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Re: Structural steel--isolated joints

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> From: "Rand Holtham, P.E." <rand(--nospam--at)sigmaengineers.com>

> I'm looking at an existing manufacturing building that has experienced
> fatigue fracturing of some 6" pipe columns that are connected to a crane
> rail beam by a 1/4" plate. It is apparent that the vibration of the 32 ton
> crane has caused the fractures.

I don't think that this is apparent at all. It may be simple static
stress rather than fatigue related. You need to review the operating
cycles of the crane systems on both sides of the post.

> Luckily the columns are easily replaced but
> when reattaching the lateral support for the crane rail beam I would like to
> incorporate an end plate to end plate connection that has isolators or
> rubber washers or something to dissipate some of the vibration. Does anyone
> have any leads that would help me fine tune this design?

It sounds like you are describing the runway beam tie-backs (1/4" plates
to the pipe column). These are notorious for having stiffness in the
wrong direction, flexibility where it is undesireable and generally poor
detailing. The tie-back cannot be considered without the rest of the
support system.

The tie-backs are intended to resist the lateral forces that the crane
inposes on the runway system. The design codes (e.g. ASCE 7) will
recommend 20%x(lifted load + trolley weight) as a static design force to
be applied perpendicular to the top of the rail, distributed to each
side according to the stiffness of the support structure on each side.
For your 32 Ton crane, this is about 14 kips to be split to each
support.

The lattice column with the common centre post, above, is not unusual.
However, it seems that the centre post design may not have considered
the local stresses imposed by the crane. This, unfortunately, is also
not unusual.

The fact that the tie-back is only about a foot above the post base
means that the post is stiff there but the material is light for the
forces imposed. Inappropriate welded connection design can cause
significant problems with tie-backs.

It is not apparent if the failure is due to excessive stresses or due to
fatigue (e.g. vibration as you described). Possibly, a very high stress
range leading to a low fatigue cycle life. If you do not rethink the
entire assembly, the problem will recur.


> From: "Rand Holtham, P.E." <rand(--nospam--at)sigmaengineers.com>

> > are these 1/4" plates used to brace the top flange to
> > the column, etc., etc.
> 
> Yes

> > Can you also describe the connection which I seem to understand you to say
> > is causing the column to fracture.  Just generally it would seem that a 6"
> > pipe column is extremely light to support a 32 ton crane.
> 
> It's not supporting the gravity load of the crane. The connection more or
> less seems to keep the the rail beam from rolling over.

> stiff. It could be reasoned that the attachment is the main culprit to the
> damage therefore don't reattach the new column right?

Unless you have another plan for supporting the top of the runway beam,
ensure that there is always something connected there. Fix the whole
detail or runway, not just the tie-back.

> Well I'd rather not go
> that route... but if I can reattach it loosely, let the rail beam do it's
> thing but restrain the beam if it walks too far.

That defeats the purpose of the connection.

> I witnessed the rail beam's
> top flange moving at the rail beam splice as the crane passed, and it was
> loaded with maybe 10 tons

If you can see top-flange lateral movement, with normal longitudinal
travel, there is a serious problem.

Has anybody set up a transit/laser to check the alignment of the columns
(lower and upper), the runway beam AND the rail on both sides of the
runway?

Try to get some service history of the structure and crane. That may
speak volumes.


> From: Christopher Wright <chrisw(--nospam--at)skypoint.com>

> I'll stick to my guns after seeing the crack at the connection. This 
> isn't anything like a typical crane rail connection. If this were my 
> problem I'd look at the whole system including a dynamic analysis.

Dynamic analysis may be way more than it needs, to start. A whole system
assessment is valid.

> I think the lateral loading was underestimated. You can probably make a
> pretty good estimate of the dynamic amplification as arising from a 
> suddenly imposed displacement of the scale of the misalignment of the
> rails.

Start with the standard lateral crane loads per the codes or standards.

> If I'm not seeing things, it looks like someone tried weld repairs to the
> connector plates.

Did I miss a picture somewhere?

> I can't remember a situation like this where weld repairs did anything but
> make it worse.

Ditto.

> I'll risk raining on the parade and say that making that connection any
> more flexible is just going to make the problem worse. 

Ditto.

-- 
Paul Ransom, P. Eng.
Civil/Structural/Project/International
Burlington, Ontario, Canada
<mailto:ad026(--nospam--at)hwcn.org> <http://www.hwcn.org/~ad026/civil.html>

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