MULLION ALLOWABLE DEFLECTION

SKYLIGHT MULLION ALLOWABLE DEFLECTION LIMITS UNEARTHED AT 307 ANNEX

The 2009 IBC is rather silent relating to specific deflection limits for mullions.  You can start to get a flavor for limits associated with sunroom additions and pattio covers as listed in footnote "h" of table 1604.3 but then you are left hungry for more, with no direction.

So here is the scoop.  We have downloaded a document titled "Maximum Allowable Deflection of Framing Systems for Building Cladding Components at Design Wind Loads" and have placed it at G:\Codes\TIR A-11-04 for all to use.  The document suggests design deflection limits for span "L" (which it denotes as clear span, i.e., length of member between reaction points) of L/175 for spans up to 13'-6". and L/240 +1/4" for spans up to about 40'.  These deflections are only a starting point though as deflections are further restricted to prevent tearing of sealant, etc.  You will have to review the document to fully unearth it's hidden secrets.

Ben

Lateral-Torsional Buckling in Wide Flange beams, I-joists and similar shapes

Hello all,

Recently, someone mentioned proper bracing of cantilevered members; that the bracing should always be installed on the compression face.  This came up on a project I worked on a few years ago, and when I looked into it I found that bracing the compression face of a cantilever can potentially lead to failures.

The AISC Engineering Journal published a document discussing the relationship between bracing location and lateral-torsional buckling in wide flange sections.
http://www.aisc.org/store/p-1108-lateraltorsional-buckling-of-wide-flange-cantilever-beams-pdf.aspx

As mentioned in the abstract, they determined that tension face bracing of cantilevered wide flange beams is more effective than compression face bracing.

With all of this said, I haven't encountered very many situations where cantilevered members with compression side bracing that do not also have a diaphragm or perpendicular members at the tension face.  It can, however, reduce the construction costs and architectural issues by eliminating the need for bracing at the underside of a large cantilevered roof.

Edit 3/19/2013:

For additional discussion, see
http://www.eng-tips.com/viewthread.cfm?qid=275285
and
http://www.eng-tips.com/viewthread.cfm?qid=212541

Concentrated loads on metal joist chords

Hello all,

I went to http://www.vulcraft.com/engineers/information/  and found a document titled "Concentrated Loads on Joist Chords" which states "SJI conducted research and determined that loads up to 100 pounds placed between panel points have a negligible effect on the overall performance on the joist"

This is quite a usefull little document when you are supporting light suspended ceilings or mechanical equipment! 

Retaining Wall Bearing Eccentricity

Enercalc creates a flag if our resultant force is falling outside the middle third of the footing.  If all global stability and bearing pressures meet the proper safety factors, is this a concern?  When the resultant falls outside the mid-third, will the footing lift up making the design unacceptable?  I would be curious what others have to say.

Shotcrete Worker Certification

A week or so ago, during a Precon meeting, I was asked by the City of Seattle if I cared if some of the shotcrete nozzle men or Blow-pipe men were trainees.  Without knowing any code requirements on the subject, I said that I did not care as long as they did it right.

It was interesting to me that the City of Seattle asked and recorded the names of all shotcrete workers.  Now as I review the shotcrete shop drawings, I see that all nozzle men and Blow-pipe men are listed along with years of experience and state they meet or exceed the ACI Qualification requrements.  Apparently there is a certification we can request: https://www.shotcrete.org/ASAcertification.htm

Does anyone have further knowledge on the subject of certification or on the design considerations and limitations of using shotcrete applications in lieu of pouring?

SFRS Special Inspections for One- or Two- Family Dwellings

We should be specifying in our construction documents what items need special inspection (SI).  I have seen an itemized list in our general notes (for large projects...not Single Family though) that covers special inspection for general construction (through IBC 1704).  However, we are required per IBC 1705.3 to also include the Seismic-force-resisting system (SFRS).  If we are designing a detached one-or-two family dwelling, we can avoid the SI requirement if our SFRS does not contain certain horizontal or vertical irregularities (listed in Exception#3 of 1705.3).  If it does have any of these irregularities, the SFRS SI is triggered.

Here is the perplexing requirement worth blogging and coming to a consensus on: 

Section 1705.3 requires SI of the "seismic-force-resisting systems..." (Section 1705.31).  

