Laterally supported beams 2

[HanStrulo]

Hi Everyone,

I have a W shape vertical beam inside a concrete mix (0.4MPA) inside of a borehole.

I am trying to size the beam to resist the lateral earth pressure but I am not sure if the beam is continuously supported against lateral torsional buckling or not.

Does the configuration I have described constitute a supported beam? What is the criteria for considering a beam supported against lateral torsional buckling?

Thank you in advance

 

[phamENG]

If it's in a borehole, how is it resisting earth pressures? Wouldn't they just be resisted by the earth pressures on the other side of the hole? Maybe I'm missing a regional nomenclature thing. A sketch might help?

 

[HanStrulo]

Hi,

sorry if my description is misleading.

To simplify. the beam is vertical and it has a uniformly distributed load (earth pressure). The beam is inside a concrete filled borehole. I want to treat it like a continuously supported beam against torsional buckling of the compression flange since it is surrounded by concrete and soil but iam not sure if there is a limit to what is acceptable as support.

I hope this is clearer.

Thank you.

 

[phamENG]

Nope. I still picture a hole in the ground with a concrete encased steel member in it, and the earth pressures would transfer around the section or through it in direct bearing on the other side.

Since I don't understand the specific case, I'll try to comment on the general case of a LTB of a concrete encased steel beam. If you have access to the ASCE Library, you may find this paper useful. In short, I believe the answer is no, you can't consider it laterally braced. I suppose if you have enough concrete and reinforcing around it you may be able to compare the torsional stiffness of the concrete "tube" to the bracing requirements that your code mandates (AISC has an appendix for beam and column bracing, much of which is based on the work of Yura).

 

[Settingsun]

Are you describing the embedded part of a post-and-panel retaining wall? Picture below.

Why is the concrete so weak? 0.4 MPa sounds like a lightly stabilised pavement base course rather than something to embed a post in.

 

[PEinc]

This question would have been more easily and properly answered in the Earth Retention Engineering Forum.

If this is a soldier beam concreted inside a drill hole, with only the front flange and a small, side portion of the beam sides exposed near the front flange, consider the soldier beam fully laterally supported. Lagging, lean concrete or flowable fill, and a permanent facing all laterally brace the soldier beams.

If the beam in the drill hole is backfilled with loosely dumped, uncompacted fill soils or gravel (NOT RECOMMENDED, BUT OFTEN DONE), the beam may not be fully laterally supported. Drill spoils or gravel backfill are not recommended because they can fall out when excavating to install lagging. This lost material can then cause settlement behind the wall and excessive bending of the soldier beams and lagging when tiebacks are stressed and tested.

W/R/T the 0.4 MPa, AASHTO calls for f'c = 0.35 MPa min. (51 psi) of cementitious backfill in the drill hole. It needs to be weak enough to allow it to be minimally removed in order to install lagging between the soldier beams. At 0.35 MPa, this "weak" material is usually much stronger than the soil it replaces, even for the soldier beam toe embedment in the drill hole below subgrade.

http://www.PeirceEngineering.com

 

[HanStrulo]

PEinc has it right.

I am new to earth retaining field and came from a building background.

The beam is indeed a soldier beam. I am not sure if there is a limit to what kind of soil would make it supported and what kind would make it unsupported.

Is there a resource i could consult to know more about how to decide on the lateral support? An empirical method?

I saw in some calculations that the beam is supported and the resisting moment is the plastic moment Mp but in others, the design is done with the full length of the beam.

Thank you so much in advance.

 

[BridgeSmith]

In the 'concrete' (more like what we would call flowable fill), I think the W beam can be considered braced for LTB. Above that, even if there is some soil in front of the wall, it would be very questionable to consider it braced. AASHTO allows driven steel piles to be braced 5' below the embedment depth, which would be, in the case of a soldier beam of a retaining wall, in front of the wall. You could maybe consider the flange on the back (retained soil side) of the the wall braced by the soil, but that's irrelevant, since that's the tension flange. I think the beam should be considered an unbraced cantilever above the 'concrete' backfill.

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

 

[PEinc]

Let's not make this more difficult than need be. The soldier beam is considered fully laterally braced - unless you have loose, drill hole backfill that runs out when you install the lagging. I can't even remember this issue being questioned over thousands of designs.

http://www.PeirceEngineering.com

 

[jayrod12]

I'm with PEinc. All of my soldier pile and lagging designs assume the lagging adequately braces the compression flange. No issues and I've done some fairly substantial ones. Nothing like PEinc's experience but enough that I'm confident in that assumption.

 

[PEinc]

Photo of installing lagging for drilled-in soldier beams that were backfilled for their full length with low strength (black) flowable fill.

http://www.PeirceEngineering.com

 

[BridgeSmith]

All of my soldier pile and lagging designs assume the lagging adequately braces the compression flange.

I hadn't really thought about that; never really had to, I guess. I suppose you're right, though; the friction on the back of the flange should be more than adequate for bracing the flange.

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

 

[hotmailbox]

We typical design the soldier beams to resist active pressure and surcharge per soils engineer's recommendations but I think in reality they rarely experience such forces in full in OK soils.

Back to the topic, would you consider it braced if it is back lagging (lagging tucked behind the back flanges)?

 

[PEinc]

If back-lagged at the rear flange, not laterally or not sufficiently laterally supported - unless some additional inter-beam lacing is added to the front flanges.

http://www.PeirceEngineering.com

 

[jayrod12]

Agreed if it were back lagging, that's a different animal. But for standard wood lagging, I've never had that be the case. The lagging is always fairly tightly fitted between the soldier piles.