Tuesday, November 19, 2013

Codes Changes Creating a Cladding Mess


If you’re a glazing contractor or a building envelop contractor; an estimator or project manager, you’ve probably tried to size your storefront and curtain wall members off of a wind design pressure.  

The wind design criteria are usually given on the contract structural drawings.   Sometimes design wind pressures for components and cladding are given as well.  However, the changes with the 2012 IBC have created much confusion regarding the actual design pressures to be used.

Many times the components and cladding pressures listed on the structural drawings are given in terms of “Ultimate strength design”, which is not used in any of the wind load charts given by manufactures.   Using these pressures will ultimately lead to overly conservative design.

Designers need to be able to calculate the allowable strength design loads from the information given off of the structural drawings.

If you have a question about what is on a structural drawing, give us a call.   

Stewart Jeske
816-734-8345
 

Friday, August 24, 2012

Can you break me down the difference in Explosive Weight I and Explosive Weight II?


Explosive Weight I and Explosive Weight II?

Question

Can you break me down the difference in Explosive Weight I and Explosive Weight II?

 Answer

This is the criteria from the 2012 UFC:

Explosive weight I is associated with a larger weight of TNT for a van or truck bomb outside of the base perimeter.   Its based on the assumption that a vehicle bomb will not be able to penetrate the base perimeter and/or will be detected at the gate.  If we have a facility that is less than 200 feet from the base perimeter/fence, then we must consider explosive weight I in addition to explosive weight II and its pressure/impulse may govern over the explosive weight II.

Explosive weight II is associated with a smaller weight of TNT simulating a back-pack or smaller bomb that gets past the base gate.  The standoff for this explosive weight is no longer the 82 feet typical of the 2007 criteria.  Now it is based on the new conventional stand-offs that are listed in the tables based on construction type and load bearing/non-load bearing walls.  The actual stand-off must be used which is from the parking lot to the window/wall in question.


Hope this helps.  Please call if you have any questions.


Stewart P. Jeske, P.E.

JEI Structural - Glazing Systems Engineering


816-734-8345
 

Wednesday, March 21, 2012

Updated UFC 4-010-01 (2012) - Impact to Glazing Contractors

Based on a quick review, here are the major changes/impact that I see:

1) The biggest change – Conventional Construction Stand-Off Distances. This is a huge change! They have reidentified the conventional construction stand-off distances in Table B-2 based on the wall type construction. I anticipate the net effect will be a moving to wall construction which minimizes the construction stand-off due to spacial constraints associated with military bases. So for reinforce masonry and concrete and wood studs w/brick that means about 30 feet and 16 feet and 36 ft respectively! Glazing systems with these requirements will not likely be able to have static equivalent calculations performed – They will be forced to dynamic analysis. I anticipate that these types of installations will start showing up late summer to fall of this year.

2) Another major impact – Testing Requirements. This is also huge! The requirements state that the testing must meet the appropriate pressure and impulse from the applicable standoff and explosive weight. So, given the changes with the Conventional Construction Stand-Off Distances most all of the manufacture’s blast testing (at 82′ Ex weight II and 144′ Ex weight I) will be obsolete! I feel badly for all of the manufacture’s who have invested a great deal of effort and resource to testing in the past 5 years.

3) Dynamic analysis is pushed. Under paragraph B-3.1.3 ASTM F 2248 Design Approach (This is the 3-second static equivalent method) they state, “In order to reduce the conservatism associated with using the ASTM methodology, the window systems may be designed using dynamic analysis or tested …”

4) Framing calculations under the static equivalent method is changed. Framing must now be shown adequate with a design load of 2x the glazing resistance instead of the 3-second equivalent blast load. In some cases (with small glass lites governing – Its usually the case that the Architect is unaware) this will be the difference between 50psf (typical 3-sec load) and 400 – 1200 psf! L/60 is used as a deflection limit now instead of L/160, but I don’t think that will help much – sections will need to be much stronger using this method.

5) Connections and anchors under static equivalent method is changed. The connections and anchors must be designed for either 1x or 2x the glazing resistance instead of the typical loop holes that we have seen 2x the 3-second load (usually 100psf). Anchors and connections at these loads will be difficult if the controlling glazing lite in an elevation is small. I think anchors into masonry and wood and light-gage steel will not be workable with this method.

6) We are now able to Use ultimate strength design instead of allowable stress, but again this helps little in light of the major changes above.



 Stewart Jeske, PE
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Tuesday, January 31, 2012

Gathering Missing Info on Blast Projects Saves Time.

Many blast project specs do not provide enough information to come up with the equivalent 3-second blast design pressure from ASTM F2248. Glazing contractors can save time by requesting the following information in advance.
 
  • Explosive Weight - (Typically for most projects explosive weight II)
  • Standoff Distance - (For most projects it is the Conventional Construction Stand Off = 82 ft)
  • Level of Protection - (Typically Low Level of Protection)
 
This criteria is usually determined during the design phase of a project by the using agency or the design team.
  
For more helpful information on blast projects see JEI Structural Engineering WINGARD, Windas, Window Blast, Blast Mitigation Design

Thursday, January 26, 2012

lateral-torsional buckling on storefronts

Regulations have changed so there is a need to design for the lateral-torsional buckling on storefronts now.   

