PT Design: Stud Rails: Limited capacity!!!

Recent discussions about studrail design refers.

Note that Ram Concept has the ability to design studrails according to the provisions of the ACI 318-08 (or previous versions if you're so inclined) . Previous codes had "speed bumps" allocated such that

• Vc =< 2(f'c)^0.5 (Concrete strength) and,
• Vn =< 6(f'c)^0.5. 

 In current (08) code, this has been bumped up to

• Vc =< 3(f'c)^0.5 and 
• Vn =< 8(f'c)^0.5. 

 (See ACI 318-08 [11.11.3](general shear rebar condition) and [11.11.5] (New section for headed stud rails). This change is in line with comments from ACI 421-99 which has some suggestions for parameters for studrails applicability.

The issue is this: The Decon Studrail ICC report limits the Vc to =< 1.5(f'c)^0.5...!!!! WTH... That means that we could conceivably design stud rails all the way at 3(f'c)^0.5 - when in fact the product to be installed (including all look-alikes) are limited to 1.5(f'c)^0.5 by approved ICC docs. ICC claims that this limit is inherent in the ACI 421 code which references that limit in relation to earthquake loads... not sure of the context... do not have a copy of this doc...!

So given the lack of clarity and direction on this issue at this point (i called Decon and ICC - no real direction coming from any of them!) - we have decided at CTE to limit our studrail designs to the 318-05 version - which includes the 2* and 6* limits...

PT Design: Ram Concept: Tips and Tricks

I've designed a few PT slabs recently, and have battled a bit with the software, and its various tweaks. Here's what i've learned recently (if you know this stuff... sorry in advance!):

1. Steps in slabs are awkward: Concept does the modelling OK and comes up with the FE units easily enough, but then it tries to get smart and figure out the sections along each design strip to be able to apply design rules and calc stresses. There are various kinds of "section trimming" methods that are available in the "design strip" manager/editor, and these can be confusing. This "trimming" can lead to bogus results or unreasonable steel/section requirements. I've had conditions where i have to check punching shear using one type of trim and tensile rebar using another type of trim. Not sure what the solution here is, but we should discuss design methodology going forward so we have some kind of standard out there.
2. Setting of tendon profiles: It seems that Concept snaps tendons at the slab perimeter to slab mid-height - so no need to spec a profile point here - FYI
3. Concept Rebar Design to "code": Concept can design rebar to "code" requirements(set under the "design strips"). Only problem is that when it does, you sometimes get unusual results. Like all things, we have to apply judgement to each case. In reading the manual, i noted that this option can be set to none (ie don't apply code reinforcement rules). This does NOT have the effect of leaving off the rebar, but merely stops Concept from trying to apply the "code rules" and coming up with bogus or "collated" results. It will still come up with steel requirements but only at the locations where stresses dictate - not coordinated with code rebar rules... not rocket science (i don't think)... (ps the reason they are trying to approximate the code design is to better approximate rebar quantities in their take-off - thats my take anyway...)... (pps - the manual says that this should produce results similar to v2.1)
4.  Notes on Design Methodology: In the overview of design in PT, i still look to deflections and top rebar requirements to adjust and "massage" tendons - also looking at mid-height prestress to check on P/A (or do a hand calc). Once the slab is behaving OK, then i would look to design strips for determining appropriate rebar requirements. As mentioned, you may need to consider 2 "trim methods" in order to determine -punching - span rebar - other min/max stress related rebar...

 Anyway - thats all i got for now... Feel free to set me right if i "strayed off the path..."

Build it Green - Advanced Framing

 Today i was asked about "Advanced Framing" and whether it would affect our design and our fee. I asked Chuck. We did not know what that meant. That made us feel dumb!

So i called up Hugh Schaeffer and asked him about it. The link below describes a bit more about it. 
http://www.builditgreen.org/attachments/wysiwyg/22/Advanced-Framing.pdf

The idea is to use "smart techniques" to minimize use of materials, and to maximize energy efficiency in "common sense" ways...

I'd be interested what y'all think about this... Of course the overriding question is... LEED or no LEED?

ENERCALC V6 - slender masonry walls - maximum reinforcement ratio

Ever wonder how much steel you can stuff in slender masonry walls these days. Did you think that ENERCALC V6 would perform an internal check? Think again. See my question and ENERCALC's responce (for your enjoyment).


MY QUESTION:

Dear Technical Support:
video and users guide is silent relating to the max % of rho balanced entry under the material properties of the general tab.
Please see attachment (ACI-530-08)Commentary section 3.3.3.5. How is the program determining rho balanced? I assume that the equations in the attached commentary section are used with alpha = 1 (1.0*yield strain). I also assume that P/db in the numerator of the equations is calculated according to the service level load combination P=D+.75L+.525Qe. Please comment on the value assumed for the third term (ie .525Qe). Thanks


Benjamin J. McCann P.E.
bmccann@ctengineering.com

CT ENGINEERING Inc.
Structural Engineers
▪ 180 Nickerson Street ▪ Suite 302 ▪ ▪ Seattle, Washington 98109 ▪ ▪ Phone: 206-285-4512 ext: 312 ▪ Fax: 206-285-0618 ▪


ENERCALC REPLY:

Good Morning Ben,

Sorry for the delay in getting back to you on this one, but it is an excellent question and one that has required some research on my part and some conversations with the developers.

