Ice Load on Conveyor Box Truss & Tension-only Members

[ARS97]

A few questions.

1) When designing box trusses for an elevated conveyor system, it is my opinion that consideration of ice load (ASCE 7-10 Chapter 10) would be appropriate. I consider these trusses to be "ice-sensitive" structures due to the exposure and relatively small members used. In some cases, this ice load can be appreciable, depending on the location and member profiles used. Does anyone agree, or am I over-designing?

2) What's everyone's thoughts on using tension-only members within a box truss, particularly if the members are supporting transverse loads, such as selfweight & ice?

3) Wind load pressures typically get fairly high (normally 50-65 psf for most applications that I deal with) for box trusses due to the force coefficient used. (I typically use figure 29.5-2 in ASCE 7-10.) I will simply apply this pressure to the first-exposed face of the truss as uniform loads on each member of that face. The load is obviously dependant on the members profile dimension. I've seen some engineers simply apply the wind load as nodal loads on the panel points. I guess I'm just looking for everybody's opinion on what's the most appropriate.

Thanks for taking the time..........please feel free to provide your thoughts!

 

[ARS97]

Anybody?

 

[SteelPE]

Since nobody is going to answer I will give it a try,

I'm not entirely sure what you are looking at. However I will tell you that in my 12+ year career I have rarely been concerned with ice loadings. Once when I was looking at the design of a clock tower with exposed steel framing and the other was when considering attaching a "green screen" to the side of a building I was EOR of. I said no to the "green screen" because it was being attached to the skin of a insulated metal panel system. I accommodated the loading in the design of the clock tower. This may be wrong, but I pretty much concluded that ice would become a concern if there were many densely packed items in a system (basically places for the ice to collect).

-If you feel it is a viable case then include ice loading in your analysis.
-I would be concerned about tension only members and how they distribute loading to their supports w/o failure..... I don't know how I would handle this though.... maybe take the safe way out and adjust the design accordingly. Distributing wind loads is up to you.
-Depending on the length of the members, only applying loads at panel points may create problems with the members driving the loads to the panel points. If you have a long member bending due to wind may be a controlling factor.

Just my $0.02

 

[ARS97]

I'm actually evaluating an existing design, so it's somewhat difficult to determine what the original assumptions were.

The project is located near Wheeling, WV. Not only is this a region where a higher ice load is called out in ASCE 7-10, but I've lived in southwest PA all my life........ice events aren't uncommon here. That's why I'm more inclined to consider ice in this region.

The tension-only members - I agree. I'm not a fan of them in this application, but the original designer HAD to make the assumption in order for the truss frame to be code-compliant. This truss is a 226' span gallery truss.....14' wide, 15' deep. There are L3x3x1/4 horizontal X-braces between the top chords. These L3x3x1/4 angles are 20.5' long......minimal compressive capacity at best.

On a side note - I'm not sure if any of you have heard about the recent truss collapse at the Bailey Mine in PA, but this is somewhat related. I've been evaluating an overland conveyor system on another project completed by the same contractor. Needless to say, it's astounding to see the level of negligence.

 

[SteelPE]

How old is the truss that you are evaluating? Is it new?

 

[ARS97]

Yeah.....a few years old at the most.

 

[SteelPE]

I'm not sure where ice loads were first introduced into the code. I do know that ASCE7-02 has a section on ice loading of members. Does ASCE 7 govern the design of these trusses?

 

[ARS97]

No code has been specified that I'm aware of. So, in the absence of any specified code, I used ASCE 7, since I'm not really aware of any other specification that deals with environmental loads such as this........

 

[SteelPE]

I'm just not sure there is a code that specifically addresses conveyors.

I assume there is a set of documents you are looking at. Is the code called out on those documents? It should be.

 

[ToadJones]

so the tension only members are the top and bottom horizontal truss portions of the box?

 

[ARS97]

I know it should be.......but it's not. Alot of times these coal mines do not produce very in-depth project documents, so most of the time it's left to the engineer to simply use their professional judgment. Yes, it's quite interesting sometimes.

 

[ARS97]

ToadJones - yes, except for the W6x15 compression struts that tie the chords together. The diagonal bracing members are L3x3x1/4 x-bracing and quite slender (kl/r = 415) due to the length.

 

[SteelPE]

Do the conveyors run 24-7?

 

[ARS97]

It depends......sometimes it will be used heavily, other times, nothing. Why do you ask? Fatigue issues from vibration of the slender angles?

 

[SteelPE]

I am just wondering if they will not use the system when they are having ice problems. I know some engineers I work with sometimes say that balconies can not have their full occupancy load at the time they will have the full snow load because the snow load will need to be removed in order to place the live load on (nobody large group of people are going to stand in 2' of snow and have a party).

 

[ishvaaag]

Approaches to safety in your case,

A case where cables have seemingly yielded but not broken ...
likely unsuitable for a conveyor belt...

 

[ishvaaag]

and one where the cables seem to have remained elastic under heavy loads ...

more useful, it seems.

