Deflection analysis aproach? Please suggest. 1

[Hemi79]

I have a Girder spanning 46'. This girder is a 6 2"x10" member spliced together by a split ring and I assume shear plate. The girder holds up a second floor, a floor consisting of 2"x10" joists for decking. The joist spans 20' and are 16" O.C. Yes this floor is a problem, but in being accurate with analyzing it, specifically deflection. What do you all recommend?

Especially the Girder. It has 34 point loads of over 600 lbs, assuming 48 psf Live and Dead, through out the 46+ ft. span. I have used the Moment Area Theorem(2) to find deflection but man it was tiring and putting into excel I think was worst. I've just downloaded Mathcad express, but is there anything you all can suggest to better my situation? I want to be able to get a better feel for results in terms of error and accuracy. For the girder my calculations have varied from 9 to 13 inches in deflection, obviously over the allowable, but I need more confirmation.

Thanks in advance for any suggestions.

 

[Archie264]

I'm not sure I fully understand but I will make a couple of comments:

1. 34 point loads are effectively a uniformly-applied load so treat that aspect of it as such.

2. 46' is a really long span for any timber member and 2x10's - even six of them - have no hope of making it per any code or objective standard. A 2x10 can't even carry it's own self-weight over that span and meet most deflection criteria. Am I misunderstanding the scenario?

 

[ztengguy]

Whats its deflection right now with just DL?

 

[BAretired]

I agree with Archie on both points. I'm amazed the thing is still standing.

BA

 

[JAE]

This is weird. I agree that 10" 2x wood beams can't span 46 feet. Do these beams have steel plates in them? Even so, a steel wide flange beam spanning 46 feet would typically need to be about 21" deep to work.

Also - your dead loads are for the partition load and the decking.

What about the self weight of the joists? And any ceiling load applied below (if any)?
Are there mechanical/electrical loads to include? Flooring? Self weight of the girder?

I don't think you have enough dead load considered here.

 

[Hemi79]

First of all thanks to every one for the replies!

Archie 264: Ok in doing so, using the 34 point loads as a uniform load, and doing typical simply supported calculations give me crazy insane numbers. in the extreme. Deflection particularly. And as to the span and code adhering also yes. This building has been there since 1912 and the new owners want to know what to change. The 46' span approach was out the moment we saw it, along with the 20' O.C. spacing, but in designing something new there are several approaches that can be taken. I still need to present something that supports the existing condition.

BAretired: I want to say that ever since the the building was constructed, in 1912, there have been a good amount of years that it was vacant. That's my only reasoning, because walking on that second floor and inspecting the aesthetic breakdowns due to the floor was a little scary.

JAE: I assume that they do have shear plates within the split ring connection. Aside from that possibility, No, I went in that diaphragm almost in total darkness and inspected the span. At least the splicing did not coincide within any of the girders, and I was happy to see that. It was me, my tablet and iphone light so I had a hard time but got some good info. It is connected to the column by more split ring/shear plates by a knee brace approach. You are totally right on the self weight dead load. I have not included that and I will I've just been so focused on getting the right approach for the calculations. There are no mechanical/electrical for this floor. Yes the 2psf for the ceiling will be included as well.

 

[IDS]

I have attached a spreadsheet that will do the deflection calculation with multiple point-loads quickly, but I agree with the comment that with so many equally spaced loads the difference from a UDL will be negligible. I get about 30 inches deflection with the loads as specified. I used an EI value of 1750000 kip.in^2, but I don't do timber design, or work in Imperial units, so check that is in the right ballpark.

The calculation is on the "SSSpanU Example" sheet (and gives almost the same result as the standard formula for a simply supported beam under UDL)



Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[hokie66]

So how much deflection actually exists?

Hard to believe this was actually built that way in 1912. Could the internal columns have been removed at some time?

 

[Hemi79]

IDS: Thank you for the attachment. Very impressive. I can never be that organized in excel. Its great! Im gonna look it over extensively. Now as simple as it is, and maybe its that I haven't slept for a while, but what is the UDL you are using when calculating by the simply supported deflection formula? (5wl^4/384*EI)Mine are off by 100 percentages. Again thank you.

hokie66: when I visually inspected the second floor there are sections with up to 3 inch slope declines. And its all over the 2nd floor area. I did ask the owners if they knew of any renovations and they could assure me only MEP. Its a huge area without vertical supports inbetweens.

RFreund: sorry Im lost in your question. Can you elaborate?

 

[IDS]

Deflection calculation:

E = 1.74 x 10^6 lbf/in^2
I = 1000 in^4
w = 600/16 = 37.5 lbf/in
L = 46 X 12 = 552 in

deflection = 5/384 x (wL^4)/EI = 26.1 in


Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[hokie66]

And the stress would be about 7000 psi. Must be good wood. The floor is in danger of collapse.

 

[Hemi79]

Yeah everything is in incredibly bad shape. However, I will mention, although the second floor is scary, based on the deflection results that we are getting, there should be more of an extreme conditions to look at and physically come in contact with when inspecting. Terrible design, yes. Now in all of this, besides thanking everyone for their help and suggestions, I wanted to post some pics of when I went into that 2nd floor diaphragm. Lighting was terrible, and it was hot to a suffocating condition so please don't judge my pictures, but I did make not of this bracing system within the diaphragm that I figured was just very old school.
See attached.

Thanks!

 

[ztengguy]

So is this part of a truss or something?

 

[Hemi79]

No its the brace from the column to the girder system and you can see a steel pipe/tube diagonally coming down that braces to the other column on the wall 46 ft. Away.

 

[msquared48]

So there is a knee-brace frame system here? The connection of the brace to the bottom of the built-up beam is not visible and could be questionable.

That being said, if, and I did say "if", this system is functioning properly, this could reduce the effective span by
4 X 2 X 2 = 16 feet or so making it about 30 feet instead of 46 feet, assuming the knee braces are adequate, at 45 degrees, and drop about 4 feet to the column from the built-up beam. You would have to analyze the knee-braced system.

Then the results could become closer to reasonable, but it would have to be thoroughly checked out.

Mike McCann
MMC Engineering

 

[Hemi79]

This is VERY interesting. I have the NDS 2005 edition. would you mind pin pointing some specs in it so I can better evaluate this issue?

 

[Hemi79]

Heres is another, sorry for putting it in a different post. I'm trying to post several pics in just one, but I've had no luck learning how.

 

[msquared48]

Are those piles for plumbing or do they appear to be structural?

Got to thinking and if structural in nature, they could be part of a harped system to lift up on the middle of the span in conjunction with the knee braces.

Mike McCann
MMC Engineering

 

[Hemi79]

I don't believe they are for plumbing. The hardest thing about this is we don't have authorization to open up more of the buildings areas to confirm these kind of things. There is more of this piping within the 2nd floor diaphragm, but it runs horizontal and it's obvious there is other use for it not being structural. How does this harped system work? The pipes run from one end to the other at an angle and in one direction. Say they start at the girder elevation on one end and end up around 4 to 5 ft. underneath the girder's end on the opposite side. I had hope of a post tension system, but after looking carefully there were no cables.

 

[msquared48]

At 4 to 5 feet under the girder's end at the opposite end of the 46 foot long girder, it is not a harped (PT) system. Could be vertically diagonal struts to take horizontal forces from wind to the location below, but I really tend to doubt it without further details.

Mike McCann
MMC Engineering