cantilvered column details R=2.5 OMF SMF Timber Frame partially open structure. 1

[bdruehl]

this has come up before....

for open on one side structure, please offer comments to an interior (braced) wood columns with cantilevered steel side columns, flexible roof diaphragm system. all wood except for side columns (therefore a dual framing system) - (no shear walls possible)
seismic zone e. approx 45' 35' by 25 high. interior timber "truss" construction for roof framing on approx 14 foot centers, for looks.

questions include:

- its always quite baffling how big these loads get. r=1.25, (or should it be 1.5)?
- overstrength requirements
- special detailing? for the fixed moment base? what else? for the connection of the wood to the top of the cantilvered column?
- and what kind of "requirements" (table 12.2-1 7-05 ) are for detailing for OMF? how do i get qualified for a SMF? obviously, this would be ideal because 2 1/2 is much better than 1.25 for R. but even so, the middle post is considered a "timber frame", so, im stuck with 1.5 for R anyhow. forgo the wood column in the center and go with all steel to get that R=2.5 perhaps?

regards... thnx in advance for comments with no rulers to the knuckles.

 

[bdruehl]

GETTING DEEPER into this now...

Encountered this little blurb.. from aisc 341 so an "Ordinary" moment frame not allowed in Design cat D or E....

E5.ORDINARY CANTILEVER COLUMN SYSTEMS (OCCS)1.Scope
Ordinary cantilever column systems (OCCS) must satisfy all the applicable require-ments of Chapters A, B, C, D, I and J of these Provisions, as well as the requirementsin Section E5.
2.Basis of Design
ASCE/SEI 7 (ASCE, 2010) includes two types of cantilever column systems, ordi-nary and special. OCCS are intended to provide a minimal level of inelastic rotationcapability at the base of the column. This system is permitted in seismic design cat-egories B and C only, and to heights not exceeding 35 ft. A low seismic responsemodification coefficient,
R
, of 1.25 is assigned due to the system’s limited inelasticcapacity and lack of redundancy. The OCCS has no requirements beyond those in the
Specification
except as noted in Section E5.4a.
4.System Requirements4a.Columns
ASCE/SEI 7 (ASCE, 2010) limits the required axial load on columns in these sys-tems under the load combinations including amplified seismic load to 15% of the (insert axial capacity)...

Com

 

[bdruehl]

2.Stability Bracing of Beams
When required in Chapters E, F, G and H, stability bracing shall be provided asrequired in this section to restrain lateral-torsional buckling of structural steel or con-crete-encased beams subject to flexure and designated as moderately ductilemembers or highly ductile members.
User Note:
In addition to the requirements in Chapters E, F, G and H to providestability bracing for various beam members such as intermediate and
specialmoment frame
beams, stability bracing is also required for columns in the
specialcantilever column system

 

[bdruehl]

ASCE says ordinary is allowed up to 35 feet tall..as long as DL does not exceed 35 lb/sf in D or E, which applies to this project. more "light" reading to do

 

[bdruehl]

just posting out loud here...415 excerpt

Required Flexural Strength
Where column bases are designed as moment connections to the foundation, the
required flexural strength
of column bases that are designated as part of the SFRS,including their attachment to the foundation, shall be the summation of the requiredconnection strengths of the steel elements that are connected to the column base asfollowsa)For diagonal braces, the required flexural strength shall be at least equal to therequired flexural strength of diagonal brace connections.(b)For columns, the required flexural strength shall be at least equal to the lesser of the followingi)1.1

(ASD), as applicable, of the column, or(ii)the moment calculated using the load combinations of the applicable build-ing code, including the amplified seismic load.
User Note:
Moments at column to column base connections designed as simpleconnections may be ignored.

 

[bdruehl]

SO, ARE TUBE STEEL COLUMNS ALLOWED??

thread507-290892

SEE THAT THREAD THERE

 

[bdruehl]

and a full penetration weld makes it a "special" ... to get that r= 2.5?

 

[wannabeSE]

AISC 341-10 includes Special Cantilever Column Systems (SCCS). You might consider using the newer seismic provisions even if the project is using an older code.

http://www.aisc.org/content.aspx?id=2884#seismic

 

[bdruehl]

excerpt from 341-05

Structural steel systems in seismic regions are generally expected to dissipate
seismic input energy through controlled inelastic deformations of the structure.
These Provisions supplement ANSI/AISC 360 for such applications. The seismic
design loads specified in the building codes have been developed considering
the energy dissipation generated during inelastic resp onse.
The Provisions are intended to be mandatory for structures where ANSI/AISC
341 has been specifically referenced when defining an R factor in SEI/ASCE 7
(ASCE, 2005). Typically this occurs in seismic design category D and above,
where the R factor is greater than 3. However, there are instances where an R
factor of less than 3 is assigned to a system and ANSI/AISC 341 is still required.
These limited cases occur in Table 12.2–1 (ASCE, 2005) for cantilevered column
systems and Table 15.4–1 for intermediate and ordinary moment frames
with height increases. For these systems with R factors less than 3, the use of
these Provisions is required.[/b] In general, for structures in seismic design category
A to C the designer is given a choice to either solely use ANSI/AISC 360 and the
R factor given for structural steel buildings not specifically detailed for seismic
resistance (typically, a factor of 3) or the designer may choose to assign a higher
R factor to a system detailed for seismic resistance and follow the requirements
of these Provisions.

 

[bdruehl]

aisc 341-10 specifically states under E5 "ordinary cantilever column systems".. that they are allowed in seis des cat B and C only. Currently in in 05 they are allowed. ASCE 7-05 also allowed in D and E with some load restrictions... which are not hard to satisfy.

clear as mud

 

[bdruehl]

and then there is the problem of the anchorage into concrete ... in seismic zone E... i got an omega'd load (1.25x) of say 5000 lbs on top of a 9 foot "ordinary" (or maybe its special?) cantilevered column system, proving rediculous.

and this post is getting no love...

 

[MJB315]

Could you boil this down? What's the actual question?

"We shape our buildings, thereafter they shape us." -WSC