Joist Extension Correction RFI

[oengineer]

I have an RFI that I need some help on. Here is the RFI:

"Contractor has identified an issue where the Gym joists that rest on the wall along gridline 12 have an incorrect chord extension due to an error by the joist manufacturer. As fabricated these joists' top chords only extend 2'-11 3/4" from the outside face of the wall, wheras they should extend 3'-4 7/8". They are correcting the issue and designing for joist extensions but need the following questions answered to do this:Can you provide us the actual DL and LL that the joist extensions need to be designed for? I am having trouble getting a section to work using the 1000 plf load that is shown on the main span."

Here is the plan view:

https://files.engineering.com/getfile.aspx?folder=820a7e5e-4a6d-423c-8428-c31f827a96a3&file=RFI_.pdf

 

[BAretired]

@oengineer

That's good! Have a nice day.

BA

 

[oengineer]

Would any one know of a provision in the SJI CODE OF STANDARD PRACTICE FOR STEEL JOIST AND JOIST GIRDERS that would yield the 1000 plf total shown for the scissor joist for this particular condition?

 

[BAretired]

Would any one know of a provision in the SJI CODE OF STANDARD PRACTICE FOR STEEL JOIST AND JOIST GIRDERS that would yield the 1000 plf total shown for the scissor joist for this particular condition?

I doubt that there is any such provision, but when you review the RFI with the EOR, make sure that you review the practicality of the roof design before responding to the Contractor. I have concerns about the following quote from your General Notes (in blue):

THE ROOF DECK IS STANDING SEAM AND WILL NOT BRACE THE TOP CHORD OF THE BAR JOISTS. EXTRA BRIDGING IS SHOWN ON THE PLAN, BUT THE JOIST MANUFACTURER IS RESPONSIBLE FOR VERIFYING BRIDGING SPACING AND DESIGNING THE TOP CHORD AS REQUIRED FOR THE BRACING SPACING.

Please consider the following:
.1 Does an acoustic standing seam deck exist which spans 18.25' for the stipulated roof load?
.2 Will the deck be single, double or triple span? Double and triple seems unlikely.
.3 Is it reasonable to assign the design of unbraced scissor joists spaced at 18.25' to the joist manufacturer?

BA

 

[oengineer]

.2 Will the deck be single, double or triple span? Double and triple seems unlikely.

From the notes - METAL ROOF DECK IS DESIGNED FOR 3 SPAN CONDITIONS. IT IS THE RESPONSIBILITY OF THE DECK SUPPLIER TO ADJUST THE THICKNESS (GAUGE)
OF THE ROOF DECK AT SINGLE OR DOUBLE SPAN CONDITIONS AS REQUIRED BY THE STEEL DECK INSTITUTE.

.1 Does an acoustic standing seam deck exist which spans 18.25' for the stipulated roof load?
.

I will verify

.3 Is it reasonable to assign the design of unbraced scissor joists spaced at 18.25' to the joist manufacturer?

I will verify

 

[jayrod12]

You can't get 3 span continuous sheets spanning 25 ft per bay. 45 feet is typically the maximum, and the drawings already indicate 16 gauge, what are they supposed to adjust the thickness to? There's nothing thicker to adjust to.

This would be another question to bring up to the EOR. And I am with BA, seriously can an acoustic deck span 18.25 feet? 3" deck around here caps out at 13 ft.

 

[oengineer]

@BAretired & @jayrod12

Here is the roof deck design info:

https://files.engineering.com/getfi...-594b0bdd714d&file=V3.5LSAASD_-_Deck_Info.pdf

Based on this design spec, the deck can span 19 ft. It can do 2 spans, but 3 spans is not possible at a length of 18.5 ft.

 

[jayrod12]

I see it does work, albeit barely.

Thanks for following up, still not a giant fan of that system.

