Concrete Encased Steel Beam 1

[Brandon]

I have been tasked with the review of a soldier pile type wall design wherein the design-build contractor is proposing to use steel railroad rails placed into a drilled hole and filled with structural concrete. I am trying to determine if this can be treated as a composite beam based on the latest AISC Specifications.

The drill hole is 36-inch diameter, with a 132 lb/yd steel rail placed near the tension face with 3" cover. The depth of the steel rail is 7.125 inches, and based on my preliminary calcs, assuming a cracked composite section, the neutral axis is located such that the steel rail is located entirely within the tension zone, i.e. the entire steel rail is in tension. I have attached a section. No shear connectors, ties, or longitudinal bars are proposed.

Referring to AISC Specification, 14th Ed., it appears that this would be considered an encased beam, the flexural strength of which is covered in section I3.3. In that section, three methods are presented to determine flexural strength - (a) yielding based on elastic stress distribution on the composite section, (b) yielding based on a plastic stress distribution on the steel section alone, or (c) yielding based on a plastic stress distribution on the composite section provided steel anchors are provided. This seems to imply that shear anchors (i.e. studs) are not needed if either method (a) or (b) is used. The commentary for this section states that "method (c) is applicable only when shear anchors are provided along the steel section and reinforcement of the concrete encasement meets the specified detailing requirements."

The questions I have are:
1. In your opinion, can this be treated as a fully composite beam without shear connectors? and
2. What are the "detailing requirements" mentioned in the commentary? There are none listed in Section I3 which deals with flexure, which seems to mean that there are none for pure beams (which this is). In section I2 "Axial Load" there are detailing requirements for the encasement for composite columns, but there is no mention in the code that these should apply to encased beams.

Thanks in advance for your thoughts!

 

[JStephen]

Just a thought- is this new rail? Or rails removed from railroad service? If the latter, the tops of the rail do get beat down and ground and there should be some reduction in section modulus for that.

 

[dcarr82775]

I am not sure what constitutes an 'encased section'. I suspect it is more intended for floors than isolated cantilever beams. Try contacting AISC.

I wouldn't assume composite section for strength calcs, but I know a shoring contractor or two that does, but primarily only on the deflection side of things.

 

[Brandon]

Thanks for your thoughts dcarr... I have submitted the question to AISC.

 

[dik]

Can you consider it as 'big' non-deformed rebar? <G> Bond may be an issue.

The change in section properties from rail loading should be minimal; the caps are hardened and don't exhibit much 'flattening'. The flexural capacity of rail is not overly great; they are not proportioned for this type of use.

Is fatigue an issue? Is the loading sustained?

Dik

 

[Brandon]

Considering it as a big plain rebar is certainly an option. It seems to me, thought, that this would not really be different than assuming it acts as a composite section, except that would be discounting any flexural capacity the section might have. And, as you pointed out, you still have the problem of bond, though I believe some of the older ACI codes that still included ASD have values for maximum bond stress for plain bars.

The loading will be sustained in this application.

 

[dhengr]

Brandon:
Doesn’t “design-build contractor” imply that that contractor prove/show that it will work, and comply with the building codes, from the structural design standpoint; and then build it and warrantee it, for the service intended? What do their calcs. and report on the design of soldier piles like this show to justify them? What’s the real story here? Would they have to haul this rail off site and dispose of it as scrape if they didn’t dump it in these 36" drilled piers? Are you reviewing their design or trying to design these piers this way for them? Contractors have some pretty crazy ideas sometimes, and you should be careful to use some good engineering judgement when being “tasked with” trying to make their ideas work, particularly if it is your stamp or reputation which is on the line.

You say.... “I am trying to determine if this can be treated as a composite beam based on the latest AISC Specifications.” It is a chunk of steel encased in concrete, but what makes the two act together as an integral unit? Is the rail embedded and does it have sufficient development length to develop its full tensile strength. What is its Fy and Fu and eleongation? Without any shear connectors, what is the bond strength btwn. the steel and the cracked concrete? Why not just dump an equivalent tonnage of scrape steel, randomly in the pier height, in the concrete as you pour it, and call it good. Why would you use AISC as the design code for this? Why not treat this like the reinforced concrete column which it is supposed to be, and use ACI as your design guide. Is the total volume of steel sufficiently distributed w.r.t. the concrete to make it act as intended? Does it have sufficient development length? Are there any ties to confine the concrete and to hold the vert. stl. in place?

If they used whole sections of track, left the ties in place and spaced the piers properly, they would only need half as much lagging. I’m surprised they didn’t think of that.

 

[Brandon]

dhengr:

I appreciate your concerns regarding the ethics and judgement involved. However, my questions were mostly of a technical nature, and you are just asking many of the same questions I am asking and have asked myself in that regard. I am reviewing their design which, admittedly, is not 100% clear with respect to their structural capacity calculations. I am trying to get as much information as I can to be able to justify my position (at least in my mind) when we respond. By "tasked with", I simply mean this was the task assigned by my supervisor. If I am not comfortable with the contractor's proposed solution, I will certainly tell my supervisor this and the reasons why, and offer suggestions on a more appropriate solution.

It is a chunk of steel encased in concrete, but what makes the two act together as an integral unit?

As stated in the AISC code, there is some research which indicates composite action is achieved for encased members without shear connectors under certain conditions, which appears to be what the provisions for flexural strength of encased members is based on. But I'm not sure if that applies to this particular case.

 

[jsha811]

I feel it is not a very good idea to use the rail since they are not good for bending. There are times we use a wide flange encased by concrete when the excavation is deep. But in that case, we only rely on the wide flange. I doubted it can be considered as a fully composite section.You may let them look at whether it will work or not if only rely on the rail. Plus you need to make sure how the soil pressure gets transferred to the rail using some strut and tie approach maybe. I think at the end it is all about the load path. Just my two cents.


Moment curvature analysis,reinforced concrete column design and prestressed concrete pile design at

http://www.structurexpress.com

 

[dik]

In response to an earlier query I posted regarding welding rail. The attachment of shear connectors, if necessary, may be problematic.

Dik

 

[kingnero]

true, dik. CE of a rail is typically ~1 (where anything above .4 needs preheating, and anything above .6 is regarded as difficult to weld)

 

[dik]

"The King is alive!"... thanks, it was something I didn't know and was surprised that it wasn't just a simple matter of welding rail. Ended up bolting...

 

[johnbridge231]

For such cases you can use a generic software aoolication for arbitrary cross sections.

Analysis and Design of arbitrary cross sections
Reinforcement design to all major codes
Moment Curvature analysis

http://www.engissol.com/cross-section-analysis-design.html

 

[Brandon]

I posed this question to AISC - here is their response:

The design requirements in AISC 360 Chapter I for encased members are typically for composite columns with wide flange sections encased in concrete. There are additional reinforcing and shear transfer requirements. The composite beam design approaches in Chapter I do require shear connectors to transfer shear between the steel and concrete. Simply casting a steel shape in concrete will not qualify it as a composite member.