whose responsibility is it? design responsibility - … · whose responsibility is it? ... • not...

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Common Pitfalls in Steel Joist Specification

Whose responsibility is it?

• Joist supplier?• Steel fabricator?• Contractor?• Other trades?• Specifier?

Design Responsibility

• Design of standard joists per Steel Joist Institute Specifications

• Design of special joists for specified load case(s)• Determining bridging requirements

– Wind uplift– Lateral bracing of top chord if required (SSR)

Joist Supplier’s responsibilities include

Standard Assumptions

• SJI Standards apply– Seat depth– Live load deflection is span/360– Camber per chart to offset dead load

• Joists are laterally braced by roof deck• All joist design requirements are specifically stated

on structural drawings• Loads are distributed to panel points

SJI Code of Practice assumes

Steel Joist Institute

• SJI Specification is a performance spec– References load table– Specifies how joist is to be designed

• No requirement for any physical size• Bearing depths• Bridging requirements

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• Determining framing scheme• Calculating wind pressure from wind speed • Determining load requirements or load

combinations from building codes• Determining loads from mechanical drawings• Designing lateral bracing system• Connection design

Joist supplier’s responsibilities do not include

LRFD Load Combinations

• Special design joists• Specifying professional will need to spell out

– Unfactored loads• D – dead• L – live (occupancy & equipment)• Lr – roof live • S – snow live• R – rainwater

– Load combinations– ASCE 7 or other cases as deemed appropriate

Deflection

• SJI load tables show a live load which will produce a deflection of L/360 – Part of the standard design process

• More or less severe limits can be specified– L/600 (within reason)– L/240 for total load

Horizontal Movement

• Scissor joists and arches will move horizontally under load

• We can control, but not eliminate this deflection

Some deflection criteria we can’t meet…

• “…no deflection under live load…”• For KCS joist, “Maximum deflection shall be

limited to…”• “Deflection at end of joist extension shall be

limited to…”• Deflection limit of L/600 on long joist


• Optimizing framing scheme• Sizing joists for special loads• Coordination with sprinkler systems• Dimensional information

– Web layouts– Special configurations– Seat depths

Vulcraft can assist with

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Drawing Review

• Engineer of record– Loads– Structural issues

• Contractor– Building dimensions– Attachment locations

• Steel detailer– Bearing details– Dimensions

All verified items must be answered

Design Calculations

• For approval?– Data used is generally incomplete– Loads and dimensions still to be verified– Preparation at this point requires extra work for joist

supplier

• For record?– Data actually used for joist production– Reflects what was supplied– No additional steps in joist supplier’s work flow

What causes dimensional conflicts?

• Sloped top chords• Bearing depths• Bearing lengths

Depth of Joists with Sloped Chords

• SJI designation depth is at centerline

• Exception is offset double pitch – ridgeline depth

• Not limited to catalog depths

Joist Depth

Nominal Depth

Nominal Depth

Nominal Depth

Bearing Conditions

• SJI Minimum Bearing Length for Standard Conditions

SteelMasonry/Concrete

K and KCS 2 ½ ” 4 ”

LH02 to 06 incl 2 ½ ” 6 ”

LH/DLH07 to 17 incl 4 ” 6 ”

LH/DLH18 to 25 incl 6 ” 6 ”

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Dimension Definitions• Base length outside of seat to outside of seat

4” 4”

Base Length

For K series


4”

Base Length

Minimum

Bearing on Masonry

Reaction

Prevent joist from loading

the face of the wall

Bearing Conditions

Base Length

2”

R

• SJI Standard Bearing Length -Masonry or Concrete

2” @ K

4” @ LH, G

Bearing Conditions

• SJI Standard Bearing Length -Masonry or Concrete

Base Length

Assume to be zero

2” 2” @ K

4” @ LH, G

R


• SJI Standard Bearing Length -Steel

Base Length

2”

R

2½” @ K, LH02-06 2” @ LH7-17

Always better to have as much

bearing as possible up to

maximum based on type of joist.

4” @ LH18-25


4” 4”

Base Length TCXTCX

Overall Length

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• Minimum bearing– Ensures reaction will not overstress support– Not too close to face of wall– Not too near toe of beam flange

• Alternatives– EOR verify support design– Stagger joist bearing– Both can complicate approval process


• Maximum bearing– Ensures web will not foul with support– Reaction cannot move back due to geometry– Bearing capacity is not normally the issue

• Alternative– Deeper bearing– Increases clearance– Requires coordination

• Avoid bearing on top chord extension

Wider Supports• Base length outside of seat to outside of seat

6”

What if more bearing is required?