This boggles Chuck and me.   If one was able to clearly delineate out the SFRS in the overall Bearing Wall System, only a few sticks would be left, in my opinion, that are used for gravity only and, subsequently, need not be checked.  We would be asking the special inspector to spend 2 days on the house checking every nail that resides as a component of the SFRS, for shearwalls, diaphragms, etc.

Chuck suggested that perhaps only the exceptions triggering the SI of the SFRS (along with any other important components) be specified for SI.  That seems logical to me.  Perhaps we could substantiate this preference or "interpretation" by alluding to the fact that the phrase "seismic-force-resisting systems" from section 1705.31 is plural and only certain seismic force resisting systems, or "components", require inspection.  

Thoughts?

Concrete Gravity Columns

So I was looking through the ACI 318-08 at the tie requirements for concrete gravity columns and saw that the entire Chapter 21 was re-written.  So don't go looking for revision bars along the side of the pages because there aren't any.  Some changes have been made to the tie requirements that I thought would be nice to summarize.  I'm going to pull information straight from the code so please forgive if it is too redundant.  Thanks

Concrete gravity columns by definition are not part of the lateral force resisting system and therefore are detailed per section 21.13.

Depending on the design method and level of forces, columns are to be details per 21.13.3 or 21.13.4.  If one determines the design displacement, the resulting member and shear forces on the column, and if the capacity is greater than the demand then 21.13.3 can be followed.  Otherwise 21.13.4 must be followed which provides a level of ductility and strength that match a column which is part of the LFRS.

21.13.3 - where induced moments/shears from the design displacements are less than the member moment/shear strength capacity then to following sections shall be satisfied.

• 21.13.3.1 - for lightly loaded members where Pu < Ag f'c/ 10 shall satisfy
• 21.5.2.1 - provides minimum reinforcement for flexural members. 
• Stirrups shall be placed no more then d/2 for length of member.
• 21.13.3.2 - for moderately loaded members where Pu > Ag f'c/10 shall satisfy
• 21.6.3.1 - longitudinal reinforcement to be between 1 and 6 percent.
• 21.6.4.2 - arrangement of hoops and crossties in column, 14" max.
• 21.6.5    - shear strength requirements at faces of joint for column.
• Tie spacing So shall not exceed smaller of 6 times diameter of smallest longitudinal bar and 6 inches.
• 21.13.3.3 - for heavily loaded members where Pu > .35Po shall satisfy
• 21.13.3.2 - see above for moderately loaded column.
• 21.6.4.7 -  cover limitation of 4" along with additional transverse reinforcement to confine additional cover.
• Amount of transverse reinforcement shall be 1/2 that required by 21.6.4.4 but shall be spaced not greater than So for the full member length.

 21.13.4 - where induced moments/shears are not calculated or where the demand exceeds the capacity of the column then the following sections shall apply.  Note that this would be our typical design approach.

• 21.13.4.1 - materials shall satisfy
• 21.1.4.2 - compressive strength f'c shall not be less than 3000 psi.
• 21.1.4.3 - compressive strength for light weight shall not be greater than 5000 psi.
• 21.1.5.2 - deformed reinforcement shall be ASTM A706.
• 21.1.5.5 - value of fy or fyt shall not exceed 60 ksi. 
• Mechanical splices shall satisfy 21.1.6 and welded splices shall satisfy 21.1.7.1
• 21.13.4.2 - for column load Pu < Ag f'c/10 shall satisfy
• 21.5.2.1 - provides minimum reinforcement for flexural members.
• 21.5.4 - shear strength requirements for flexural member
• Stirrups shall be placed no more than d/2 for length of member.
• 21.13.4.3 -  Where gravity axial load exceeds Ag f'c/10 the following shall be satisfied.
• 21.6.3.1 - longitudinal reinforcement to be between 1 and 6 percent.
• 21.6.3.2 - lap splices are permitted only within the center half of the member length, shall be designed as tension lap splices, shall be enclosed transverse reinforcement conforming to 21.6.4.2 and 21.6.4.3.  This is a major change from previous codes. 
• 21.6.4 - all sections must be followed therefore tie reinforcement similar to column that is part of the LFRS.
• 21.6.5 - all sections must be followed therefore shear strength requirements similar to column that is part of the LFRS.
• 21.7.3.1 - joint reinforcement shall satisfy
• 21.6.4.4(a) or 21.6.4.4(b)
• 21.6.4.2
• 21.6.4.3
• 21.6.4.7
• except as permitted by 21.7.3.2 

As can tell from section 21.13.4.3 when the design displacement forces are not checked the detailing requirements become much more significant.