HSS members used as beams 5

[Struct1206]

I've noticed recently there is very little information out there that supports HSS members used as beams. My connection design software doesn't support it and there are few connection design examples out there. AISC tabulates information (total UDL) for WF beams and other shapes but not for HSS shapes, just to name a few examples. Is there some reason why it is undesirable to use an HSS member as a beam?

 

[strucbells]

There's plenty of information regarding HSS members used as beams - check your steel manual!

Generally wide flanges or channels are preferred compared to HSS sections for use as beams because they use the material more efficiently for strong-axis bending and the connections are easier/cheaper. Thick flanges, thin webs = more material away from the neutral axis. For a given weight of beam, a wide flange beam will have significantly higher stiffness and section modulus.

However, there are cases where HSS members do make sense:

1. HSS members generally are less susceptible to lateral torsional buckling so where you don't have constant or periodic bracing from a metal deck or regular joist framing it may be more efficient to spec an HSS beam.

2. For significant biaxial bending conditions (IE a lintel over a large door in an endwall resisting wind loads OOP and gravity loads above the door) HSS sections can be more efficient than WFs.

3. Sometimes the smaller scale HSS members just fit better when working at smaller scales for non-building structures/pipe supports/etc.

 

[BridgeSmith]

Certainly HHS sections are not as efficient for members primarily bending primarily in one direction as a wide flange or I-shaped section. They are, however, typically more stable against some buckling modes, such as lateral torsional buckling. So, the lack of resources to design them as beams is likely due to their limited use as such, and also due to fewer and simpler failure modes to check.

If the HSS is a compact shape, which most are unless the wall thickness is really thin relative to the size of the section, designing it as a bending member is fairly straightforward.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[strucbells]

Since you asked about connections specifically, Chapters K and J of the AISC specification cover HSS connection limit states, and the FR moment connection chapter definitely has some HSS to WF connections that can be rotated in either direction with some rational analysis (ie HSS beam, WF column or vice versa).

Some common shear connections include a shop welded knife plate that can be field bolted to a shear tab or other connecting element, any seated connection, or a thick end plate welded all around to the tube end with a tab welded to that for mating with a shear tab or other connecting element. You will need to consider the effects of eccentricities.

NCSEA just released a free webinar (at least to members) explaining how many of the limit states that used to be in Chapter K in AISC 360-05 and 360-10 have been moved to the manual and/or rely on more general provisions in Chapter J in the updated AISC 360-16.

 

[Tomfh]

Also, HSS connections are often more of a hassle than open section connections.

 

[SlideRuleEra]

Is there some reason why it is undesirable to use an HSS member as a beam?

In an industrial or other corrosive environment (say, near the ocean), preventing and monitoring corrosion from the inside-out is not practical. Same applies even if HSS is used for columns.

http://www.SlideRuleEra.net

 

[strucbells]

SRE- I've been lurking here long enough to understand your experience level. Have you (or others on this board) come across this issue in your career?

The reason I ask is that I have come across some conflicting information on this topic. On the one hand, there are a couple of studies that seem to suggest that internal corrosion of HSS members is not an issue providing they are adequately sealed to the exterior environment, and that even in cases where the seal isn't perfect, it shouldn't be an issue (see reference to Blodgett's comments in the first paper below).

https://falconrme.com/wp-content/uploads/2019/06/Tubular-steel-vs-channel-steel-research-Part-1.pdf

https://www.aisc.org/globalassets/a...osion-of-structural-steel-closed-sections.pdf

On the other hand, it is commonly stated as a "rule of thumb" not to use sections that can't be easily inspected in corrosive environments, which makes sense.

I suspect the answer has to do with proper detailing..providing end caps and limiting penetrations through the closed sections as much as possible, but would like to hear from anyone with practical experience.

 

[r13]

On the other hand, it is commonly stated as a "rule of thumb" not to use sections that can't be easily inspected in corrosive environments, which makes sense.

Metal product often contains impurities,

Apart from gold, platinum and a few others, metals do not occur in the nature in their pure form. They are normally chemically bound to other substances in ores, such as sulphides, oxides, etc. Energy must be expended (e.g. in a blast furnace) to extract the metals from the sulphides, oxides, etc to obtain pure metals.

As all material in the universe strives to return to its lowest energy state, pure metals also strive to revert to their lowest energy state which they had as sulphides or oxides. One of the ways in which metals can revert to a low energy level is by corrosion. The products of corrosion of metals are often sulphides or oxides.

The premise of "Blodgett's comments" is key on "sealed section" without chance of replenish fresh air and allow condensation to occur, a costly proposition. Also, the galvanization process would require the caps be welded after ...

 

[SlideRuleEra]

Have you... come across this issue in your career?

Thank you. Short answer, "no", I have not encountered internal corrosion of hollow sections... but I never used or even saw hollow members at our electric generating stations.

If hollow sections were sealed, agree that internal corrosion should not be an issue. Thing is, at an essential operating industrial plant there is no reason to expect hollow members to remain sealed. Repairs and modifications are a way of life and you can't have unusual limitations (such as, no welding / drilling on columns / beams).

Most of our stations were coal fired, very corrosive - I have had to deal with repair / replacement of heavy WF sections destroyed by corrosion. Would shudder to think how a hollow section would have performed... and we may not have known what was happening until it was too late.