The answer is that the Masonry Slender Wall module is currently using reinforced concrete concepts (and 1.0 x steel yield strain) to determine rho balanced. It then respects the user-specified "max % of rho balanced" as the upper limit on reinforcing. This is workable, provided that the user observes the calculated value of rho balanced and enters a meaningful factor as the "max % of rho balanced" to arrive at the desired upper limit.

I'm glad you asked the question, though, because it has given me reason to propose to the developers that we should implement the provisions in ACI 530, which are more specific to concrete masonry. So I would expect that we will soon be seeing the reference to rho balanced disappear from masonry modules, as that term is not used in ACI 530. I would also expect that we will soon be determining the upper limit on reinforcing in masonry sections by using 1.5 x steel yield strain, along with usable masonry strains as defined by ACI 530 rather than as defined for concrete sections in ACI 318.

At present, none of the masonry modules are incorporating the axial load from the specified load combination into the determination of rho max.

· The Masonry Lintel module does not perform a check on the reinforcing limits. I’m going to provide the developers with some recommendations on this. But even if a reinforcing limit check gets implemented in this module, it will not consider the axial load from the specified load combination, because the beam modules in the Structural Engineering Library do not incorporate axial load in any way.
· The Masonry Column module currently bases its maximum reinforcing limit check on 0.04An. ACI 530 specifies that value as an absolute upper limit on the area of reinforcing in a column, but it also says that designs should still respect the value of the upper limit as determined in section 3.3.3.5. I’m going to provide the developers with some recommendations on this, including the incorporation of the axial load effects from the specified load combination.
· As mentioned above, the Masonry Slender Wall module is basing its maximum reinforcing limit check on the user-specified "max % of rho balanced" multiplied by the calculated value of rho balanced. I’m going to provide the developers with some recommendations on this, including the incorporation of the axial load effects from the specified load combination.

Finally, you asked for comment on the value assumed for the 0.525Qe term in the load combination specified for determining axial load on masonry elements. The factor of 0.525 occurs in Basic Combination #6 in section 2.4.1 of ASCE 7-05, which is (Equation 16-13) of IBC 06 and IBC 09. However, the combination specified for determining axial load on masonry elements differs slightly from the equation in ASCE 7 and IBC, because it omits the roof live load or snow term. So I will need to provide the developers with some guidance on how to handle this requirement, and at the present time, I think I am going to point them to the expanded form of this equation as presented in section 12.4.2.3 of ASCE 7-05. I think I will eliminate the roof live load or snow load term, and ask that they calculate a value of axial load based on all of the remaining terms, including the vertical component of the seismic load.

Thanks for raising great question, and keep your eyes peeled for some nice enhancements in the masonry modules as a result.

Cheers,
Chris

Chris Conrad, P.E.
Director of Development
ENERCALC, Inc.
cconrad@enercalc.com
Technical Services: 949-645-0441
Fax: 949-645-3881
Business Office: 949-645-0151, 800-424-2252
Web : www.enercalc.com

Shoring Alternative from Craig!

fyi - an alternate shoring design that Rob Ward is advocating for house addition in west seattle I'm working on. 

From: John McDowell [mailto:JohnM@PileKing.com]
Sent: Tuesday, June 28, 2011 7:19 AM
To: croberts@ctengineering.com
Subject: driven wide flange shoring

Hi Craig,

Please see attached a general description of the driven wide flange shoring system as well as a few pictures of jobs we have done.

Beams are driven using a hydraulic impact hammer that strikes the pile at about 250-400 blow per minute.  For temporary cuts up to 8’ cantilever we oftentimes see W6x25 beams at 2’ centers, but for deeper excavations we recommend a W8x31 or W8x35.  Lastly, the maximum length beam we can drive without a splice is 30’.

At the end of the job beams are left in the ground as sacrificial.  If necessary beams may be cut off several feet below grade to insure they don’t interfere with future landscaping.

If you have any comments or questions then please do not hesitate to call or write.

Regards,

 John McDowell

       McDowell

Pre fabricated wood stairs

Here are a few photos from my "Seattle stair and design" field trip last Friday afternoon. Contact information on card photo.

These guys are craftsmen to say the least and have been in business in the local area for around 30 years.

If this post works I can follow up with some more information.

Ben

Check the links to the CTE blog

You all should be getting the links to postings whenever they now arise... (there were some bugs!)

Should all be good now!