TECHNICAL BULLETIN
TB-808
July 2015 (Expires 7/2017)
Bottom Flange Trus
Joist®
TJI®
Joist Attachment Connections
This technical bulletin provides designers and engineers with guidelines for specifying withdrawal connections into the bottom
flange of residential Trus Joist® TJI® joists with wood screws or lag screws. Examples of items commonly attached to the bottom
flange include furnaces, ductwork, bike racks, garage door motors/tracks, lights, etc. The information in this bulletin is not
appropriate for sprinkler attachments, which must comply with NFPA 13. For sprinkler details, refer to Weyerhaeuser’s Fire-Rated
Assemblies and Sprinkler Systems, reorder 1500.
Design considerations for bottom flange attachments include:
•
Maximum allowable bottom flange load is 500 lbs every 5 feet (250 lbs on each side of flange).
•
•
Minimum edge and end distances shall follow 2012 NDS® guidelines.
Lead holes are required as follows:
o Lag screws ¼" to ½" diameter = 50% of diameter.
o Wood screws larger than #8 = 70% of root diameter.
•
Withdrawal values shall be determined using a specific gravity, G, of 0.50 and taken as:
o
Lag Screws: 2012 NDS® Table 11.2A or W = 1800G (3 2 ) D (3 4 ) (lbs/in of thread penetration)
Wood Screws: 2012 NDS® Table 11.2B or W = 2850G 2 D (lbs/in of thread penetration)
Spacing of fasteners shall be taken as:
o Lag Screws: 2012 NDS® Table 11.5.1E
o Wood Screws: 2012 NDS® Table C11.1.5.7 or sufficient to prevent splitting of the flange.
o
•
•
•
Penetration into the web is permitted.
Loads applied or attached to the bottom flange must be accounted for in the design of the joist.
Table 1: Residential TJI® Flange Sizes
Joist Series
TJI® 110
TJI® 210
TJI® 230
TJI® 360
TJI® 560
Minimum Flange Thickness (in)
1.25
1.25
1.25
1.375
1.375
Flange Width (in)
1.75
2.08
2.30
2.30
3.50
Note: For sprinkler attachment
details, refer to 1500 (Fire-Rated
Assemblies and Sprinkler Systems)
Table 2: Common Screw Withdrawal Capacities
Fastener
Lead Hole
W(1) (in/lbs)
7 gauge Wood Screw
8 gauge Wood Screw
9 gauge Wood Screw
10 gauge Wood Screw
12 gauge Wood Screw
14 gauge Wood Screw
¼" Lag Screw3
5/16" Lag Screw3
⅜" Lag Screw3
7/16" Lag Screw3,4
½" Lag Screw3,4
3/32"
3/32"
7/64"
7/64"
1/8"
9/64"
1/8"
5/32"
3/16"
7/32"
1/4"
107
117
126
135
154
172
225
266
305
342
378
1.
2.
TJI® 110
134
146
158
169
193
215
281
-
WMax(2) (lbs)
TJI® 210, 230
134
146
158
169
193
215
281
333
381
428
473
TJI® 360, 560
147
161
173
186
212
237
309
366
419
470
500(5)
Withdrawal capacity from 2012 NDS® Tables 11.2A and 11.2B based on the fastener diameter and a specific gravity of 0.50. Capacity shown is in
pounds per inch of thread penetration into the main member, excluding the tapered tip of the fastener.
Maximum withdrawal capacity is based on the threaded portion of the fastener engaging the full thickness of the flange, which may not occur with many
fastener lengths and connection configurations.
*
1.888.453.8358 ● www.woodbywy.com
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TECHNICAL BULLETIN
TB-808
3.
4.
5.
July 2015 (Expires 7/2017)
Clearance holes may not penetrate into the flange.
Requires a 10% reduction to design moment values.
Withdrawal capacity capped by the limit of 500 lbs for flange tear off.
Design Example:
Given:
•
TJI® 230 joist series.
•
¼" x 1½" lag screw thru ¼" metal plate into bottom flange.
Find:
•
What is maximum withdrawal capacity of lag screw?
Solution:
•
From 2012 NDS® Table L2, the dimensions for a ¼" x 1½" lag
screw are as follows:
o
H = 11/64"
o
L = 1 ½"
o
D = ¼"
o
Dr = 0.173"
o
S = ¼"
o
T - E = 1 3/32"
o
E = 5/32"
Figure 1: ¼" x 1½" Lag Screw
•
From Table 2, the reference design withdrawal value is 225 lbs/in of thread penetration.
•
From Table 1, the flange thickness of a TJI® 230 joist is 1¼".
•
Determine the length of thread engaged in the flange:
o
Determine if the screw shank dimension, S, penetrates into the flange. In this example, the screw dimension "S"
is the same as the metal side member, so the shank does not penetrate the main member.
o
Determine length of thread in the main member. Since the shank is not in the main member, the amount of
thread in the main member is the dimension "T - E", 1 3/32". In this case, the flange thickness is greater than the
"T - E" dimension (1¼" > 13/32"), so "T – E" is the length of thread engaged in the flange. Had the "T - E"
dimension been greater than the flange thickness, the flange thickness would be used as the length of thread
engaged. .
•
Lag screw capacity = 1 3/32" x 225 lbs/in = 245 lbs of withdrawal capacity.
•
Check end, edge, and spacing requirements per 2012 NDS®:
o
Minimum End Distance = 4D = 1"
o
Minimum Edge Distance = 1.5D = 3/8"
o
Minimum Spacing = 4D = 1"
,’Weyerhaeuser,’Forte,’Javelin,’Microllam,’Parallam,’Silent’Floor,’TimberStrand,’TJI,’TJ,’Trus’Joist,’Edge,’and’Edge’Gold’are’registered’trademarks’
and’TJ-Pro’is’a’trademark’of’Weyerhaeuser’NR.’©’2015’Weyerhaeuser’NR’Company.’All’rights’reserved.
1.888.453.8358 ● www.woodbywy.com
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