Repetitive Member factor in built up girders
Repetitive Member factor in built up girders
(OP)
Can anyone tell me or refer me to documentation that allows (or not) using the repetitive member factor for built up 2x wood girders?
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS Come Join Us!Are you an
Engineering professional? Join Eng-Tips Forums!
*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail. Posting GuidelinesJobs |
Repetitive Member factor in built up girders
|
Repetitive Member factor in built up girdersRepetitive Member factor in built up girders(OP)
Can anyone tell me or refer me to documentation that allows (or not) using the repetitive member factor for built up 2x wood girders?
Red Flag SubmittedThank you for helping keep Eng-Tips Forums free from inappropriate posts. Reply To This ThreadPosting in the Eng-Tips forums is a member-only feature.Click Here to join Eng-Tips and talk with other members! |
ResourcesThe world has changed considerably since the 1980s, when CAD first started displacing drafting tables. Download Now
Prototyping has always been a critical part of product development. Download Now
As the cloud is increasingly adopted for product development, questions remain as to just how cloud software tools compare to on-premise solutions. Download Now
Engineering-centric businesses face a number of challenges today, but unmanageable design and change processes don’t need to be counted among them. Download Now
|
RE: Repetitive Member factor in built up girders
As a side note, in the 1994 UBC, in note 4 of table 23-I-A-3, the Reperiotive Member factor is mentioned for 2" to 4" dimension members of trusses.
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: Repetitive Member factor in built up girders
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: Repetitive Member factor in built up girders
I know that in the NDS it says that I can use this factor for joists, rafters etc. where the members are 24" or less on center. I just don't recall it saying specifically that it can be used on members attached to one another.
RE: Repetitive Member factor in built up girders
4.3.9 Repetitive Member Factor,
"members which are in contact or spaced not more than 24" on centers, are not less than three in number......"
Missed the "In Contact" part. Guess I should read more carefully.
Don't have a copy of the latest NDS handy. I'll have to check to see if anything has changed.
Thanks again Mike
RE: Repetitive Member factor in built up girders
RE: Repetitive Member factor in built up girders
Jason A. Partain, P.E.
www.myconstructionfinds.blogspot.com
RE: Repetitive Member factor in built up girders
RE: Repetitive Member factor in built up girders
RE: Repetitive Member factor in built up girders
Per the NDS (1997 commentary) section 4.3.4-Repetitive Member Factor "...The adjustment recommended in ASTM D245 (18), is based on the increased in load-carrying capacity and stiffness obtained when multiple framing members are fastened together or appropriately joined by transverse load distributing elements..."
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.
RE: Repetitive Member factor in built up girders
RE: Repetitive Member factor in built up girders
RE: Repetitive Member factor in built up girders
The repetitive member factor is used to adjust the allowable design stress for the member you are designing. The allowable design stresses for wood are based on the 95th percentile of actual load capacity. So for every 100 joists loaded, 95 will have an actual strength greater than the design strength. When designing a single member, there is a probability that the member will fall into the lower 5th percentile. When you have 3 or more members that can share load, what is the likelihood that all three will fall into the lower 5th percentile? Through physical testing, a 15% strength increase was calculated to account for this probability.
Based on this premise, as long as the load can be shared among 3 or more members, the repetitive member factor can apply. Whether the 3 members are attached to each other or spaced up to 24", the members still share the load. A 3-2x10 beam has the same allowable bending stress as a 2x10 joist. The capacity is different because it is bigger, but the basic allowable stress does not change.
Note that this factor applies to sawn lumber only. Engineered Lumber does not allow increases to be taken because it is already factored into the design. Also, being an engineered product results in a smaller coefficient of variation for design stresses.
Jason A. Partain, P.E.
www.myconstructionfinds.blogspot.com
RE: Repetitive Member factor in built up girders
No. Must be at least three.
per the 2005 NDS
4.3.9 Repetitive Member Factor,
"members which are in contact or spaced not more than 24" on centers, are not less than three in number......"
RE: Repetitive Member factor in built up girders
To be quite honest (I don't do a bunch of wood design), I wasn't aware that the repetitive member factor only applied to dimension lumber. It's curious that when I select Beam - LVL n-ply in Woodworks Sizer (v6.4), it activates and selects the repetitive member check box! It also does it for Lumber n-ply, regardless of the number of plies... kind of a fake-out.
Again, learn something new every day!