Section 8.4.B.3.a of the ‘Aluminum Windos and Doors, and Glazing’ specifications states the following:



The AAMA standard dealing with storefront design is ‘AAMA SFM-1-87:  Aluminum Storefront and Entrance Manual’.

Engineering Section 6.0 (shown below) directs us to the current Aluminum Association’s Aluminum Design Manual (2010):
The Aluminum Design Manual, section F.2 (shown below), indicates that when framing members are subject to lateral buckling, the nominal flexural strength must consider the un-braced length of the framing in question:

We want you to have this information to give you a better understanding of why we are designing open shaped verticals mullions to prevent lateral-torsional buckling.   This increases the stablity and life of the storefront wall installation providing a higher level of client statisfaction to the end user.


Tuesday, January 3, 2012

Glass Roads - the prototype

Solar powered glass highways on zooming toward you.   How will it impact the glazing industry?



http://www.jeistructural.com/blog/?p=300

Friday, August 12, 2011

FUTURE CHANGES TO UFC CODE / ASTM F2248 STANDARD

For the past several years, the Unifed Facilities Criteria (UFC 4-010-01) has been the governing code for all U.S. Department of Defense (DoD) blast mitigation projects.  Referencing the current ASTM F2248-03 standard, the UFC provides a guideline for determining an appropriate static design blast pressure for both framing and connections of blast resistant glazing systems.
Surprisingly, many engineers and glazing contractors are unaware of the requirements set forth by ASTM F2248-03 for the design of framing connections for blast resistant glazing systems.  ASTM F2248-03 specifies connection design loads of at least 2.0 times the magnitude of the 3-second equivalent design load or the glazing resistance as determined from ASTM E1300, whichever is greater.  Often the glazing system connections to the main structure are only designed to resist 2.0 times the 3-second equivalent design load, despite the glazing resistance of the system.

The UFC 4-010-01 is currently undergoing revisions that should clairify blast design loads and reference a more stringent version of the ASTM F2248 standard (ASTM F2248-09). The revised ASTM F2248-09 sets forth the following criteria for the design of blast resistant framing connections to the main structure:

a.       2.0 times the magnitude of the load resistance of the blast resistant glazing if the maximum air blast pressure is greater than one half the magnitude of the load resistance of the blast resistant glazing.

b.      1.0 times the magnitude of the load resistance of the blast resistant glazing if the maximum air blast pressure is less than one half the magnitude of the load resistance of the blast resistant glazing.

 Currently, UFC 4-010-01 (2007 revision) references ASTM F2248-03 and not the more up-to-date F2248-09 edition.  It is our understanding that ASTM F2248-09 is not required in the design of blast resistant systems until referenced in the most current version of the UFC which is anticipated this year or early next year.

The changes may be difficult to accommodate with static equivalent analysis and may require a larger push for dynamic blast analysis to maintain reasonable connections.

Written by Matt Quinlivan, E.I.T.




Saturday, July 16, 2011

Be Careful With Manufacture Storefront Design Charts

Design wind load charts put out by manufactures are usually only good for estimating overall span deflection of a mullion, and do not typically consider proper methods for stress design.

The 2010 Aluminum Code has new criteria for considering the unbraced length of open sections. The unbraced length for a vertical mullion is usually considered to be the distance between horizontal mullions. However, design wind load charts put out by many manufactures of storefront systems are often based on the assumption that the mullion has full lateral buckling support and an unbraced length of zero. How can this be?

I believe that the manufacturers are considering lateral bracing from the glass and the mechanical gaskets. However, after review of many industry specs it is clear that mechanical gaskets should not be considered as a means of lateral bracing for open shaped storefront mullions. Therefore, the charts error on the side of being too liberal. When it comes to the calculations, they can’t match up and the mullions usually don’t perform as well as the charts indicate.

Unfortunately, it’s in the interest of the manufactures to keep the charts the same because they are competing against one another for framing systems with the highest performance standards.

Glaziers should keep this in mind when selecting open shaped vertical mullions and stay well under the curve projections that are indicated. If glaziers use the charts, as is, reinforcing structural glazing or heavier mullions will likely be needed.

Automatic Sliding Doors Have Operational Limitations

It’s important for glazing contractors to get information from the automatic sliding door manufacturers, in advance, on operational limitations for their systems. 

Some automatic door manufacturers have stringent criteria on how much the supporting header and jambs can deflect from wind load and how much can be supported vertically on top of the header.

Floor Movement & Glass Railing

For interior glass railing one of the things you need to watch out for is glass railing on stairs that traverse floor levels. If the floors are designed to deflect with live load, care is needed to make sure load doesn’t transfer to the glass railing system.


We saw a project with glass railing that goes between floors and the top portion of the metal handrail continued to the second floor wall framing. There is the possibility that the second floor framing can deflect and if you have a continuous rail that goes between the second floor wall and is attached to the stair, load can be transferred into the glass causing breakage.

It’s more appropriate to break the railing at the floor transition or provide a joint that slides.



Title: Staircase Glass Railing Designs, Glass Railing, Glass Balustrade

Description: JEI offers staircase glass railing designs, glass railing, glass balustrade and glass railing calculations for glazing contractors.