Lateral displacement of deck beams

sometimes that bearing area needs to be larger okay now we're going to actually get to the tricky part of this session this code section the attachment must also resist horizontal displacement so that's really all we know the goal is to keep the post under the beam or keep the beam on top of the post but we don't know what is trying to knock it out we don't know what that load is or how much needs to be resisted is it the sway on the deck that's translating to the post is that what we're trying to resist well we talked about that in course two and we know that we don't know what that load is so that doesn't help us at all is it a force in this direction due to the torsional moment of the beam right we learned about that with the joist that that's that effect of them wanting to roll over well i could be a part of it but that is a much more complicated discussion with beams and it's nothing the code eludes us to so what about this is somebody leaning on the post is that the force i mean yeah of course right it should be strong enough for that but i didn't get that from the code i just got that from what is the concern in deck construction well for guards we have to resist 200 pounds laterally so should that be it should that be the lateral load that we're trying to resist i mean honestly i'm asking here i'm reaching out because you know what the code doesn't give us this information and if we ask these two guys we're probably going to get some very very different answers to what will resist lateral displacement and what what the target even is all right so if we read further in this section though the code does provide some this ought to be good enough kind of provisions that can help out with it it points us to two figures with some connection details now why do i say that these are well that ought to be good enough provisions is because we don't know what forces these details are even working against so anyway let's go over the first one it's a generic installation to depict an approved post cap and approved is defined as acceptable to the building official so how would a building official find this acceptable well in the previous session about joyce we looked at the load capacities right because the hanger has a pretty big responsibility of transferring those very well-known live or snow loads over to the ledger or beam but for beams those known vertical loads are expected to be bearing directly on a post or maybe you could create a design where they bear in a hanger onto the post and we go about that hanger route remember just be sure to follow installation instructions load ratings that idea is beyond this course but you should have enough information if you wanted to pursue it so when it comes to resisting horizontal displacement how do you go about approving the many many common post caps on the market like this ac6 i've seen a million times so let's go to the load table and let's look at the notes for the ac6 top cap that i just showed you and again this is just an example from a manufacturer i chose now the ac6 is included here with a few other two-part type of top cap connectors and the table notes that the loads only apply the the load capacities in this table only apply when these are installed in pairs and then you've got lots of information here and it's the stuff you should read for whatever products you choose to use or if you're inspecting or designing one of these but what i just want to use is this to talk about the loads that are tested for and look at this we find it they've tested these for a lateral load capacity that is awesome i just don't know what it means because i don't know what lateral o's were resisting now this ties back to course two even more about lateral bracing of the deck and why do i say this is because if you read the general notes for all post caps from this example manufacturer you're going to find this it says allowable lateral loads for post caps can only be achieved if one of the members the post or beam is supported laterally by other means so what it means is if you have a wiggly post and a wiggly beam and you connect them together they're just going to wiggle together so unless this post is somehow braced on its own at this location that then the loading on the beam in this direction is basically saying it doesn't even apply that's okay we don't know what it is all right so let's go back and look at some more of these specific product notes we see that this one particular post cap is only tested for directions parallel to the beam but we just canceled out that direction at least when it comes to sway of the deck so this is all saying that this load capacity for that particular hanger doesn't apply to loads in this direction and hey that ac6 is not installed in pairs is that a problem so what does that mean but what does that mean with this lpc post cap right that can't take those lateral loads perpendicular well that's in the simpson deck guide so are we to say that it's this particular post cap is not acceptable to resist horizontal displacement because the load capacities don't tell us that well if that's what you think it is not because you heard me say it okay and just in the same way i've got nothing to say about this concealed beam tied thing here that i've never seen until creating this slide my question is simply the same as simpson posts here resisting horizontal and lateral displacement so remember those two don't look anything like this in the picture in the code and again that's not the point this is just illustrative so let's go back again to the simpson table because there's still more here to dig up notice here that it says these post caps are not designed to transfer tension loads between spliced members and actually you find this note in the general notes for all post caps so back to the irc figure again now this note here about splices has nothing to do with the connection it's just a reminder that you have to put all beam splices over a post or bearing area and i got to be honest i can't say i've seen many builders splice a three-ply beam like this over a post but we should know that the post caps are not going to resist tension for this exact beam splice pictured