Science of Simple Spans of Floor Joists

the science of simple spans let's learn about the science of simple spans and this information was provided by glenn mathewson who is a consultant and educator with buildingcodecollege.com maximum joist spam seems like a pretty simple concept but there's obviously a natural limit to how far wood can span without breaking under a particular load but when inspecting a house we don't want to see any floor joists getting anywhere near that breaking point we don't even want a joist to bend all that much so when we talk talk about maximum joist spans as the international residential code defines them we're not really talking about pushing things to the limit we're largely talking about what it takes to have a functional floor one that doesn't overly bounce or won't rattle when we walk on it check out this sketch about joists spans load compression and tension loading a joist creates bending stress that puts wood fibers at the top in compression and at the bottom in tension the height of the joist and the species of the lumber affects how well it handles these stresses taller joists and denser species deflect less and allow to span farther the irc's span tables make it easy for contractors and inspectors to pick up a joist to bridge a simple span one without intermediate supports or cantilevers or to get the biggest simple span out of the wood available but span is just one of many roles relating to joists there are other rules for blocking notching and bearing lengths and all of these roles can be read alone but when taken together and mixed with a little science they start to make sense not as arbitrary rules but as the means to reach the expected ends home inspectors don't need an engineering degree to understand how these various joist-related provisions in the code work together so let's discuss them informally the first step in designing a floor system for a house is to determine how much you expect it to support and that's load the heavier the load imposed on a joist or beam the shorter the distance it can safely and functionally span this shortening isn't because the code is concerned about the floor breaking it doesn't want it to bend too much the flexion rather than strength is the foremost limiter to span in conventional wood frame construction maximum deflection is the most a building system or component is allowed to bend under its maximum design load floors have historically been held to l divided by 360 where you divide the joist's length in inches by 360 and you get the maximum deflection allowed for a given span we can refer to table r 301.7 and the irc we can also take internachi's free online structural issues course for home inspectors to calculate deflection of a joist as an example for any 14-foot joist the deflection limit is about a half an inch that's 168 divided by 360 approximately the l divided by 360 deflection limit for floor joists exists for at least two good reasons one it should prevent the drywall or the plaster ceiling below from cracking it's also about how stiff the occupant expects the floor to feel when they walk on it while a trampoline can support the 40 pound per square foot live load prescribed for residential living areas it would make a rather impractical kitchen floor if more deflection than l360 were allowed the allowable spans would increase but the floor wouldn't be as stiff if you spam less than the maximum allowed for a given joist it will deflect less and feel stiffer so what gives a joist stiffness when a load is put on a joist the fibers in the top of the joist compress and those in the bottom are under tension and in the middle what engineers call the neutral plane these forces diminish the zero the ability of the wood fibers to resist being scrunched by compression or stretched out by tension directly translates to how much a joist deflects and is why denser wood species can span farther they're better at resisting the stretching and the scrunching and therefore they deflect less still height can make up for the deficiencies of a species the more mass is distributed away from the center of a joist or toward the tension and compression edges in other words the taller the joist the better it resists deflection thus a two by ten can span farther than a two by eight of the same gradient species and this is also why i can span extreme distances without a major increase in depth much if not most of their mass is in the top and bottom flanges these tension and compression stresses are also what moves the load to the ends of the joist for transfer to whatever's below typically beams hangers or plates and for this transfer of load to work the joists must be held in a state of stress a joist not stressing is a joist deflecting joists will lay down on the job if permitted to and a joist laid flat like decking can't span very far without bending along its thickness given the way houses are built that isn't what happens instead joists especially when over spanned or overloaded try to flop over in the middle of the span joist thickness is a large part of what presents this kind of flop while the height handles the stress the thickness of the joist helps keep it upright and handles the stress the ratio between thickness and height is important and most obvious in beams where additional plies and larger sawn