 

[BAretired]

From the notes - METAL ROOF DECK IS DESIGNED FOR 3 SPAN CONDITIONS. IT IS THE RESPONSIBILITY OF THE DECK SUPPLIER TO ADJUST THE THICKNESS (GAUGE)
OF THE ROOF DECK AT SINGLE OR DOUBLE SPAN CONDITIONS AS REQUIRED BY THE STEEL DECK INSTITUTE.

It is not clear where roof deck is to be spliced in the area with 18.25' spans. They cannot be 3 span as stated in the General Note above. Are they all single span? Are some of them double?

For 2 span conditions, the reaction to the middle joist is greater than 18.25(DL + LL) because of continuity.


BA

 

[oengineer]

It is not clear where roof deck is to be spliced in the area with 18.25' spans. They cannot be 3 span as stated in the General Note above. Are they all single span? Are some of them double?

For 2 span conditions, the reaction to the middle joist is greater than 18.25(DL + LL) because of continuity.

This type of deck is Double Span, TRIPLE SPAN is not possible. This is shown in the this quote:

Here is the roof deck design info:

https://files.engineering.com/getfile.aspx?folder=...

Based on this design spec, the deck can span 19 ft. It can do 2 spans, but 3 spans is not possible at a length of 18.5 ft.

@anyone
What would be the deference in weights for a regular joist verses a scissor joist?

 

[StrucEngg]

Do you work for a AEC firm in Texas with multiple offices which designs schools?

 

[oengineer]

@BAretired
Thank you for your help and guidance! I really appreciate it!


@Everyone
Thank you all for your input!

 

[BAretired]

The Roof Framing Plan calls for 18 ga. deck in the bay nearest Gridline T and 16 ga. for the other four bays. Why you do not use the same gauge throughout is a puzzle to me. You have five bays. If 16 gauge is used in all five bays, it would be best to alternate double and single spans as follows:

1,2,2​

2,2,1​

In that way, adjacent deck splices never occur side by side.

Alternatively, if you stick to 18 ga. deck in one bay, you would have

2,2,1​

2,2,1​

And all joints are lining up with heavier load going to the middle joist in each of the two span cases...absolutely unacceptable.

BA

 

[BAretired]

@anyone
What would be the deference in weights for a regular joist verses a scissor joist?

I assume you mean difference, not deference. Since there have been no takers on this question, I'll comment briefly on it.

In 53 years of consulting, I have never to my recollection specified a scissor joist or truss. For that reason, I cannot give you a comparison based on experience. At a rough guess, I might start with a scissor joist having an average depth equal to a parallel chord joist. That may be nowhere near the correct depth and needs to be checked for strength and deflection (particularly horizontal deflection, but also vertical deflection). Easiest way to do that is by computer.

Your span is approximately 74' if I added correctly. I looked at two catalogs which I used to use regularly, namely Great West Steel Industries (GWS) and Canam Steel Works (CSW). Admittedly, GWS is no longer in business, but their catalog is still helpful.

For a 74' span, GWS lists a variety of joists, the heaviest of which is 72N (72 is the depth in inches) weighing 26#/' which carries a total safe load of 621 plf, not nearly adequate for your load of 1000#/'.

For a 72' span, CSW lists a 72" deep joist weighing 30.5#/' which carries a service load of 680 plf, again not adequate for your project.

The Roof Framing Plan on your project indicates the elevation of bottom of deck at each wall and the ridge, but there is no indication of bottom chord elevations which would be required to make an assessment of comparative weight with a standard parallel chord joist.

If you can provide a typical elevation of the scissor joist, someone on Eng-Tips may be able to answer your question.

BA

 

[BAretired]

@BAretired
Thank you for your help and guidance! I really appreciate it!


@Everyone
Thank you all for your input!

You are welcome, but I have an uneasy feeling that there will be some serious problems ahead.

BA

 

[XR250]

1.)ALL ROOF FRAMING TOP CHORD EXTENSIONS OF STEEL JOIST SHALL BE DESIGNED BY THE JOIST ENGINEER FOR 45 PSF TOTAL, 20 PSF LIVE
UNIFORM GRAVITY LOADING, AND 75 PSF WIND UPLIFT (TYPICAL ALL TOP CHORD EXTENSIONS UNO).

