Application of loads to wood truss

[AELLC]

See attached girder truss.

The psf loads in the lower right corner (TC LL=20 psf, TC DL=14 psf, etc,) are all proper and tidy - however, look at Special Loads in the upper left hand corner - where I have red arrows - and then look at the drawing, at the bottom chord on the right, where I have colored it red.

The majority of the dead and live load does exist on the roof deck, near the top chord, but the common trusses at 24" oc that impose the their loads to this girder apply it to the BOTTOM chord because that is where the truss hangers are.

Now doesn't that signify that these calculations are garbage, i.e., the web forces are in reality completely different, and the top and bottom chord forces are slightly off?

The design of the truss plates for web to chord is completely incorrect as a result?

Or am I completely not seeing that these calculations are indeed correct, because the truss plates H1315 and H1014 are relatively large?

These truss calculations, for some reason, do not identify each member and what axial force exists in it.

 

[AELLC]

Additionally, look at upper right hand corner Nail Schedule 0.131" x3", min. nails

Top Chord: 1 Row @ 5.75" o.c.

Bottom Chord: 1 Row @ 12" o.c.

I would think the opposite be specified for the reasons I give in above post

I don't know exactly what criteria they use for designing these nail spacings...and this is not a drag truss.

 

[AELLC]

**bump**

 

[AELLC]

***bump***

I actually need this ASAP.

 

[woodman88]

You are correct if the supported trusses are hung from the bottm chord. Also the added bending to the bottom chord should cause it to fail. The only way the girder truss will work, is if the supported trusses are hung or bear on the top chord of the girder truss.

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.

 

[AELLC]

GArth,

The truss design software doesn't seem to take that major issue into account.

I learned recently to not trust any software, however expensive.

 

[AELLC]

Garth,
I see you have a lot of truss design credentials, is it possible I give a you a ring on the phone? I am a one-man shop in Chandler, residential only.

 

[woodman88]

I do not want to break the rules here by posting my phone,e-mail, etc. I am not sure how you can contact me by phone as I am not in the phone book.
That being said, per the 2012 IBC (the same also applies to the 2012 IRC) Note number 11 below. So the truss designs should show the truss to truss connection.

"2303.4.1.1 Truss design drawings.
The written, graphic and pictorial depiction of each individual truss shall be provided to the building official for approval prior to installation. Truss design drawings shall also be provided with the shipment of trusses delivered to the job site. Truss design drawings shall include, at a minimum, the information specified below:
1. Slope or depth, span and spacing;
2. Location of all joints and support locations;
3. Number of plies if greater than one;
4. Required bearing widths;
5. Design loads as applicable, including;
5.1. Top chord live load;
5.2. Top chord dead load;
5.3. Bottom chord live load;
5.4. Bottom chord dead load;
5.5. Additional loads and locations; and
5.6. Environmental design criteria and loads (wind, rain, snow, seismic, etc.).
6. Other lateral loads, including drag strut loads;
7. Adjustments to wood member and metal connector plate design value for conditions of use;
8. Maximum reaction force and direction, including maximum uplift reaction forces where applicable;
9. Metal-connector-plate type, size and thickness or gage, and the dimensioned location of each metal connector plate except where symmetrically located relative to the joint interface;
10. Size, species and grade for each wood member;
11. Truss-to-truss connections and truss field assembly requirements;
12. Calculated span-to-deflection ratio and maximum vertical and horizontal deflection for live and total load as applicable;
13. Maximum axial tension and compression forces in the truss members; and
14. Required permanent individual truss member restraint location and the method and details of restraint/bracing to be used in accordance with Section 2303.4.1.2."

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.

 

[AELLC]

Garth,

Thanks for the above info.

I was having a s****storm of truss problems in the last 2 weeks and it is driving me starkers.

The definition of a structural engineer: overdesign by a factor of 1.999, instead of the usual 2.

 

[AELLC]

I didn't mean post phone # here...we get enough junk calls on the 3 phones here as is.

The definition of a structural engineer: overdesign by a factor of 1.999, instead of the usual 2.

 

[charliealphabravo]

I have talked to the engineers at Alpine in the past when I had questions about a truss design. That would be the best place to get an answer and they seemed to be pretty accessible.

 

[ztengguy]

Did you call the truss company and discuss?

 

[AELLC]

They won't connect me to anyone by phone, and they are ignoring emails.

I think their software is flawed, and they aren't getting any approval for their truss calculation submittal from me, so they can hire another EOR, I am done wasting my time with those idiots.

The definition of a structural engineer: overdesign by a factor of 1.999, instead of the usual 2.

 

[ztengguy]

Reject it, ask for more info and let the contractor deal with them, He is the customer. Dont make things so difficult for yourself, make it difficult for them if they wont pick up the phone.

 

[AELLC]

I already rejected it.

I have been payed for this job.

If they don't want to cooperate, they can find another EOR. If the want to contest my actions, I have a string of emails and a printed letter to back myself up.

If this was life-safety issue, I would handle it otherwise.

I am sure this software has been used in all 50 states and has not had any repercussions from what I think I may have detected as a software error.

Also, the Bldg Dept is aware of this entire saga, and they have told me that truss mfrs have been very uncooperative with them, also.

The definition of a structural engineer: overdesign by a factor of 1.999, instead of the usual 2.

 

[AELLC]

Also, this is a very strange job.

It 100% completed. Somehow, the Owner forgot to walk the truss waiver thru, and somehow, the field inspector signed off on the final inspection.

Somehow, someone at the Bldg Dept realized the truss waiver was not signed off by me, so here comes the Owner to me, asking me to approve the truss layout and calculations.

The definition of a structural engineer: overdesign by a factor of 1.999, instead of the usual 2.

 

[ztengguy]

So really the truss types have nothing to loose. They have sold their product.

 

[charliealphabravo]

If I remember correctly, the design of girders is not done with the flagship Alpine or Mitek application. I think they were using a standalone tool and even manual tabulation in some cases to apply carried truss loads and calculate reactions.

At any rate the expertise is not with the truss manufacturer but with the plate manufacturer who owns the software and issues the truss engineering. I always tried to maintain a good relationship with at least one truss manufacturer so that I could get access to the software, but any serious questions about design always go to Alpine or Mitek.

 

[hokie66]

Was there an attachment in this thread? I don't see one now.

 

[AELLC]

For some reason, the attachment was deleted (after set time???)

Here it is again

The definition of a structural engineer: overdesign by a factor of 1.999, instead of the usual 2.