 

[aggman]

In regards to ice loading on conveyor structures, we take the approach that they are a real load and as such must be accounted for. This means that we consider the ice load when needed on the members that will see the ice load. The ice load is a distributed load to the members that see ice. Just because eventually the load will get to the panel point does not mean that it does not induce bending or catenary tension in the member to get there first. It depends on the structure and location as to the issue of controlling the design or if it will even have any effect on the design. My experience has proven that most of the time (95% or more) it does not change anything. Most typical small box trusses it will not matter. Large gallery trusses may be a different story.

In regards to tension only members on long span box trusses. For a 226' span I am not sure I would be comfortable with tension only plan bracing, at least on the top chords. From a lateral load analysis I see no issue. From a top chord buckling perspective I am not as comfortable with it. It takes a certain stiffness and strength to brace the top chord. Certainly the L3x3x1/4 is probably strong enough in tension but you have to think about the compressive deflection of the top frame. The axial deflection of the top chord will cause the tension angles to go into compression and likely buckle based on your slenderness ratio. To brace the chords the frame would need to drift enough to remove the buckle (compressive stress) from the angle and force a tension load into it. This may be more movement than what you think. On the other hand the bottom chord will also have axial deflection. Remember that the diagonals are attached to the panel points and provide stiffness to the joint. The deflection of the chords due to axial load will induce tension in the diagonals as they try to restrain the joint from moving as well. You may find that the axial load on the diagonals is much higher than what you would anticipate if only accounting for the lateral load. The member may not fail but the connections could. This would all have to be analyzed. I have seen trusses like what you are describing and they seem to work. They may work for a lot of reasons other than what the designer was originally assuming. I do not use tension only bracing in the plan views for these type of structures. I am not personally comfortable with it based on quite of bit of research that I have reviewed. I would guess your chord forces are 400k - 500k based on what little information I have. If you model the truss in a 3D program with the tension only members it will likely give you instabilities on the P-delta analysis. This should get you thinking.

Wind loads are like ice loads and must be accounted for. Wind does not jump around and magically go to the panel point. It blows directly on the exposed members based on the angle of attack, shielding, wind speed, and shape factor. It may be reasonable to assume from an analysis standpoint that the truss is only loaded at the panel point and you may have a safe structure. The reality in the field is that the wind blows on the exposed members. From a practical standpoint we calculate the wind load and distribute it equally to all exposed members on windward and leeward faces of the box truss based on the individual exposed area of each member. It is under loading the windward face and overloading the leeward face. Our justification is that we do not know how much shielding will actually happen and at what level so no matter what we do it's a WAG at best. We just feel like we are in the ballpark on it.

From a code perspective it would not be unreasonable to assume that no code was followed and the structures were hopefully designed by an engineer that made good judgements. If the structures were not stamped by a PE it would not be uncommon that many important features would have been simple overlooked. We see this a lot. It is not uncommon to see companies using mechanical engineers to design these type of conveyors. This is not a statement to start a war about who is smarter in engineering. It is just an observation of mine that I have seen engineers designing structural engineering type structures that are not qualified to do so. It doesn't mean that a mechanical guy can't do it, but they may miss some important design issues from lack of knowledge. Don't ask me to design your huge crusher and I will not ask you to design my long span box truss. Bottom line, code or no code, you need to account for any foreseeable loads that the structure will be exposed to and have enough reserve in the structure to stay in the air and function. This is the requirement of any engineer in any field. It does not matter what you design for or what code is used. If the structure performs over it's life as expected without issues than you have done your job. If it does not then you will be dragged into depositions for three years and deal with whatever comes with a structural failure. IF you follow a code then at least you can argue that you met the standard of minimum care of the time the structure was designed. It may or may not be enough to keep you out of trouble.

 

[aggman]

One more thing I forgot to add. The following references may be useful.

1) Guidelines for Seismic Evaluation of Petrochemical Facilities - second edition
2) Wind Loads for Petrochemical and Other Industrial Structures

These are used pretty extensively in addition to the ASCE 7 and other ASCE publications in the design of bulk material handling structures.

 

[ARS97]

aggman;

I think we see things in a very similar light......

I agree 100% about the application of the ice & wind loads. I never understood the panel point load application when in reality, the load must first get there.

One thing that I'd like to hear more on is the distribution of the wind pressure to both windward & leeward faces. See, from my understanding, the force coefficients from ASCE 7-10 Figure 29.5-2 are higher than normal in order to account for the multiple surfaces. From reading the notes in Fig. 29.5-2, it almost sounds as if applying the wind pressure to the windward side only is what they intended.......but I'm not sure. I've debated this very thing in the past......so I like hearing other peoples opinion on it as well.

I'm not comfortable with tension-only bracing on a truss of this size.....but I need to have justification as to why. Technically, this is one of those "gray-areas" that would be hard to argue against due to the abstract nature of the topic.

By the way.....the chord forces are in the 350k range.....so you were close.