsatisfactory evaluations for 74 of the 76 bays with complaints (97.4% accurate predictions). It correctly predicted satisfactory evaluations for 28 of the 29 bays without complaints (96.6% accurate predictions). If engineers are using the guidance provided in our design guide, there shouldn’t be any vibration concerns.

Quick tips for architects to consider with respect to vibration

According to AISC’s Facts for Steel Buildings - Number 5 – Vibration (Section 2.8, page 4):

• In open-area office layouts, long walking paths and walking paths perpendicular to the beam or joist span at mid-bay should be avoided. While deeper members may be required, other ways exist to mitigate occupant-induced vibrations.

• For floors supporting rhythmic activity, floor natural frequency is the most important parameter. To achieve a specific frequency, the required total load deflection magnitude is the same regardless of span. For long-span floors, while deeper members may be required, other ways exist to mitigate floor vibration.
• Computer monitors or other items supported on relatively flexible arms may jiggle, causing user complaints, although these vibrations may not be associated with floor motion.
• Vibration is usually maximal near the center of the bay, so locating sensitive equipment as close as possible to girders or columns should be considered.
  
  

Example of predicted vibration mode shape for floor framing. In this model, warm colors indicate upward deflections and cool colors indicate downward deflections. 

Question

Will using green steel make my project more expensive and take longer to procure?

Answer

No. In fact, if your project uses wide-flange members, your budget and schedule are already based on green steel.

The term “green steel” typically refers to steel produced from recycled ferrous scrap rather than mined iron ore, coke, and limestone. Electric arc furnaces (EAF) use recycled steel scrap, instead of iron ore and coke, to make new steel beams and columns. In the U.S., all hot-rolled steel members are produced in sustainable electric arc furnaces using electricity as the primary energy source.

Using scrap metal as its primary source component, the steel-making process continuously recycles steel into new structural steel products – a sustainable cradle-to-cradle life cycle.

So even if you aren’t specifically specifying green steel, as long as you’re using domestically produced steel, you’re going to get it! In fact, the average new steel member from a domestic mill contains an average 93% recycled material. And steel can be recycled over and over again with no loss of properties.

However, structural steel produced outside the U.S. still often comes from old mills that use iron ore, coke, and limestone. In addition, not all EAF mills are the same. In fact, a typical Chinese mill results in around twice the global warming potential per ton of steel as a domestic mill. That’s why we recommend always specifying domestically produced structural steel.

Choosing green steel doesn’t involve any compromises. Steel construction products made from green steel meet the same metallurgical and performance standards as products from traditional production methods, and they have the same prices and typical schedules, as well.

In addition to keeping building materials out of landfills, green steel’s global warming potential is one-quarter to one-third that of traditional production methods.

More on the sustainability of structural steel, including a Sustainability Designer Toolkit, can be found at aisc.org/sustainability.

Question

Does structural steel need to be primed or painted if it is enclosed or covered? Is corrosion a concern if the steel is not primed or painted?

Response

No, corrosion is not typically a concern with enclosed steel and there is usually no need to prime or paint steel that is enclosed or covered. Painting steel unnecessarily results in increased costs while also producing a negative environmental impact.

AISC’s longstanding recommendation is that in building structures, steel need not be primed or painted if it will be enclosed by building finish, coated with a contact-type fireproofing, or covered with concrete.

As stated in section M3.1 of the AISC Specification for Structural Steel Buildings (1993, 1999, 2005, 2010, 2016, and 2022): “Shop paint is not required unless specified by the contract documents.” The commentary then elaborates that: “The surface condition of unpainted steel framing of long-standing buildings that have been demolished has been found to be unchanged from the time of its erection, except at isolated spots where leakage may have occurred. Even in the presence of leakage, the shop coat is of minor influence (Bigos et al., 1954).”

In addition, priming or painting can have a negative effect on performance when the steel is going to be fireproofed (either by the application of a spray-applied cementitious coating or intumescent coatings) because the primer can decrease the adhesion of the fire-protective material.

There are two exceptions to the advice not to prime or paint steel in enclosed structures: corrosion protection should be specified when the critical relative humidity level is expected to be above 70% as well as in industrial structures where corroding chemicals are present.

Sketch by AISC Architecture Center

Question

Can structural steel be curved into very tight radii, such as corkscrew or helical shapes?
And if so, do you have to heat it to be able to curve it?

Response

(Supplied by Chicago Metal Rolled Products):

Question 1: Yes, steel can be curved into very tight radii.
Question 2: No, you don’t always have to heat the steel to bend or curve it.

There are many machines available today to assist in bending or rolling steel, from thin sheet metal and flat bars to large beams and tubes. By applying enough pressure—and sometimes heat—most metals can be bent relatively easily. However, the quality of a bend is influenced by many variables, and it is crucial to consider the end use of the material to determine the most suitable bending or rolling method for the desired outcome.

