Strengthening Cover Plate - Detailing? 2

[psychedomination]

Hi there,

I'm working on a project where the client is trying to put a heavy piece of equipment on a roof. I ran a model and found that the beams the equipment will be resting on would be overstressed (~15%) and fail in bending (sagging only).

To strengthen the beam, I was looking to bolt a partial length cover plate to the bottom of the existing beam flange. I haven't done this type of strengthening before. In theory this seems quite simple, the additional section increases the section modulus, which then increases the moment capacity. However, when it comes to the detailing best practice, I can't find much guidance on this?

A few questions I have is :

1. What is the minimum spacing of bolts to take the shear flow. When I carried out the calculation, to take the shear flow the bolts only needed to be spaced 12" apart. However, I read online here :

https://www.bgstructuralengineering.com/BGSCM13/BGSCM008/Design/BGSCM0080602.htm

that the bolts should not be spaced more than the flange width apart. I went with this approach conservatively but I was wondering if this is general practice?

2. The cover plate is partial length (total length of beam is 25' but the cover plate only needs to be ~12' based on the applied moments). I have the end taper as 1' long (tapering from 6" to 2.5") with a fillet welded all around the tapered portion. Is there any guidance on sizing the end detail that someone can provide?

3. Am I missing anything here or is there anything I should be concerned about when considering construction? I oversized the width of the new plate slightly, allowing a 1/2" overhang on each end to allow the contractor to carry out a top down tack weld if they wanted to keep it in place when bolting. I'm not sure what other considerations I should include to ease construction?

Details I was thinking of using are below :

Any assistance/advice or guidance would be appreciated.

 

[BridgeSmith]

@Dik I didn't think there would be any fatigue issues as the HVAC equipment will have vibration isolators installed.

The isolators may take care of the changes in stress due to the vibration of the machine (or not), but they probably won't do much for the wind load. As far as I know, AASHTO doesn't really address fatigue stress limits in situations where the permanent load produces large tension forces and the cyclic loads are small (not really a situation that comes up for highway bridges), so I'm not sure what the stress limits would be, but it's something to look into before you get too far into it.

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

 

[SlideRuleEra]

psyche - Others are providing good issues to look into. I'll offer a few more to consider:

As I mentioned earlier, a W14x22 is a pretty wimpy beam. I would not want alter it too much.

With 12% overstress, the W14 will not fail (collapse) even is you did nothing. I'm in no way suggesting leaving it "as-is", but be careful not to do too much.

Even though a 3/8" cover plate increases section modulus only 12% (I quietly checked your calcs), the moment of inertia for the composite is increased 41%. With a span-to-depth ratio over 21:1 (25' span, 14" beam) the significant increase in moment of inertia will improve the composite's rigidity & deflection and maybe vibration issues.

If bolting a cover plate is reconsidered, be sure to properly clean (probably sandblasting) the W14's bottom of the bottom flange to prepare it for slip-critical bolts. Of course, the cover plate needs to be equally clean and will have to be installed very soon after cleaning is completed to minimize corrosion in your environment.

What I would do is make the cover plate substantially longer than the minimum calculated. In exchange, I would not use the proposed tapering detail at the end of the cover plate. Keep it simple, just stop the plate (gets back my wimpy beam concern and aversion to doing too much to it).

 

[ldeem]

I'd add to SlideRuleEra suggestion to make the reinforcement the full length (I second that idea) is to include a full pen weld detail for a plate splice. It is unlikely they will get a continuous piece of plate and I have seen people just butt them together with no weld in the field too many times.

 

[ldeem]

One more thing for your future reference. A WT section gives a lot more bang for the buck as a reinforcement section. Also placing solid rods in the flange to web fillet is often easier than plates. You can do all 4 corners and avoid the issues with moving the neutral axis.