In another application, I'm in a coastal region with hurricane storm surge (seawater) from time to time. I have seen reports of elevated buildings partially collapsing a few years after storm surge flooded under the building and then, unseen, corroding hollow supporting members from the inside.

http://www.SlideRuleEra.net

 

[EdStainless]

Of have large enough windows in them to galv after welding.
If the closed sections are not continuously exposed to moisture and able to drain freely the risk is minimized.
I recall some overhead steel that was closed sections (chosen for good reasons).
Just because of ambient heating/cooling each day they collected enough condensate internally that they dripped all of the time.
We had one redesigned that would be open continuously, and then it was galv.
We lost space because of the size of the new beams, but we didn't have rust streaks down the side of the building.
And we didn't have to worry about how long it would last.
I'll add that we considered stainless, but the connections would have been complicated and there still would have been water dripping.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy

 

[TrussBridgeboy]

If you galvanize the tube, then sealing it shouldn't be necessary since the inside is protected by zinc. If you don't galvanize and attempt to seal a tube, I've seen many instances where water will be sucked into the tube at a weld pinhole. So it better be sealed and inspected for absolutely no pinholes (for outdoor applications). What happens is when rain hits a warm tube, it cools and slowly sucks in the moisture. If the pinhole is anywhere but the bottom, then it stays in the tube and air is blown out the hole as it warms the next day. It usually takes a year or two to fill it up enough to finally freeze and form a square into a round and split. I prefer to drill a small hole, say 1/4", in the bottom of a "sealed" tube to vent it, or inspect it thoroughly if no vent is provided.

 

[STrctPono]

I can think of very few instances where I would design an outdoor structure with HSS tubing and not hot-dip galvanize it. If they galvanize an HSS with sealed top plates, they are going to drill holes through it anyway to allow the flux solution to fill the inside. You can always plug them with zinc plugs afterwards but sealing at that point is not as big of a concern since you have some protection on the inside of your tube.

To answer the OP's original question... The reason that most people don't use HSS sections for beams is because of cost (they are more expensive pound for pound to a wide flange), reduced efficiency compared to Wide Flanges (lower section modulus compared to equal weight wide flange), and more difficulties with end connections. It's not as simple as using a single shear tab connection plate.

They are superior to wide flange beams, however, in cases where you need torsional rigidity, have buckling concerns, or perhaps are concerned about aesthetics.

 

[southard2]

For situations where you can not brace the flanges of wide flange beam HSS beams are a fantastic alternate. Under the older steel specifications there wasn't an slenderness limitations but I believe in the latest AISC specifications there are limitations on how far you can go with an HSS member and leave it laterally unbraced. I can't remember the specifics at the moment but there is a span limitation based on Iy.

I have also found them hand in situations where I can not brace the bottom flange and the beam will have considerable uplift. Again an HSS section can come in handy. Usually though you can get a wide flange section to still work. But you have to go with wide flanged shapes and often they get pretty heavy if you can't brace the bottom flange.

The other MAJOR advantage of HSS sections as beams is that they are much better when biaxially loaded which happens all the time when designing all these fancy entrance towers architects like to design these days.

Generally speaking I will connect them to columns using stiffened seated connections with top and bottom angles. This connection works well if the beam is biaxially loaded.

This is a bit out of date now

https://www.aisc.org/products/engineering-journal/lrfd-beam-tables-for-structural-tubes/

For tube to Column connections the Hollow Structural Sections Connections Manual is pretty nice. I personally don't do any real fancy connections. But this manual will help with things like connecting a wide flange beam moment connection to an HSS section, cap plates, base plates, cantilevered connections.

Usually though I'll just use stiffened seated connections. I'll use Desconwin softward just using a wideflange beam of similar size and than swap it out for an HSS shape. After all web crippling isn't going to be an issue with HSS sections and all the other checks would remain the same for the stiffened plates.



John Southard, M.S., P.E.

https://www.pdhlibrary.com/

 

[dauwerda]

strucbells, I have been in a few substations built in the 70's that utilized painted HSS, some of these structures had to be replaced due to internal corrosion. Compromised structures would exhibit some rust spotting at the base plate to column connection and upon further investigation we would discover that the entire section was compromised (I haven't seen an actual failure though). I have never seen an issue when the structures were hot dipped galvanized (base plates are not grouted, allowing for any moisture that does get inside the tube to freely drain out through the galvanizing flow holes). The utility I worked for at the time would use paint over galvanizing when doing any additional work in substations that had all painted structures. This enabled them to match the existing structures as well as ensure better corrosion protection than just the paint offered.

 

[moltenmetal]

HSS has several problems:

- it can fill with water and then freeze, so if it is ever used for beams OR columns outdoors in cold environments, it must have adequate weep holes

- it is a poor choice to use if you have to galvanize

- it has an internal surface which can corrode from the inside out, and which cannot be easily inspected

- it is an inefficient way to deploy steel if you know that the forces and moments will only act in particular directions, ever

HSS also has numerous advantages:

- although the use of steel is inefficient in terms of pounds of steel to support a given load at a given distance, you can often use a shallower member and hence it can be more space efficient

- attachment details between HSS members are relatively simple, if you use your brain when selecting the sections

- its exterior surface is very easy to paint, and to inspect afterward

- it provides lots of normal surfaces for attachment of secondary supports

- it has incredible torsional strength, so is a wonderful way to support loads which may come from numerous different directions

We use HSS sections therefore almost exclusively to build the non-building structural frameworks for our modular chemical plants, used all over the world for decades. We do have clients who insist on galvanized bolted construction, and that system is (usually) superior in corrosion resistance, but comes at a huge cost adder.

Our plants aren't designed for 50+ durability, so in practical terms we've never had an issue with a structural member corroding sufficiently to need replacement within the expected service lifetime of the plant.

Our module frames must be strong enough to be loaded and shipped laying down, then off loaded tilted up into final position. That's a complex problem when you're trying to design with members that are only strong in one direction, and invariably results in the excessive use of steel irrespective of which kind of section you use. But numerous problems encountered by the use of W and S and L and C sections are solved, trivially, by using HSS instead on our plants.