2.)JOIST ENGINEER SHALL DESIGN AND PROVIDE JOIST SYSTEMS WITH CANTILEVERED “JOIST SUBSTITUTE” JOISTS FOR ALL CONDITIONS SHOWN ON
THE FRAMING PLANS, EVEN IF THE BACKSPAN LENGTHS ARE LESS THAN THE UNSUPPORTED CANTILEVERED LENGTHS AND EVEN IF THERE IS A
DEVIATION FROM SJI DEFAULT STANDARDS. THE CONTRACTOR SHALL INCLUDE ALL COSTS ASSOCIATED WITH THIS REQUIREMENT IN THE
PROPOSAL. THE JOIST ENGINEER SHALL DETERMINE THE REACTIONS AT JOIST SUBSTITUTE JOISTS SUPPORT POINTS AND PROVIDE SUFFICIENT
VERTICAL RESISTANCE (UP AND DOWN) IN JOIST CONNECTIONS AND IN OTHER JOIST MEMBERS THAT SUPPORT JOIST SUBSTITUTE JOISTS.

3.)ROOF JOIST SUPPORTING ROOFTOP MECHANICAL EQUIPMENT (RTU'S OR OTHERWISE) SHALL BE DESIGNED BY THE JOIST ENGINEER TO SUPPORT
THE INDICATED ADD-LOADS, AS DEFINED IN THE SJI CODE OF STANDARD PRACTICE FOR STEEL JOIST AND JOIST GIRDERS, PLUS A UNIFORM LOAD OF
45 PSF TOTAL LOAD (20 PSF LIVE LOAD). THE MAXIMUM DEFLECTION OF ANY ROOF JOIST SHALL BE L/240 UNDER TOTAL LOAD AND L/360 UNDER LIVE
LOAD. LOADINGS FOR MECHANICAL EQUIPMENT ARE BASED ON THE UNIT(S) SHOWN ON THE STRUCTURAL DRAWINGS. ANY CHANGES IN TYPE, SIZE,
WEIGHT OR NUMBER OF UNIT(S) SHALL BE REPORTED TO THE ARCHITECT PRIOR TO FABRICATION OR INSTALLATION OF STRUCTURAL MEMBERS OR
MECHANICAL EQUIPMENT.

4.)UNLESS INDICATED OTHERWISE ON THE DRAWINGS, WHERE JOISTS ARE SUPPORTING WALL LOADING, THE ADDITIONAL LOAD THE JOISTS ARE
REQUIRED TO SUPPORT SHALL BE DETERMNED BY THE JOIST ENGINEER BASED ON THE FOLOWING ASSUMED WALL LOADING:
A. MASONRY VENEER: 39 PSF X HEIGHT OF WALL SUPPORTED
B. CFMF WALL: 12 PSF X HEIGHT OF WALL SUPPORTED
C. CMU WALL: 75 PSF X HEIGHT OF WALL SUPPORTED

5.)THE JOIST GIRDERS SHALL BE DESIGNED AS A SIMPLE SPAN WITH THE UNFACTORED JOIST REACTIONS INDICATED BY THE JOIST GIRDER
DESIGNATION (DEAD LOAD AND LIVE LOAD).

STEEL JOISTS SHALL CONFORM TO THE REQUIREMENTS OF THE STEEL JOIST INSTITUTE STANDARD SPECIFICATIONS FOR OPEN WEB JOISTS.
MATERIAL SHALL BE DOMESTIC STEEL WITH ANGLES FOR BOTTOM CHORDS. BRIDGING SHALL BE PROVIDED FOR ALL JOISTS IN ACCORDANCE WITH
SJI REQUIREMENTS.

METAL ROOF DECK SHALL BE PROVIDED IN ACCORDANCE WITH THE PROJECT SPECIFICATIONS AND AS INDICATED ON THE PLANS. METAL ROOF DECK
SHALL BE ATTACHED TO SUPPORTING MEMBERS PER THE PROJECT SPECIFICATIONS.

