Messin' with a Glulam 2

[msquared48]

This thread got me thinking...

Repair 7"x14" parallam beam that has been notched on the tension side?
thread507-302004

The question is, if the beam was a Glulam, say a 24FV4, with the tension lam on the bottom, and only a 1/2" to 1" notch was taken out of the bottom 1.5" lam, as in that thread, short of using only 1200 psi for the design bending stress, is there any documentation saying that some of the tension can still be considered effective and the allowable moment varied per the depth of cut?

Not that I'd use it, but just for the sake of discussion...


Mike McCann
MMC Engineering

 

[Ron]

Mike...probably not, because it would be outside the manufacturer's recommendations for modification. Also APA-EWS does not allow notching on the tension side except at bearing ends.

The IBC does not allow notching of engineered wood products, specifically including glued-laminated timber and I-joists, except where the manufacturer allows it and even then it must be part of the engineering, done by a licensed design professional.

The IBC also does not allow tension side notching of a bending member in the middle third of the span.

Attached is the APA-EWS publication on notching Glulams.

 

[Ron]

Sorry...that should be EWA, not EWS. Can't type.

 

[woodman88]

Yes,
Per the IBC Chapter 16, Structural Design, Section 1604.4 Analysis "...Any system or method of construction to be used shall be based on a rational analysis in accordance with well-established principles of mechanics...”
Also per Chapter 23 Wood, Section 2308.10.7 Engineered wood products “...or where the effects of such alterations are specifically considered in the design of the member by a registered design professional.”
By which you go back to the basics of engineering and design the member or you can use the 1200 psi or any higher values that you can justify in your analysis.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[Ron]

Garth,
When the design accommodates the notch, you are correct. If done after the fact by a contractor, then the design professional has to rationalize acceptance of a subverted design or come up with a repair method. This places the design engineer in a higher liability position because there are few, if any, guidelines on such repairs by the manufacturers. At the very least, the notch would require modification by radiusing the corners and possibly by "spiking" the potential crack plane; for each of which he must take personal responsibility.

Here's the whole 2308 paragraph...

[highlight]2308.8.2.1 Engineered wood products. Cuts, notches and holes bored in trusses, structural composite lumber, structural glue-laminated members or I-joists are not permitted except where permitted by the manufacturer’s recommendations or where the effects of such alterations are specifically considered in the design of the member by a registered design professional.[/highlight]

 

[woodman88]

Ron

What is the difference between designing it with a notch before construction starts or after construction starts? You would have the same professional liability in each case or am I missing something?

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[Ron]

Garth,
In a way, there's no difference, provided you can achieve the same result; however, when a contractor notches something in the field, it is usually done with less care and overview than if designed into the member. If you design the notch in the member, then you can set a hold point for layout, inspection, etc. Further, the notch can be cut by jigging if necessary, and using proper tools, to achieve proper radii and the cut can be done on the ground, not in place. If notched in place for convenience, you're stuck with whatever happens and you have to rectify at that point. In my opinion and experience, that usually results in some compromise to quality and greater risk on the part of the engineer.

Ron

 

[woodman88]

Ron,
Whether done on the ground or in place there is some compromise to quality. So you add a factor of safety, like designing the notch 1/2” deeper in case they make it 1/4” deeper. If you can't come up with a factor of safety, then tell them it can't be done and do a repair.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[Lion06]

Also, if I know they are going to be notching the tension side of a bending member, you can bet that I'm going to use a lower fb/Fb (of the notched section) ratio than if the member is not notched. I'll account for the notch by reducing the section properties accordingly, but I'm going to bump the factor of safety, too.

If you know the notch is coming, though, I go back to the question that I mentioned in an earlier post, why not just design the member for the depth at the notch for the entire length?

 

[Lion06]

Sorry, I made that reference in the other thread.

 

[msquared48]

That is a logical consideration Lion, but the question is, in the case of the glulam, what stress would you use for the design?

Mike McCann
MMC Engineering

 

[Lion06]

Mike,

I apologize. My head was in LVL/PSL world. I don't think I would notch a glulam.

 

[msquared48]

Thanks for the input guys...

Mike McCann
MMC Engineering