4”

Base LengthBearing Seat

Reaction

Anything past the 4” bearing seat is a tcx

Overall Length

2”

TCX

Deeper Bearing• Base length outside of seat to outside of seat

4”Bearing Seat

Reaction

Seat depth can be increased, but elevations

must be coordinated

Base LengthClear Bearing6”

Deeper Bearing

• High uplift can require deeper seats

4”Bearing Seat

Reaction

Compression in end web too large for rodMust use double angles

Base Length

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Girder Seats

• SJI Standard seat depth– Most Joist Girders 7 ½ inches

• Left of blue line

– Large Joist Girders 10 inches or more• Right of blue line

See pages 103-111 of Vulcraft manual

Different Bearing Depths in One Location

• Increase seat depth of smaller member• Never try to decrease the seat depth of the

larger member• Reasons should be clear from previous

discussion

Extended Ends

• Frequently same depth as bearing seat• SJI type “R”• Bearing seat extends back one panel to develop

capacity

K series 2 ½”LH series 5”

Extended Ends

• Short extensions may only be top chord• SJI type “S”


Extended Ends

• Short extensions may only be top chord• SJI type “S”• Bearing seat may extend out for dimensional

reasons, but is not considered in design


Extended Ends

• Specifying type “S” can be confusing• Is it important that the seat not continue?• Or is it only load that is meant?


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Extended Ends

• “If the loading diagram for any condition is not shown, the joist manufacturer will design the extension to support the uniform load indicated in the K-Series Joist Load Table for the span of the joist.”

• What can happen– 10K1 x 12-00 supports 200 plf total load– From the SJI load table, wTL = 550– Top chord extension 3-00– Deflection for 550 plf exceeds L/120 with the largest members

available

• But we could design for 200 plf with no problem

Extended Ends

• Best option for long extension• Define loads on contract documents• Deflection requirements must include loads

What about sloped joist bearing seats?

• When required?• What will the seat depth be?

Sloped Bearing

• Increased depth required – See page 42 or 47 of

Vulcraft manual• Depths shown

– Minimums (more is OK)– Clearance for end web– Clearance for extension

• Contact Vulcraft for extreme slope rates (6:12 or more)

Sloped Bearing

• Sloped seats required– Assumed 4” bearing– Maximum gap is 1/8”– Slope greater than 3/8:12

• Sloped seats not required– Slope is less that 3/8:12– No top chord extension, or

top chord only

gap

Sloped Bearing

• Sloped seats required– Long bearing length– R type extension– Maximum slope for

standard bearing will become less than 3/8:12

• Similar for LH joists

gap

bearing

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Sloped Bearing

• Sloped seat depth– End of base length– Not necessarily centerline

of bearing• Similar for LH joists

Seat Depth

5”

4”

c. l.

How to design for concentrated loads?

• Double joist? • Standard joist?• KCS series?• Special joist?

– Add load– Load diagram

Option 1

Use Double Joists• Generally conservative• Simple• Not very efficient• Potential problems with

framing & bridging

WTL = 240/120 plfWDRIFT= 120 plfP = 2 @ 600 lbs eaRL = 6413 lbsRR = 6187 lbsMMAX = 70584 k-ft @ 23.22 ft

Compute W EQ and W LL

WEQ1 = 2(6413)/44 = 292plfWEQ2 = 8(70584)/442 = 292plfWLL = 172 plf

L = 44-00

23.22 FT

Shear Diagram

14’ 4’ 12’

Option 2

Select a Standard Joist

Using WEQ = 292 plf WLL = 172 plf with a span of 44-00from the SJI Load Table

Joist TL/LL Weight/ft22K11 311/146 13.824K10 298/154 13.1

� 28K9 295/181 13.0 30K8 291/192 13.228LH05 297/180 13.0

Option 2

Select a Standard JoistWTL = 240 plfWDRIFT = 120 plfP = 2 @ 600 lbs eaRMAX = 6413 lbsWEQ = 292 plf

28K9WTL = 295 plfR = 6490 lbsVMIN =0.25(6490) = 1622 lbs

23.22 FT

14’ 4’ 12’

Required shear not covered by selection

Option 2

Select a Standard Joist

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JOIST DESIGNATION 10KCS1 10KCS2 10KCS3 12KCS1 12KCS2 12KCS3 14KCS1 14CSS2 14KCS3

16KCS2 16KCS3 16KCS4 16KCS5

DEPTH (inches) 10 10 10 12 12 12 14 14 14 16 16 16 16

MOMENT CAPACITY (inch-kips) 172 225 296 209 274 362 247 324 428 349 470 720 934

SHEAR CAPACITY (lbs) 2000 2500 3000 2400 3000 3500 2900 3400 3900 4000 4800 5300 5800

APPROX WEIGHT (lbs/ft) 6.0 7.5 10.0 6.0 8.0 10.0 6.5 8.0 10.0 8.5 10.5 14.5 18.0

MOM. OF INERTIA (in^4) 29 37 47 43 55 71 59 77 99 99 128 192 245

BRD ROW SEC 1 1 1 3 5 5 4 6 6 6 9 9 9

Option 3

Select a KCS-Series Steel Joist

Designed in accordance with the StandardSpecifications for K-series joists, plus. . .

– Constant moment capacity.– Constant shear capacity.– Interior webs designed for 100% stress

reversal.– Top chord end panel designed for axial

load based on the force in the end web.– No check for secondary bending.