ALL JOISTS SHALL HAVE BOTTOM CHORD UPLIFT BRIDGING AT EACH END AND BE DESIGNED FOR 35 POUNDS PER SQUARE FOOT NET UPLIFT.

FOR ALL ROOF PENETRATIONS THROUGH METAL DECK 8” OR LARGER PROVIDE AN L3X3X1/4 AROUND THE OPENING AND FRAME ALL SIDES (EXCEPT
WHERE OPENING EDGE IS WITHIN 1'-0” OF ANOTHER FRAMING MEMBER - JOIST/BEAM) WITH A STRUCTURAL MEMBER. FRAMING NOT REQUIRED FOR
ROOF OPENINGS SMALLER THAN 8”.

THE ROOF DECK IS STANDING SEAM AND WILL NOT BRACE THE TOP CHORD OF THE BAR JOISTS. EXTRA BRIDGING IS SHOWN ON THE PLAN, BUT THE
JOIST MANUFACTURER IS RESPONSIBLE FOR VERIFYING BRIDGING SPACING AND DESIGNING THE TOP CHORD AS REQUIRED FOR THE BRACING
SPACING.

LOCATE CONCENTRATED LOADS ON JOISTS AT PANEL POINTS. PROVIDE ANGLE WEB MEMBERS TO CREATE INTERMEDIATE PANEL POINTS AS
REQUIRED. MANUFACTURER SHALL DIRECT INSTALLER AS TO METHOD OF INSTALLATION AND MATERIAL REQUIRED. JOISTS AND JOIST GIRDERS
SHALL BE SHOP REINFORCED FOR ALL LOADS PROVIDED ON DRAWINGS. FIELD REINFORCING SHALL BE PROVIDED AS DETAILED ON THE DRAWINGS.

METAL ROOF DECK IS DESIGNED FOR 3 SPAN CONDITIONS. IT IS THE RESPONSIBILITY OF THE DECK SUPPLIER TO ADJUST THE THICKNESS (GAUGE)
OF THE ROOF DECK AT SINGLE OR DOUBLE SPAN CONDITIONS AS REQUIRED BY THE STEEL DECK INSTITUTE.

PROVIDE SUPPORT FOR EDGES OF DECK WHETHER SHOWN ON THE DRAWINGS OR NOT.

UNLESS INDICATED OTHERWISE ON THE DRAWINGS, WHERE BRACES ARE SHOWN TO ATTACH TO THE ROOF JOIST, THE ENGINEER SHALL ASSUME
THE LOAD IN AXIAL LOAD IS EQUAL TO A COMPRESSION OR TENSION SERVICE LEVEL WIND LOAD OF 2 KIP.

Wow, seems like I am doing it wrong. I spend way too much time actually engineering things.
My new Structural Notes:
1) Metal Deck Supplier to design the roof and floor deck and attachments for all required gravity, lateral, wind and seismic loadings
2) Steel supplier to design all members and attachments for all required gravity, wind, lateral and seismic loadings
3) Concrete supplier to design slabs, footings and rebar for all required gravity, wind, lateral and seismic loadings
4) Excavation contractor to provide ample sized footing excavations to support all required gravity, wind, lateral and seismic loadings

LOL!

 

[BAretired]

@XR250,

You forgot the following:

Contractor to retain Engineer of Record (EOR) to check all shop drawings, perform construction review as deemed necessary by EOR and prepare reports to Owner. Fees to be commensurate with hourly fees set out in Engineering Schedule of Fees shown in specification or as otherwise determined by EOR.

All errors by contractor to be corrected as instructed by EOR at no cost to the Owner.
All errors by EOR, Architect or any of their friends to be corrected at no cost to the Owner.

BA

 

[XR250]

Yes!

 

[BAretired]

@oengineer,
Can you provide a cross section through the building showing the configuration of the scissor joist?

BA