The Golden Rules of how to design a steel frame structure

the design of steel is an art form that only comes with time and practice and it's not just about sizing a structure effectively to better resist the loads when you are designing steel you also need to know where to spend your time so how much time should you be spending into the design into the connection and the detailing this is where the art form comes in i'll be going through some of my best advice i can give you to help you improve your still design so you can shortcut some of this long learning experience but it really does require you to go out there and actively apply some of these principles i'll break down this video into two parts the first part i'll go through is the analysis portion and using rules of thumb to be able to size up your structure rules of thumb are not only important for sizing your structure but also checking designs you've done from analysis software to ensure they just got the correct answer it's highly important to know these rules not only when you're scheming a structure but also when you're assessing the design as well and the second part i'll be focusing on some of the important aspects you need to know for designing connections in steel frame buildings when designing steel you need to understand the difference between analysis and design as analysis is the simple design of a structure to be able to resist the loads that are imparted upon it where design not only needs to incorporate the analysis results so the structure can resist these loads but also needs to incorporate site conditions how they're going to put it together is it actually buildable so there's lots of considerations that you need to impart on your design design is not actually only about just making sure it's buildable but also imparting that knowledge to the people that need to know it and this is really where the art form comes in designing steel the analysis part is really simple that's where you should be spending the least of your time most of the time should be coming down to the detailing the connections how they're framing it up and making sure it's buildable this is highly important when you are starting still design to understand that you need to be spending most of your time in the connections and detailing of steel structures all comes down those connections if the connection fails the structure fails has very little redundancy compared to a concrete structure when you're documenting the design make sure you're specifying the different elements that are inside your structure and then you clearly understand how they're meant to behave so for example a column is a primary support so typically supporting a beam or loads over however where mullion is normally facade support so it's normally got slotted holes at the top that allows it to move vertically but takes that horizontal load from the facade loads and things like rafters and purlins so typically a rafter is running with the line of the roof where prelims are running parallel to the line of the roof and prelims are generally a secondary element as well so they're normally a lighter structure than typical roof beams and then also specifying if you have roof trusses and if you have wall headers there's another one that often gets missed so wall head is normally taking those horizontal loads for wind but not taking very many if at all vertical loads so just clearly to find those elements for the purpose that they're there for will help both yourself when you're designing it but also when someone's putting it together understand the actual true purpose of that element just makes it easy to build put together and make sure you haven't missed anything so when we're starting off any design it's important to know where the building joints are going to be you need to allow for thermal contraction and expansion when the steel is heated up it generally expands when it cools down it contracts and over a large steel structure either that flooring or roof this can lead to some quite big movements inside your structure and lead to significant forces in your members so you need to allow for joints throughout your building to allow for this thermal construction and expansion the rough spacing of these joints is roughly about 150 meters in both directions so when you're framing up your building get out the architectural floor plans work out where a good place is to have these joints then make sure they're no further apart than every 150 meters or so now either side of these joints as they do allow for contraction expansion at these points you're likely to need to have additional bracing frames around these areas this is a good place to start off with to make sure your structure is stable getting those three sides of supports and allowing for these movement joints throughout your building in addition to placing your movement joints 150 centers at each way it's also important to look outside your frame to make sure it's both stable in your temporary and permanent conditions but know what you might be saying surely that's for the temporary works engineer however with simple design changes you can both make your structure easier to erect and cheaper to actually build as they do not need to put those temporary frame inside there for example designs on columns especially the top level is having four bolts instead of two this wherever possible as the four bolts allows you to stand up that column and have it stable without the need of temporary works and also when you're starting off a brace frame at either side it's starting off with angle braces as opposed to your rod cross bracing as rod cross raising typically needs your structure to be temporary propped until the whole structure is built where you started that first frame with angle bracing you've made that first base solid and it's stable so they do not need temporary bracing until they finish your the whole build as you've made a stable structure from date nord they're two simple things that can help make your structure easier to build and cheaper and your steel fabricator will like you for it so something to discuss early on with the project especially when you get your steel fabricator on board is some of those potential changes that will add a little bit more material but make it quicker