Option 3

Select a KCS-series Steel Joist

Moment

Shear

Option 3


V

• Determine the required maximum end reaction and moment.

• Calculate the equivalent uniform load from the maximum moment.

• Find a KCS-series joist from the Load Table that has an end reaction and moment greater than the required.

• Check the required number of bridging rows using the standard K-series bridging chart.

M

Option 3


WTL = 240 plfWDRIFT= 120 plfP = 2 @ 600 lbs eaRMAX = 6413 lbsWEQ = 292 plf

28KCS3M = 846 in-kR = 8000 lbsVMIN = 8000 lbs

23.22 FT

14’4’ 12’

KCS Series

Advantages . . .

– Provides alternative to joist supplier provided “special designs” for joists with concentrated or unusual loading conditions.

– Allows the Design Professional to know the capacity of the joist being provided for a special loading condition.

– Can accommodate moving loads or loads that are not possible to locate.

Option 3


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Disadvantages . . .

– Much heavier than standard designation or “special design” joists.

– More expensive per unit than standard K-seriesjoists. (But overall project may be more

economical.)

Option 3


REMEMBER . . .– The equivalent uniform load calculated from the

required resisting moment must not exceed 550 plf.– If the equivalent uniform load exceeds 550 plf, you

have two options :• a) Use 2 KCS-series joists to provide a total

moment and shear capacity equal to the required. • b) Use an LH-series joist selected for the

equivalent uniform load. Provide a Load Diagram for this option.

Option 3


For occasions when the TOTAL Concentrated Load

to be applied is < 2000 # and the location cannot be

determined on the structural drawings

Vulcraft will design for:Mmax = wL2/8 + PL/4

andRmax = wL/2 + P

RTU

Location?

Note that secondary bendingis NOT checked

Option 4

Specify an “Add Load”

The Structural Engineer must provide the following information:

1. The SJI Standard Joist designation, or the uniform total and live loads

2. The amount of the “Add Load”

RTU

Location?

Example:20K4SPor20K200/100SPandSP=Design for additional load of 1000# applied at any panel point.

Option 4


Can an all series be designed for an “Add Load”?

K series Girders

LH series KCS series

DLH series

YES

YES

YES

YES

NO

Option 4


The Structural Engineer must provide the following information:

1. The SJI Standard Joist designation, or the uniform total and live loads

2. The additional point loads to be included

4’12’

1.0/0.5K 1.0/0.5K

12’ 4’

20K200/100SP

20K200/100SP

Option 5

Provide Specific Load Criteria

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Regardless of the option used, secondary bending is not checked for typical concentrated load designs

Specifying Joists with Concetrated Loads

Joist chords cannot be designed for any significant local moment

Local Moments

Connect to both chords so that only axial loads are applied to the joist.

Local Moments What about lateral moments?

• Load path?• Connections?• What moments to include?• When to make the connection?

Joist Girder Frames

• A structural frame utilizing joist girders with rigid connections at the supports.

• The rigid connections allow the joist girder to carry moments in addition to the reactions from the joist girder loads.

Joist-Girder Frames

Steel Joist InstituteTechnical Digest #11July, 1999

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Joist Girder Frames

Determine approximate moment of inertia of joist girder:

IJG = 0.027 NPLdN – number of joist spacesP – panel point loadL – length in feetd – depth in inches

Joist Girder Frames

Determine approximate moment of inertia of joist girder:

Joist Girder Frame

• Determine area of joist girder cross section– Refer to Vulcraft catalog for girder weight– AJG = girder weight (plf) / steel density (490

pcf)

Joist Girder Frames

Rigid Frame with Joist Girder

Idealized Rigid Frame Analysis

Joist Girder Frames

SJI Technical Digest offers two options:1. Provide end moments and gravity loads2. Convert the end moments to axial loads

based on the joist girder effective depth and provide the chord axial loads and gravity loads.

Design information required by joist girder manufacturer.

Joist Girder Frames

Vulcraft recommends against converting the end moments to axial loads.

Therefore…..


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Joist Girder Frames


� Standard joist girder designation

� Lateral moments due to wind and/or earthquake� Live load moments� (Axial loads)� (Net uplift)

Joist Girder Frames

Connection Details

Basic Connection

NO WELD

Welded Basic Connection

BOTH ANGLES 3/16” 6”

TYP.1/4” 5”

Eccentricity with Basic Connection


TYP.1/4” 5”

e

M

F

F

Maximum Pa

4 k to 10 k

Joist Girder Frames

• Commentary and Suggestions in Vulcraft catalog

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Welded Basic Connection with Tie


2 ANGLESTYP.1/4” 5”

Girder Moment Plate

BOTH ANGLES

Joist Girder Frames

• Connection Details

Axial Loads & Lateral Moments• Can be applied to

• K-series• LH, DLH, SLH-series• Joist Girders

• Connection must transfer the load

Moment Connections

• Consider load path• No axial load capacity for

7 ½ inch deep girder seats

• Sloped seats should not transfer axial load

Moment Connections

• Any load applied after the bottom chord extension is welded creates an end moment

• Live and dead load moments are induced

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