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.

Wood Shearwalls on Podium Slab

Recently received a new comment from Seattle DPD plans reviewer Roussi Roussev. 

"The transfer slab at level 2 supports discontinued wood shear walls, and therefore is required to comply with ASCE 7.12.3.3.3.  Please provide verification of the slab for the maximum force (uplift or compression) that can develop in accordance with the seismic load combinations with overstrength factor."

Were the heck did this come from?  We've been doing two-stage analysis for years on this type of structure and have not had to consider wood shearwalls as discontinuous.  If we look back to the 2003 IBC section 1620.1.2 modified ASCE 7 and specifically exempted light-framed walls supported on concrete slabs from this type of irregularity.  Then the 2006 IBC saw most of the seismic design cut from the code and referenced ASCE 7 without modifications.

My initial response to Roussi was that since a two-stage analysis (ASCE 7 12.2.3.1) was used, only the amplified forces from the upper portion need to be designed for and that no irregularity exists.   This response was not well received.

Charlie spoke with Roussi concerning this comment and he was not really sympathetic to our way of thinking.  In fact Roussi mentioned that both John Siu and Steve Pfeiffer are taking a more rigid definition of shearwalls supported by concrete slabs and that they are indeed an irregularity that needs to be designed for.  He stated that there has been a project where the reinforcement did increase due to the seismic loading. 

In my second response to DPD I modified the PT slab model to include load combinations for seismic.  Below is a list of steps taken to modify the model.

• Added loads "Seismic Loading N-S" and "Seismic Loading E-W".
• Added the four load combinations:
• "Factored LC (N-S):  (1.2 + .2Sds)D + Omega QE +L + .2S"

• "Factored LC (E-W): (1.2 + .2Sds)D + Omega QE +L + .2S"
• "Factored LC (N-S):  (.9 - .2Sds)D + Omega QE

• "Factored LC (E-W): (.9 - .2Sds)D + Omega QE
• The seismic factor of 4.65 accounts for Omega*Rw/Rc*LRFD/ASD.  For all four of the load combinations there is an Alt. Envelope Factor.  Use the same factors except the seismic should be negative.  This has the effect of reversing the seismic load direction.
• Then for each N-S and E-W directions the holdown loads were applied at each end of the shearwall.  Sign convention used was that North and East are compression (down) loads, South and West are tension (up) loads.  Loads used are the allowable tension load from the threaded rod holdown schedule on sheet S1.4.

N-S Shearwalls



E-W Shearwalls

Modifications finished!  Run the model!

Reinforcement requirements did NOT change due to included seismic load combinations.

Provide response to comment along with PT calculations back to DPD.  If we let DPD know that the seismic load combinations don't control then maybe one day they will stop asking for it.