and easier to erect so let's move on to some rules of thumb when sizing a structure so when you're framing up your building how far can roughly every element span for example a floor beam you're sitting at about a 12 metre max span now you can go further with deeper structures but is really becoming inefficient at this point roof trusses can span upwards of 17 meters to be somewhat efficient and then you can go onto more of a space frame if you do need those big expenses which can go upwards of 60 meters so looking at your spans will dictate what size structure you need to go now these are only rules of thumb they're not hard limits they're just a good guide when you're starting to frame up your building where your columns should go and where you need those lines of support also looking at span to depth ratios is highly important here as well as this will give you a rough size of the structure that you need to look at for example a floor beam has a spanded depth ratio of somewhere between 15 to 18 roof trusses can be somewhere between 14 to 15 and space frames can achieve somewhere between 20 and 24. probably one of the biggest things when you're sizing up a steel frame what is going to be the governing factor will it be serviceability or will it be strength typically a steel structure is governed by deflection it's a lightweight structure that allows for great spans but the deflection limits or vibration limits are typically what are going to govern your design so let's start off with a typical design that's generally governed by deflection we can simply work out what size we need to frame up our building we know the loads we know the spans or we do not know the size yet so we're looking at the beam formula as i've got an example here we can rework either our simply supported beam or a continuous beam formula to work it out from our deflection limits what size section modulus we need from that we can now look at our span tables and specify a member that fits into that limit of state so as you can see by reworking the formula it gives us a minimum size of structure we need for a design that is governed by deflection one good thing about a deflection govern design is that you're able to pre-camber out some of that load you're able to pre-set the beam up to get rid of some of those deflection limits that you have in there so when you're framing up your building having a look at your dead loads and trying to pre-camber out majority of this load is recommended when you're pre-cambering you want to ensure that you're not over pre-camping your structure for two reasons firstly if you over pre-camber the member may never go back to flat so you've got a permanent hog inside your structure and secondly if you're starting to over pre-camber your design it will typically then become governed by vibration there are limits on how much precamp you need as a rough guide you roughly want to only pre-camber about 80 of your dead load out of your self load so when you're looking at design looking at yourself weight times in that by 0.8 and that's roughly the maximum pre-camber you want inside your structure so after you've framed up your structure you've taken it out about 80 of that dead load we look back at your deflection formulas and you may better see some efficiencies inside your design also with still structures do not want to be changing your members too rapidly as this will increase sight time so grouping them together such that they're easy to put together on longer span floor structures you may be governed by vibration so this is in excess of 10 to 12 meters is when you start to reach this point is that your structure may be governed by vibration so when someone walks across the floor you can feel them walking on there and typically in an open office situation this is somewhat acceptable as you can see the person there however in more residential structures we have walls people can be deterred by feeling this vibration so on longer span designs you need to be critical of your vibration limits the iso standards give you some sort of limits of vibration that you can have inside your structure there's a rough guide that you roughly want to be between four hertz and eight hertz for efficient structure if you've got your frequency below four hertz you're starting to move into the point of where walking can actually exacerbate the design through the floor bouncing too much and the lower the frequency the softer your structure is so the more likely it is to be felt through your design so by knowing that we want to try and keep it between four hertz and eight hertz for an efficient design there's a rule of thumb for roughly what vibration your structure will have and that's 18 on square root of the deflection so on your longer span designs trying to keep it to four hertz you can rework this formula now to work out what your deflection is and as we now have the deflection we can go back to the formula worked up before to work out what moments of inertia needs to be and from this we are able to quickly size up our design although most steel structures are governed by deflection there is the articulation where it's governed by string when you're sizing up your steel structure it's easy to check not only your moment of inertia but you can also reverse the formula and also check your section modulus when you're looking through your tables for the minimum size required you can frame them up such that a critical design case still governs the one important thing about designing your steel structure for strength is knowing her effective lengths especially for beams and especially for roof structures roof structures typically have gravity lows which is just the self weight of the structure and maybe some live load that is imparted upon it but they also have wind loads across the structure so wind blows across the structure to potentially uplift inside your design so you potentially have moments up and you potentially have moments down and because of this it requires different flanges to be restrained so what flange needs to be restrained for what force so if we have a tension force it's obviously pulling toward there's no way that structure can buckle from a tension load it's the compression flange as when it's compressing has potential to buckle in size and then severely