Lateral Loads on Decks 1

[GlennMathewson]

In response to discussion in this thread:

http://www.eng-tips.com/viewthread.cfm?qid=309930

Recent testing by Washington State University reveals answers regarding the ability of band joists and ledgers to resist lateral loads. I have put together a 13-minute video about the IRC (2009 and 2012) lateral load anchor detail (R507.2.3), describing how the provisions got in the code, how they affect the industry, and new information from the research studies. (as best a 13-minute video can, so see the links provided below)

Enjoy!:

http://www.youtube.com/watch?v=nP6QzyxZgAw

ADDITIONAL LINKS:

If you only want to hear about the testing, but without the context, here's a shorter video for you. I hope you'll watch the full one instead.

http://www.youtube.com/watch?v=TsbZtNBteS8

If you'd like to read the entire research report yourself, use the links below. I highly encourage the subscription.

http://www.forestprod.org/buy_publications/wood_design_focus_online.php

This is the editorial from the researcher. Take a look!

http://www.forestprod.org/buy_publi...013/Wood Design Focus 23, Number 2 Bender.pdf

Here is a link to our public comment, posted on the NADRA website.

http://nadra.org/code/RB263-13_public_comment.doc

 

[RFreund]

Glen,

Nice video and good information. I agree with what you have to say and have previously tried to discuss this as well. As there always was confusion on if that detail was required and the amount of force. I swore that this was a detail/provision driven by the Connection industry (i.e. Simpson. Also I don't get why everything has to be so prescriptive when it comes to wood. But I don't want to get side tracked.

It's interesting because I see a bit of a disconnect developing between a prescriptive method and an engineered method (I suppose either way there needs to be a load path). The problem though to me is as follows:

It seems that the holddowns 'made sense' because essentially you are treating the diaphragm like a horizontal shear wall (or a cantilevered diaphragm). So normally you would place holddowns at the end of your shear wall or in this case your cantileved diaphragm to resist your T/C Force due to the lateral load. However based on the testing done by Wash U it appears that the holddowns may not be necessary. Therefore, I could see using the lag bolts to resist the tension forces due to lateral loads. However the problem becomes (or so it seems) - how do you transfer the tension forces from the joists to the ledger board. In reality it would be through the joist hangers. However most companies do not publish values for axial loads on joist hangers. So now if an owner/architect comes to the engineer and acts them for assitance the engineer has a hard time justifying that connection with no data to back him up. I think this would need to be a big step in removing the holddowns.

EIT

http://www.HowToEngineer.com

 

[msquared48]

Next step is to say that shear walls do not need hold downs... as the sill plate anchors transmit all the hold down force. This will not happen.

Professionally, I would always use the horizontal tension ties (hold-downs) if the deck has any diaphragm or frame action whatsoever, which it should in order to stand.

Moreover, in trying to make this work, you have to employ cross grain bending in the ledger which is not allowed by code.

Mike McCann
MMC Engineering

 

[GlennMathewson]

RFreund, you are on the right track. The researchers found that the common hangers marketed for exterior construction use the "double shear" design, which does not restrain the joist end by much. They used a line of hangers with all fasteners perpendicular to the member. They also used the structural screws Simpson designed for their products. This was important to transferring all the loads possible to the ledger/rim joist connection where the load cells were located.

msquared48, The purpose of the video was in regard to the band joist specifically. There is a whole other discussion to be had about the entire lateral load path of decks, and it starts with the diaphragm that does not currently exist in the code. The 17 inch deflection of the unbraced decks in the study is a failure before you even get the loads to the ledger.

The lateral load path discussion for decks is far greater than merely two locations of large magnitude on a ledger length of untold distance and an unknown aspect ratio of the deck area. However, with the hold down "solution" in the code(as viewed by many), all this is ignored and the industry is testing products, creating technical memos, and conducting research revolved around a fear and a guess. We have code leading science, not science leading code.

Removing the "permitted" anchor from the IRC will not stop a B.O. from continuing to require it, but it will take this issue off the shoulders of the decking industry and put it back on the table for discussion and research.

Thanks for the discussion.