Engineering Question on Span for a Glulam Beam...

[jedijawa]

Hello to all,

I am remodeling a house that has already received it's occupancy permit and engineering buy-off but I think there may be a structural problem. In a room on the main floor (home has a basement, main floor and upstairs) there is a glulam beam 12" by 5" and spanning 15 feet. The beam is a simple span and has flooor joists running both directions perpendicular off of it. The tributary on the one side is 7 feet and on the other is 6 feet (tributary is 1/2 the distance of the floor span off the beam right?). On the smaller side above the beam is a full bath with jetted tub. Anyway, I am wondering if anyone out there knows where I can find the information that would tell me if the beam is up to the task or not, I get the feeling it is not. Also would 50 PSF Live Load be good to reason for the beam or should it be higher due to the bath?

I appreciate all the tips offered and offer my thanks in advance.

Thanks,
Adam

 

[SperlingPE]

Glulam beams have different designations that indicate the strength of the beam. You will need to find the species (douglas fir, southern pine, etc.) and the designation. If this is a new beam, it might be on the invoice or it can be obtained from the lumber supplier. As far loads for the beam, minimum live load for residential construction is 40 pounds per sq. ft. The dead load would be the sum of the weights of the components of the floor (typically 7-15 pounds per sq. ft.). What indications are you seeing that leads you to believe that the beam is not performing?

 

[SlideRuleEra]

You have provided a good word description of your concerns. Tributary area is as described assuming simple supports for the floor joists.
Many building codes require 40 PSF Live Load for residential construction (this would include the bathroom that you described). Allowing another 10 PSF for Dead Load and you have the 50 PSF that you mentioned.
Assuming that the Glulam beam has adequate end supports & foundation and all other members (floor joists themselves) are properly sized and connected, stresses in the beam itself (at 50 PSF) are reasonable:
Shear: <100 PSI
Bending: <1200 PSI
Deflection: < L/360
I would not expect you to have any problems with this beam.

 

[SlideRuleEra]

Back To The Drawing Board! Just realized that I misread the length (I saw it as 12 feet instead of the 15 feet you stated - sorry). At 15 feet the analysis is more dependent on the specific properties of the member as stated by SperlingPE.

 

[jedijawa]

Thank you very much for the replies,

As far as the performance indicators already: I see a 1/2 inch droop in the middle of the beam, that is the main indication I have for being worried. I have considered the idead of placing a column of three buddied up 2X6s at the 10 mark of the span, the column would rest directly on top of the bearing wall below.

As far as what material the beam is created from, I don't have any idea. It's not new, and I don't know how to distinguish one type of lumber from another... it has been installed for about eight years. It did have a very small wall under it, but not bearing in construction type just two 2X6s 16 inches apart with a top and bottom plate and rocked. Perhaps the original builder intended this panel to be supportive in nature?

Both ends of the simple span are supported on 4 2X6s, should be very adequate.

That is about all I know... so I really really appreciate the help and feedback, you guys are great.

Adam

 

[RockEngineer]

If you aren't sure what type wood the glulam is made of you are usually conservative to assume 1600psi bending strength and E=1,000,000 psi. These are the weakest glulams sown in the national design standard (NDS) used as the governing timber design document in the US. My calculation shows your glulam is borderline with this assumption. It deflects more than desired but it isn't failing. If your floor isn't bouncy you are probably ok. To know this for sure I would need to see the house and be sure some loads were not missed. Engineering over the internet always has its pitfalls because you can't use your experience to find the little problems that the untrained eye misses.

 

[sc]

jedijawa,

The beam seems on the face of it to be reasonably sized (as slideruleera notes, as rockengineer says, you need to ascertain or make assumptions to ensure that the timber type and properties are conservative).

Also as a consideration some gluelam beams are pre-cambered and sometimes builders, especially home renovators fit the beams in upside down. Not saying it has been, but just a thought.

Another simply way to check the beams behaviour is to measure the deflection with the lowest load possible on it and then load it up by filling the bath and getting a few people to stand on the floor over the beam. If the deflection increases greatly then the beam is probably undersized, If the deflection doesn't vary significantly, then it is probably OK.

All the same get a PE to have a quick look and do a quick calc to verify the beam size.

Good luck
SC

 

[FredForeman]

Input Data

Design of 5 1/8''x12'' 24F-1.8E-Un-Balanced Layup-Generic Glulam
Left Cantilever: None Main Span: 15' Right Cantilever: None
Check for repetitive use? No Tributary Width: 0' Slope: 0
Dead Load: 0 psf Live Load: 0 psf Snow Load: 0
Allow. LL Deflection: L/360 Allow. TL Deflection: L/240 (3 in Maximum) DOL: 1.000
Eb: 1800000 psi Fv: 195 psi Fb: 2400 psi

User Defined Loads

Load Case Load Type Distance(s) toStartft LoadLengthft Load atStartplf Load at Endplf
Dead Uniform 0' 15' 195
Floor Live Uniform 0' 15' 520

Design Checks

Reactionlb Bending - Xpsi Shearpsi LL Defl.in TL Defl.in
Max. Value 5362.5 1961.89 110.084 -0.4459 -0.6131
Allowable 11531.2 2400 195 0.5 0.75
% of Allow. 47 82 56 89 81
Location 0' 7'6'' 13'9-3/4'' 7'6'' 7'6''

Reactions and Bearing

Support Locationft Min. Bearing in Reaction lb
0' 2.114 5362.5
15' 2.114 5362.5

Self-weight of member is not included.
Member has an actual/allowable ratio in span 1 of 89 %.
Design is governed by live load deflection.
Governing load combination is Dead+Floor Live.
Maximum hanger forces: 5362.5 lb (Left) and 5362.5 lb (Right).
Timber design is governed by NDS 1997.


According to this calculation your beam should be adequate.
This was done using a Generic Glulam Spec.

The printout does state that the maximum allowable Live load deflection is 1/2&quot; and total load is .75&quot; I fyou are deflecting more than this than you probably have additional loading from above that is not being taken into account.

As an additional comment to SC statment above.
If your beam is precambered and place upside down, your beam will be under designed. Unless specifically ordered as a balanced layup most beams are an un-balanced layup and must be set in the right direction.

 

[RAY72]

How can we come to ANY conclusions, without knowing all the loads?
I would like to know where are the roof loads being incorporated..