reduce the moment capacity as it rolls over so you need to restrain that compression flange so when you've got your gravity forces that would be the top flange and on a typical steel frame structure you'll have your purlins on top so the top is well restrained however in the uplift force as the purlins are on the top the bottom flange is not really restrained and this is why we have flow braces so the flow braces typically inside a steel frame design are there to restrain the bottom flange for those uplift forces so there's two things you need to be careful about where you need those flow braces and the connection details that you're looking at as we're talking about purlins another important fact that's often sometimes overlooked is which way especially a zed pearl should face and there's a simple rule that you can do to remember this and that is ducks always go uphill so if you look at a purlin it's got the top that looks like the beak of the duck and the back that looks like the tail so when you're putting your pullers inside your design and looking how it's framed up making sure the ducts are always going uphill as if they're going the other way it leaves a great place for items to pull and collect at the top but this way there's very minimal dust or water collected inside your purlin structure so that's just a pro tip when you're putting your purlins up ducks always go uphill and if you're finding this content informative smash that like button so i can produce more of these videos it gives me a good update of what type of content you would like as we can see the design of a steel structure the strength and deflection is relatively simple you do not have the complexities that you do with a concrete structure where the structure changes over time and at the start or saying you should spend most of your time in the connection detailing and when you're going down to design those connections i highly recommend you manually sketching up those designs that's either by pen and paper in revit or cad or other software as well however putting it together in your head about how you're going to put together and detail it and the physical act of you putting the structure together you may see a couple of things firstly you look at especially those complex designs you may see there's a lot of section cuts you need and providing an isometric on a more complex design may actually help tell the story better as well so i encourage you to bring out those pens and paper and draw some isometric sketches of the design that you're looking at as when you're sketching it up you may realize there's some areas where they may not be able to get to bolt in so you need to modify your section design accordingly or connection design and maybe you need some additional framing in there to allow it to be built and when you're framing up those designs especially inside an existing structure it's also drawing the existing structure in the design as well as it may be constrained in how you actually be able to build your building this is really where you should be spending most of your time not only because of the lack of redundancy in a connection design but also allowing for a builder to put it together it's highly important to understand that a steel frame design is not being put together inside a factory so you have people on site so you want to not only keep it quick simple and easy to put together but also looking at those site constraints is it physically able to be put together as you've designed there are many different connection types that you can have inside your steel structure these are flexible connections cleat plates you can have flexible end plates angles heat plates bearing pads cleat angles rigid connections fully welded bolted moment end plates bolted cap plates splice connections either welded or bolted as you can see there's a lot of different connections that we can use to put together a steel frame building it's important to know the limits and constraints of each of these and where you should actually apply them so most of the time you're trying to keep your structure as pinned as much as possible as a splice connection especially a moment splash connection leads to a weak point of potential failure so wherever possible limiting your steel frame design but sometimes you do need to allow for those splices for example steel is limited by the amount that it can be transported and typically they're probably about the max spend you can get especially on a semi trailer is roughly about 19 meters they can be transported in a reasonable length and sometimes that can be as low as 12 depending on what site constraints you have so it's important to not only look at these so when you framing up your design especially long frame structures if you do need to put moments in if you do need to put moment connections in especially splice connections is making sure are inflection points wherever possible where the minimum moment will be impacted upon it so be mostly sheer going through these joints so sometimes you may not be able to get effectively 19 meter spans everywhere you may have varying spans depending on the forces they're imparted so when you're looking at connections there's two primary designs either got your welded connections or your bolted connections typically welder connections are done inside a factory and so they've put together frames that are big enough that are transportable welted together inside the factory and brought to side sometimes you do need welder connections on site be trying to avoid these as welding is a complex design and to ensure that it is done properly and typically on site you're limited by side access and the materials that you have on hand and access to the actual connection so wherever possible not only for design but also for costs is limiting your welded connections because they're both hard to do and expensive to build this brings us to the other point which is bolted connections and this is typically what you want your on-site connections to be and with bolting there's a couple of important factors to look at as not only do you have different classes of bolts we also have different bolt connection types as well so in australia we typically have two bolt types i see that 8.8 which is your high strength bolt that is 800 grade