It’s often assumed that very tight radii require heat bending, but this is not always the case. The appropriate method depends on the customer’s needs and project goals. For example, if the steel member will be covered, there is often more flexibility regarding cosmetic distortion. However, if the steel member remains exposed as a focal point, the bender-roller team selects the bending method that will minimize or control distortion as much as possible. In some cases, this may involve induction bending or heat bending.

Advancements in induction bending technology have revolutionized the bending process. For specific applications of bent and curved steel, induction bending is often preferred—or even required—over cold forming methods. Cold forming can lead to issues such as wall thinning, rippling, and ovality in pipes and tubes due to the pressure applied by steel dies during the process. Induction bending mitigates these issues by using a unique process that, while more time-consuming, allows for tighter radius bends with minimal distortion. This capability is especially critical for large tubes and pipes requiring precise bends.

Regardless of the method, involving the bender-roller at the early design stage is essential for a smooth and successful project. This proactive approach facilitates better planning, reduces potential issues, and ensures higher-quality results and greater customer satisfaction.

Courtesy of Chicago Metal Rolled Products
This CAD drawing illustrates the capabilities of a bender-roller when rolling rectangular tubes helically to a tight radius.

The AISC Steel Solutions Center has an FAQ to address fire protection of HSS. Here's an excerpt of that FAQ.

Question

We have a steel medical office building with several stairs and elevator shaft wall corners framed with hollow structural section (HSS) columns. We’ve been informed that a one-hour fire rating is required for these HSS columns and beams framing into them. Can you provide more insight into protecting HSS against fire? 

Response

When it comes to protecting HSS against fire, three options exist: coat it, cover it, or fill it.

Sketch by AISC Architecture Center

Coat It

Intumescent Coatings

Intumescent coatings are paint-like mixtures applied to the primed steel surface. When subjected to high heat, these coatings expand to many times their original thickness, forming an insulating blanket that protects the steel member from heat. These coatings allow for fire ratings of up to four hours.

Intumescent coatings provide many benefits, including reduced weight per surface area protected, durability, aesthetic appeal, and good adhesion. Aesthetics is typically the main driver for selecting this system--steel members protected with intumescent coatings are often used in architecturally exposed structural steel (AESS) applications and can be colored if desired. New intumescent paint products can be applied off-site to save on-site construction time. Maintenance of intumescent systems--cleaning the protected members and post-installation repairs--is relatively easy.

With these advantages sometimes comes a higher cost compared to other fire protection systems, particularly for higher fire ratings. One way to control that cost is to upsize the steel and thus decrease the required thickness of the intumescent coating. This not only reduces intumescent material costs but also decreases the labor and drying times involved with the application process. Additionally, there should be enough room around the steel member for the intumescent paint to expand, should a fire make that necessary.

Spray-Applied, Fire-Resistant Materials (SFRMs)

Spray-applied, fire-resistant materials (SFRMs) insulate the structural steel from rapidly rising temperatures and are typically used if steel is hidden from view, such as above a ceiling or behind drywall.

The biggest advantages of using SFRM are speed, efficiency, and cost-effectiveness. SFRMs are field-applied, and surface preparation time is minimal, only requiring the removal of dirt, oil, grease, and loose mill scale. The application of SFRM is relatively easy and fast; however, because it is a wet process, it can impact other trades. Protecting on-site areas from overspray is typically required.

Research has shown that it is unnecessary to paint structural steel when it is protected with spray-applied fire protection materials or fully enclosed between the inside and outside walls of a building.

Cover It

Gypsum is commonly used for fire protection, and it comes in a variety of formats. Adding lightweight mineral aggregates such as vermiculite and perlite can significantly increase the effectiveness of gypsum-based fire protection systems.

Gypsum board can be installed over steel framing or furring and comes in a few different varieties. Type X wallboards have specially formulated cores that provide greater fire resistance than regular wallboards of the same thickness. Gypsum board enclosures are relatively cost-effective when compared with other fire-resistant products. Gypsum board walls and ceilings are commonly used in building projects for interior finishes; thus, upgrading to a fire-resistive gypsum assembly achieves two goals simultaneously--interior finish and fire protection.

Fill It

Round, rectangular, and square hollow structural sections (HSS) and pipe can be filled with concrete to increase their fire resistance. The HSS serves as permanent formwork for the concrete, which can be reinforced by standard bars, or by adding steel fibers to the wet concrete mix. The HSS can be filled off-site or erected and filled on-site.

During a fire, heat passes through the steel to the concrete, which serves as a heat sink. As the yield strength of the steel decreases, the load is transferred to the concrete. The steel encasement and reinforcement help limit the heat effects on the concrete, such as spalling and strength degradation. Ventilation holes in the steel encasement allow for steam to be released when the concrete is heated, relieving pressure.

This method is frequently used in exposed steel applications because the steel can be easily painted.