here and i guess that doesn't matter because we don't know what that tension load is anyway so with that set up let's go ahead now and let's look at this next illustrative figure this next figure here in the code and now this one states this minimum five and a half inches and again remember this is not about bearing area for each end of the beam this is all about connections that one is and so what that part is about is it's about these edge distance requirements in this figure for a notched post to beam connection now first we've got to do a little clerical fix up okay right here where it says minimum half inch that is just a typo in the code it should read two and a half inches that's what it reads in 2018 irc but you'll probably find out soon that i'm not holding that tight to that number anyway now if we look here it specifies two half inch diameter bolts and then it says or approved right we know what that means approved equivalent connector and in any normal sense of reading code we would assume that to be equivalent to two half-inch bolts and that's probably what most people are going to think when they read that i just want to point out the irony that two half-inch bolts aren't equivalent to anything that we know of that we're trying to do they're probably good enough kind of code provisions so just as we looked at these post cap methods a minute ago from simpson well if we scroll down this picture they also provide some other cool alternative connections using structural screws now do we look at these and scrutinize them to be equal to half inch bolts or should they be equal to that wide variety of load capacities and no load capacities we found in the tested post caps or what about this alternative through bolt system from fastenmaster now they state in their literature that they can as they resist uplift and lateral displacement equal to traditional bolted connections well is that okay is that enough or does it have to be specifically equal to half inch bolts well you didn't hear me say that because there's still more to talk about here and in this figure down here we get some very specific edge distance requirements and you can look over and pour over these on your own if you'd like all of these edge distance requirements come from the nds and this is what provides the standards for wood engineering so these distance limitations are for when you need to determine a guaranteed load resistance because you probably have a load that you know of so like the tension load across this beam splice right here well those half-inch bolts would be able those edge distances would design to actually say yes we can resist that tension if we knew what it was all right so there's still one more detail and this is in the code section not in the figure and here it says that there must be a washer under the head and nut of the bolt well okay pretty specific and what this means and it was kind of intended to mean was to prohibit the use of carriage bolts in this beam to post notched post connection now why it's because technically carriage bolts are not a wood to wood fastener they aren't recognized by the nds and you can't get any engineered load resistance for that type of connection so even though we don't know what we're resisting if it's not recognized in the nds it makes it really hard to be recognized when you're developing irc code so again though the only goal we know is still this all right now there's one last portion of the code to go over and it's going to kick us back to the manufactured post connections here it states that manufactured post to beam connections shall be sized for the post and beam size now this is probably the most popular post cap design on the market and different manufacturers have similar designs well let's go look at the load table for this particular one we can see that there's dimensions for what fits right what this was designed for and for a six by six post the width that is provided for the beam is five and a half inches and for a four by four post the beam width portion is basically about three and a half inches and this brings some things to mind it's really common to see four 4x4 post caps like this used on a 3 inch wide beam just a 2 ply beam and it's kind of bent over a little bit to fit well will this still resist horizontal displacement or is it a fail what about it would that change i guess it would right if the beam plies happen to be separated by a half inch now is that suddenly if that was the same picture before would that suddenly change everything about resisting lateral displacement of an unknown load now for more fun and confusion on this take a look at this illustrative figure in the code and and it looks to me like a three-ply beam without spacers which would only be about four and a half inches wide not the five and a half inches that we just saw one example of that post cap is designed for so what if the builder again uses those half inch fillers right well now they're out to the full depth but wait a minute that's a different post cap that's the ac6 and when you look at the dimensions for this post cap the beam dimensions aren't actually so defined so is it okay then for that to bend a little bit over if we don't find it in the load capacities okay guys i think you get the point and so my final suggestion understanding everything i've talked about and how loose this kind of whole thing is my suggestion is this look at both of these details reasonably and these details you don't have to look at them so significantly and so precisely just like the post cap load capacities and notes about lateral loads you might not have to scrutinize so precisely because the goal is to resist horizontal displacement of reasonable loads that are quite undefined now a deck like this okay that's a whole nother level of lateral displacement we're talking about vertical posts this whole thing this whole time i'm expecting that engineer for this one certainly my name is glenn mathewson thanks for learning with me this course was provided to you by buildingcodecollege.com where we go beyond the words

Channel: Glenn Mathewson

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Length: 14min 56sec (896 seconds)

Published: Thu Apr 01 2021