or 4.6 which is your low strength bolt at roughly about 400 mpa there are limitations behind where you should use them obviously when you're standard bolter connection on highly loaded structures you want to use your 8.8 however there are limits on where you can use these 8.8 cannot be welded when they're welded they become brittle through the heat treatment connection so if you've got studs welded to the end of a plate they cannot be 8.8 bolts they need to be 4.6 and as they are on the weaker side as well those 4.6 bolts that typically be done on your purlins and other structures that are not going to be so much governed by string as they're a lower grade they're relatively cheaper your 4.6 volts can also be bent easily as well so typically on holding down bolts where you've got a bent frame a welded frame you'll be using those 4.6 volts and as i was stating there is also a number of different connection types as well you either have slash s slash tb or slash tf the typical connection type for both 8.8 and 4.6 is s or snug tight a snug type bolt is still quite taut you're trying to tension up as much as possible with a big wrench on site however they allow for slip and typically for your standard shear to shear connections so your pin joints is where you'll be using your snog type bolts most of your connection details will be those snug type bolts so they're not only just for your connection types but typically for connections that are done in end plates cleat plates or bolts that are just really transferring shear the second most common type is tb our tb bolts are tightened to a specific requirement and as they are tightened the shear capacity and resistance of these bolts is reduced however they're typically used in connections that are prevented primary and shear and sliding effects so typically in your splice connections is where you use these to prevent your connection from sliding as much as possible and allowing it to resist those sliding actions and then the next connection which is the least used is tf or friction time friction tight is an expensive connection detail and it only really limits slip in critical structures and surface ability limit stones a friction type bolt will still slip under ultimate forces so you should be really trying to limit where you put them they're highly expensive as you not only need to resist the slip inside your structure through tightening them up similar to tb but you also need to treat the surfaces of the two metals that are going together as you need to prevent those slips you need to make sure there is a friction on there to prevent those sliding actions so wherever possible you're trying to limit where you need those friction bolt types to go there's very limited situations you actually need your friction bolt type so two prime reactions the majority is your stock type bolts slash s then for your moment connections is tb therefore critical slip connections is only where you use tf and use it sparingly something you may not know when you're designing your skill structures something that you may not thought about is how big a hole you need inside a bolter connection this is roughly needing a two mil clearance around the annulus of your bolt which is the area of the bolt between the bolt thread and the hole of the cleat plate for example a bolt that is m16 you roughly need an 18 mil hole just something to note when you are putting together how much slip you're actually going to see another thing that i often see that is often overlooked is mismatching of different metal types this is typically mixing steel with aluminium especially on facade framing structures so what's the problem of mixing these type of metals well if you put a aluminium structure and connect it metal to metal to a steel structure you'll have a corrosive action through rusting of the steel frame building this occurs due to the aluminium being higher on the galvanic scale than your steel frame so it'll act as an anode to your aluminium structure similar to anodes that you put on steel reinforcement which will sacrificially rust over your reinforcement same thing happens between the mixing aluminium and steel however the steel has become the anode and it'll rust out and prematurely cause aging inside your structure so when you're mixing these two type of metals you need to make sure they're isolated electrically so there's neoprene pads neoprene washers neoprene covers so there is no metal to metal connection between these two elements as if you do connect them and do not put these isolating pads in you'll have the steel structure artificially age through acting as an anode to your aluminium structure this is similar to how the protection of galvanizing works as the galvanizing is placed on the outside and they typically act as an anode and rust higher to your steel sacrificing itself essentially something to be careful when you're mixing metal types ensuring that you haven't created a connection that's going to artificially age your structure through this galvanic reaction i hope you enjoyed the content and if you do have any pro tips for anyone else designing a steel frame building please comment below and i've got two reasons for you to try and smash that like button if you've made it to this point you've clearly liked the video it's all the way at the end so smash that like button but if this video gets over 500 likes i'll do a full video on the rule of thumb design for steel structures so make sure you smash that like button to get the rule of thumb on steel design if you haven't subscribed at this point just hit the subscribe button for weekly updates and to get all updates you need to ding the bell i look forward to seeing you next week bye

Channel: Brendan Hasty

Views: 206,143

Keywords: how to design steel frame structure, designing steel structures, design of a steel structure, design steel structure, how to design steel buildings, how to to design a steel building, Steel frame design, designing steel framed buildings, structural steel design, designing structural steel building, how to design a steel framed building, designing steel framed structures, brendan hasty, how to design structural steel buildings, steel frame, steel structure design

Id: KWnBNf1INfs

Length: 23min 19sec (1399 seconds)

Published: Tue Jan 19 2021