thicknesses are common sawn joists on the other hand no matter how tall are always one and a half inches thick so even though a sawn joist can handle more stress as it gets taller it also gets harder to keep it upright thanks to other code provisions and common sense the width of the joist doesn't have to do the job alone blocking rim joists or joist hangers at the ends and bearing locations are what ultimately keep the joist upright and stressed are 502.7 in the code will help other than hangers which also support the vertical load these other features do almost exclusively that blocking and rim joists resist rotation when the height of the joist compared to its width is greater than 212 the width can't hold the joists upright along their length and bridge blocking is required within the span to prevent the joists from rolling over home inspectors are not code inspectors but home inspectors can get out the code book and look at the science if you want a stiffer floor with less deflection you just need better stress management you could use larger joists height denser wood species or closer joist spacing or you can add bridge blocking with the span to reduce mid span rotation of the joist so let's say you have a 2x10 joist that's slightly over spanned its primary failure is excessive deflection and the usual reason for that is it wants to flop over in mid-span the same principles that stiffen a floor to above code deflection limits can stiffen an over-span floor up to the deflection limit the code will tell you this but we will bridge blocking can bring slightly over span joists to within deflection limits and that's a more reasonable alternative recommendation than to replacing the two by tens with two by twelves after deflection sheer is the next stress that joists need to be able to handle but it typically isn't a major consideration in wood joists shear stresses are distributed opposite of bending stresses they are greatest along the horizontal center of the joist that neutral plane and increase towards the ends shear failures and sawn joists typically appear as cracks that start at the ends and propagate along the neutral plane in other words along the grain and this kind of failure would be problematic because two small joists aren't equivalent to one big joist each smaller piece deflects more than one big one would but it's highly unusual for a song joist to fail and shear along its length shear across the grain is even rarer if not unheard of because of the way the grain is oriented it's much easier to split wood along the grain than it is to chop across the grain right research has shown joists will fail and bending long before they fail and shear which is why despite the shear stresses being largest at the end of the joists the irc allows joist ends to be notched up to one quarter of the depth of the joist no notches are allowed in the middle third of the joist where the bending stresses are the greatest and in the rest of the joist where bending stresses are smaller they're limited to one-sixth of the joist depth holes meanwhile are allowed two inches away from the edges anywhere in the span which tells you how little there is to worry about when it comes to shear stresses we also have to think about bearing once joists are loaded they have to transfer the load vertically the key here is preventing the joist or the member that the joist is bearing on from crushing wood crushes more easily across the grain like where the ends are located than it does parallel with the grain such as a post the size of the bearing area required to prevent crushing is directly related to the strength of the joist what it's bearing on and the total load being transferred distributing the load over a greater area reduces the load on any one point which can in turn allow for greater loads it's just like how snowshoes distribute your weight to keep you from sinking in the snow this is another engineering fundamental the irc simplifies for joists bearing on wood or metal section 502.6 of the code requires the full width to bear with at least one and a half inches of length this minimum area of wood necessary to transfer the loads that a floor designed using the irc's prescriptive tables can generate without overstressing the joist and crushing its ends the minimum bearing length on concrete is three inches but this longer length isn't about the wood it's about the concrete a joist bearing on concrete creates stresses that can cause the corner of the concrete or masonry to crack and spall or break off under the joist and the three inches of required bearing spreads the load over a greater area to reduce the chance of spalling and leaves enough area behind the compromised edge for the joist to bear safely if it does spawn this information was provided by glenn mathewson who is a consultant and educator with building buildingcodecollege.com i'm ben gromicko from internachi that's the international association of certified home inspectors at nachi.org thank you for learning about the science of simple spans

Channel: InterNACHI® Ben Gromicko

Views: 642,113

Keywords: internachi, home inspector, home inspection, home inspection training, home maintenance, gromicko, home inspector training, span, simple span, floor joist

Id: A-YyZAcYQCk

Length: 11min 29sec (689 seconds)

Published: Tue Feb 01 2022