Raceway support/metal roof trusses

[chris kennedy]

I was having a conversation with the plumbing contractor today and he told me it is a violation to support raceways from the bottom of the red iron.(he couldn't quote me a code) If this is indeed true, could someone point me in the right direction?

Thanks

 

[mtlogcabin]

Most bottom chords (steel or metal) are not designed for a lot of dead load. The small stuff should be okay but you may want to check with the truss engineers on the larger items.

 

[chris kennedy]

Most bottom chords (steel or metal) are not designed for a lot of dead load. The small stuff should be okay but you may want to check with the truss engineers on the larger items.

Where should one start to be concerned? The 4" in that pic weighs around 100lbs per 10'.

 

[brudgers]

Where should one start to be concerned? The 4" in that pic weighs around 100lbs per 10'.

Most Industrial uses are designed with between 5 and 20 psf for that sort of thing. Most people after thinking about it, won't take the opinions of plumbers as expert on structural matters. Though it is reasonable to check that sort of thing, it usually isn't an issue.

 

[north star]

* * * *

Where should one start to be concerned? The 4" in that pic weighsaround 100lbs per 10'.

Realistically, questions should be raised at the plan review stage.From the 2006 IBC - 1603 CONSTRUCTION DOCUMENTS



1603.1 General.

Construction documents shall show the size, section and relative locations of

structural members with floor levels, column centers and offsets dimensioned.....The

design loads and other information pertinent to the structural design required by

:Next('./icod_ibc_2006f2_16_sec003_par001.htm')'>Sections 1603.1.1 through :Next('./icod_ibc_2006f2_16_sec003_par008.htm')'>1603.1.8 shall be indicated on the construction documents.



From Section 1603.2 - Restrictions on loading.

It shall be unlawful to place, or cause or permit to be placed, on any floor or roof

of a building, structure or portion thereof, a load greater than is permitted by

these requirements.



From Section 1604.3.1 - Deflections.

The deflections of structural members shall not exceed the more restrictive of

the limitations of :Next('./icod_ibc_2006f2_16_sec004_par004.htm')'>Sections 1604.3.2 through :Next('./icod_ibc_2006f2_16_sec004_par007.htm')'>1604.3.5 or that permitted by

Table 1604.3.



* * * *

 

[ICE]

In this case, the load is spread over two trusses. Isn't there a safety factor involved with trusses that could account for the added weight?

 

[steveray]

I thought there was something in the steel joist institute literature that said..."thou shalt not hang off the bottom chord" can't find it now....when in doubt, make the engineer address it. Something about the bottom chords being a tension member and no loading allowed.....if anyone finds it please post up.....I try to bring it up at review and it becomes less of a problem...

 

[steveray]

Not official....just some info...

RE: Hanging on bottom of Steel bar joists

S L

Mon, 18 Dec 2006 10:19:26 -0800

You're right on about all of that, but the lower chords of engineered

members are more sensitive to loading because they serve to strengthen

the piece by maintaining tension. The more you load and distort the

bottom chord, the exponentially higher the overall impact to the member.

That's why 98% of the factory details you see from truss manufacturers

show connections to upper chords only. The lower chords are especially

sensitive and that's why you have to proceed with so much care and

coordination.

S L

Protection Design & Consulting

 

[brudgers]

In this case, the load is spread over two trusses. Isn't there a safety factor involved with trusses that could account for the added weight?

The safety factor is there for safety with the designed loads, not so you can add stuff to reduce the safety of the design.

 

[brudgers]

I thought there was something in the steel joist institute literature that said..."thou shalt not hang off the bottom chord" can't find it now....when in doubt, make the engineer address it. Something about the bottom chords being a tension member and no loading allowed.....if anyone finds it please post up.....I try to bring it up at review and it becomes less of a problem...

Bottom chords are typically designed with some allowable load, otherwise you could not attach a ceiling to them...or hang pipes. BTW, in most truss designs the idea is to put all the members in tension if you can because that uses material most efficiently.

 

[steveray]

Thanks Brudgers...expected a shellacing on that one without a "code" reference....bottom line is most of us cannot say what load they (bottom chord) were designed for, so the only safe bet is to not attach anything without the approval of your (an) engineer...

 

[Mark K]

The reason for concern about joists is not that they are in tension but rather because the design of these members is often very tight. Loads of this magnitude would generally be ok if supported from the panel point. When the bottom chord member is a rod as opposed to a significant angle it is probably not a good idea to support loads between the panel point. As previously noted the engineer who designed the truss should make the call.

 

[chris kennedy]

What is a panel point?

 

[Mark K]

A panel point is where the vertical and diagonal web members intersect with the top or bottom chord. Loads applied at these points do not create bending in the chords. Because they do not produce bending in the chords loads at the panel points are more efficiently transfered to the truss. Top chords of trusses are typically designed to resist the ode specified loading between the panel points

 

[alora]

... As previously noted the engineer who designed the truss should make the call.

In this case, my money would be on the fact that an engineer just selected the trusses from a catalog.

 

[brudgers]

Thanks Brudgers...expected a shellacing on that one without a "code" reference....bottom line is most of us cannot say what load they (bottom chord) were designed for, so the only safe bet is to not attach anything without the approval of your (an) engineer...

I would tend to disagree. Again, the trusses are typically sized to allow for anticipated loads attached to the bottom chord - and because trusses are a standard sizes they usually have more capacity than the actual design loads - e.g. the lightest truss in the catalog which handles a 100plf design load has a capacity of 108plf.

 

[brudgers]

A panel point is where the vertical and diagonal web members intersect with the top or bottom chord. Loads applied at these points do not create bending in the chords. Because they do not produce bending in the chords loads at the panel points are more efficiently transfered to the truss. Top chords of trusses are typically designed to resist the ode specified loading between the panel points

The kind of trusses pictured don't have vertical members.