Built up beams 1

[Gforce1]

Hi,

First time poster.

I am looking at a repair to a jetty structure where the entire top flange of a beam is to be replaced. In this instance the use of welding is not permitted, so I am proposing a bolted solution. On propping the beam the top flange, which is heavily de laminated, is to be cut off. A new flange will be added, using 2x angle sections and a flat plate. The connection between each element is to be bolted, with loads based on the shear flow. An overall buckling assessment has also been made of the new section to EN 1993.

The difficulty is that I have specified that precision bolts be fitted to avoid unintended flexibility being added to the structure. For all my years of experience I have never specified such a repair in bolted form. Am I over thinking this, would bolts in standard clearance holes be sufficient? I have used similar details in the past to strengthen welded end plates so I am aware of the difficulty their installation may cause.

Note that I am in the UK so my terminology reflects this (I am very familiar with AISC though so no issue with any references to it).

Thanks

 

[BridgeSmith]

The bolts should be fully tensioned and checked for the slip under service load (or the EN equivalent) conditions. That should ensure that in service, the components should act as rigidly connected elements.

You could reference AASHTO for the slip-critical connection criteria, friction coefficients, etc., although I assume there's an equivalent European spec.

This would allow you to use standard holes without having to account for the movement. It's what we do for bolted cover plates, replacing rivets with bolts on older built up girders, and bolted fields splices for bridge girders.

 

[Gforce1]

Hi Bridge Smith,

Thanks. I agree with your reply and its certainly the approach I would take in a bridge structure or a structure subject to cyclic loading.

In the UK I think we generally use bearing only behavior more often than in the US. This is evident in that to get this behavior the physical specification of the bolts would be different. For building structures, as a general rule, you would only expect to see slip critical design at beam splices. In this instance, as there is no cyclic loading, I was considering if I could get away with bearing only behavior. If I need to prevent slip I think for this particular application precision bolts would be more practical than preparing the surfaces for frictional transfer.

 

[JoshPlumSE]

What are "precision" bolts? That's not a term I've used before. Maybe it means using smaller sized bolt holes so that there is less potential for slip?

I don't know. There is some wiggle room in the bolt spec for diameters and such. And, if the existing beam isn't perfectly straight and the bolts are perfectly placed (in either of the angles or the web), then you could have SERIOUS construction problems. Maybe that's what the "precision" bolts mean is the placement of the holes.

Regardless, I can't believe this would be a cheaper option than just fully tightening the bolts and checking for slip at service level (or even ultimate level) loads.

 

[EdStainless]

Will anything be done to minimize the amount of water getting between the bolted sections resulting in very rapid corrosion of the repair?
There is a whole range of options depending on the life expected out of this.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[Gforce1]

Hi,

A fitted bolt is a bolt where the diameter and hole size are close to the same. A well known UK text (Cheal & Owen) describes these as follows; 'fitted bolts can be used where it is not practical to use a friction connection. Such holes must have a clearance on the nominal bolt diameter of not more than 0.15mm. They are used in conjunction with precision-bearing bolts'. This reference is to ISO precision bolts that are manufactured to a tighter tolerance. Installation will involve site drilling and then hammering the bolts into place. Essentially its a modern day rivet. They are most often used in repairs or strengthening works to end plates where load share with welds is required, or as rivet replacements. I don't think they would ever be used in new build construction. My structure is a 70 year old jetty in an oil terminal.

An alternative to this would be a resin packed bolt as per the attachment.

In the US would it be standard practice to use slip critical joints between the components of built up beams in building structures?

 

[Gforce1]

Post regarding corrosion is a good one. At the moment other than grit blasting and painting nothing in particular is planned.

Ordinarily I would have specified sealing welds prior to painting. I guess I could specify an epoxy based strengthening measure that would seal the connection. I have reservations on such products in this environment as they will severely affect the fire performance of the structure. I believe these products typically are only good to 50deg C?

 

[BridgeSmith]

If I need to prevent slip I think for this particular application precision bolts would be more practical than preparing the surfaces for frictional transfer.

You shouldn't need to prep the surfaces. There are frictional coefficients rusty surfaces, and they're actually fairly high. I'm not at work at the moment, so I don't have access to my AASHTO spec, but if I remember right, either in the spec or the commentary, it tells you what coefficient can be used for rusted surfaces.

 

[Gforce1]

I will have a look at what the spacing of m20 hsfg bolts needs to be for no slip under factored loads. The surfaces will be grit blasted, so a slip factor of 0.5 should apply.

Regarding the use of bearing bolts in clearance holes there is no consensus that this would be permissible? Given the location of the repair I am wary of imposing additional inspection requirements for the ongoing justification of the jetty.

 

[BridgeSmith]

A 0.5 slip coefficient is fairly high. I don't think I've ever had a bolted field splice where the design was controlled by slip at the service limit state.