Calculating Loading Cases To New Steel Frame Residential Balcony 1

[KyleBH]

Hello,

What is the process you would follow to calculate the loadings on a residential steel frame balcony serving 1 family?

The balcony is to be installed against the house - which is constructed of sound brickwork and the client wants two columns to the corners projecting away from the house structure(ideally no third or intermediate column). The steel member against the house brickwork is to be pinned and anchored as required.

Context: client wants a new first-floor steel frame balcony with composite decking
Dimensions: 5m x 2m (length and width - height is 2.5m to top of steel from ground
Regional specs: UK British Standards
Imposed Loading standard: 1.5kN/m2
Balustrade: not in scope

The client wants the steel member to the front to be as slim as possible and the same with the two columns to the front of each of the 5m points to the front of the balcony.

Question: with the above - how would you calculate the total loads for the steel frame balcony to the (a) two columns and (b) front and back 5m steel members. What load cases would you consider?

Kind regards,

Kyle

 

[12345abc6ttyui67]

Why are you asking? This should be a pretty trivial exercise for any engineer with a corresponding degree.

 

[KyleBH]

RandomTaskkk,
The main reason is curiosity and the second being a desire to better understand structural engineering and the ways in which problems are solved. There is a wealth of information on the internet but getting guidance here from how the question is approached saves invaluable time researching and can get me on the right track (hopefully).

 

[phamENG]

Kyle - I think what RandomTaskkk is getting at is that this question is very basic. The sort that an intern would ask the office's junior engineer during his first week. We've all been there, but the internet isn't the best place for this kind of question. If you can't get this basic level of guidance from your supervisor, you need to find a new job.

That said, I'll help as much as I can - I know nothing about UK standards apart from the fact that they are ruled by roughly the same laws of physics that we are in the States (Newton was yours, after all).

Determine what loads your code requires. For a balcony, I would expect dead load, live load, snow load, wind load, and seismic load (wind and seismic can act up or down). These are your vertical loads. You'd also need lateral loads. For load combinations where no wind or seismic lateral is included, it's always good (and may be required by your standards) to apply a percentage of the vertical load as a lateral load.

What kind of deck do you have? Concrete slab? Is it one way or two way? Is it on removable forms or metal deck? Is it a wood frame with planks and joists over your steel frame? This will dictate how the load gets from the people and furniture's feet to the steel frame. Design your floor system, and then apply the reactions to the steel frame supporting it. Use those loads to design your steel frame.

 

[KyleBH]

PhamENG,

Thanks very much for your response. Very helpful.

I guess the question is very basic but I think the answer is very complex as to the method to derive the loads and how these relate to the individual steel members. In the UK, the imposed/live load standard for residential balconies is 1.5kN/m2.

Snow loads are not considered for balconies which makes very little sense...

For simplicity, I'm just trying to understand the verticle loading cases (live and dead loads, not yet seismic or snow loads)

So say an imposed load of 1.5kn/m2 (as per code) + dead load of timber joist & decking 0.25kN/m2 = total unfactored load of 1.75kN/m2

I understand that live loads have a safety factor of 1.6x and a dead load safety factor of 1.4x

thus, 1.5kN/m2 x 1.6=2.4kN/m2 and 0.25kN/m2 x 0.35kN/m2 = total factored loads 2.75kN/m2

Total surface area of balcony = 5m x 2m = 10m/2

Total loading 2.75Kn/m2 x 10m/2 = 27.5kN/m2

So assuming that the load case is only looking at the live and dead loads of a balcony structure - would this seem a reasonable way to determine total loadings for the total area?

 

[strucbells]

Pure speculation on my part (US-based engineer doing all of my work here), but it's possible that snow loading is ignored in the UK for balconies if live load is assumed to govern (since snow loading will be relatively small due to climate).

Some of your units and numbers are jumbled above, but very generally the process is reasonable.

In terms of determining how to apply the total load to individual members, it depends on a number of things: how the balcony is framed, the deck span direction, etc.

It's about as basic as it gets in the structural engineering realm. If you've been assigned this task, ask your supervisor for direction.

 

[Ron247]

I have a question about your anchoring to brick. Are you anchoring to a brick veneer?

Also, the load to each member is based on the load path. As already stated, turn your composite deck a different direction and the load path changes as does the loads to the members.

 

[phamENG]

I do agree that determining the loading can be a bit confusing early on - my curriculum had zero insight into what loads to apply, just how to analyze for the total.

Don't let the code minimums deter you from making intelligent decisions based in engineering judgement. I don't know the wording that you're looking at that makes you think you don't need to apply snow, but I can tell you that if there's a chance for snow there's a chance for snow and live load to coexist on that balcony (live load is more than just people - it's the people, furniture, and anything else that goes on there that isn't introduced by the environment). In the US, our load combinations address this possibility.

I know the US is one of the last places in the developed world that still uses ASD regularly. In allowable stress, you have safety factors. In limit states design (which I believe is what you're using, based on the stated factors), you have something else. I'll use my US terminology again here - we call them load factors. A safety factor says: my load is X, my strength is Y, and I'll reduce that strength by dividing by a factor of safety to provide...safety. This works, but doesn't respect the fact that different load assumptions have different levels of reliability, and different strengths have different levels of reliability (be it ductility, strength, etc.). So with LRFD (LSD for you, right?), we have Load Factors which increase the load based on it's statistical variability. Live load gets 1.6 because people do crazy things. Dead load - we only use 1.4 if it's dead load only - is usually pretty reliable so it gets a much lower factor. Various strength parameters are then reduced by statistically sound decisions - flexure is predictable and only gets 0.9. Bearing on concrete is much less so, so the strength is modified by a much lower factor at 0.65. So be careful calling these safety factors - they are part of a process that serves the same purpose, but they are fundamentally different.

Going back to you the analysis of your structure - follow the load path. Apply your area loads to the decking, looking at is as a a beam with supports where the joists are. Once you've calculated those reactions, apply it the joists. Apply the joist reactions to the supporting steel beam. Then apply its reactions to the column, and the column's reaction to the footing. After you've done that, do some google searching about tributary area. Chase your loads using that method and compare 1) the results and 2) the time it takes to get to the answer. One is generally more accurate, the other is generally faster.

 

[phamENG]

Oh - and the brick. Ron brings up a good point. If it's veneer, it's not "sound" in a structural sense. If it's structural brick, then that's a different story.

 

[MIStructE_IRE]

1.5kPa is WAY too light for me, and I know its the recommended value here. However the size of some of the planters I’ve seen on balconies is frightening. I now just use 4kPa as standard.

 

[phamENG]

I just looked it up (I think in psf) - 1.5 is really light. In the states we do "same as the area being served", which for a bedroom is 30psf (about your 1.5kPa), or 2kPa for anywhere else in a house. For commercial, the recommended is 1.5x the area served, but it generally doesn't need to be more than 100psf (a hair less than 5kPa).

Given the frequent performance issues with code minimum residential construction that I see, I will very rarely design a floor in a house for less than 40psf, and I always use higher deflection standards than the code requires. So all of my balconies are instantly 2kPa, but for high end residences with large balconies (where parties for important friends are likely to occur frequently) I use the commercial 1.5x - so I'd be at 3.5kPa. Not quite as high as MIStructE_IRE's personal rule, but certainly more than code minimum.

 

[kissymoose]

Out of curiosity phamENG, what are some of these frequent performance issues you run into that could be related to live load design values?

 

[jayrod12]

1.5kPa is 30 PSF, I tend to go a bit higher than that. More like 2.4 kPa minimum. Also even though the dead load of a timber framed deck is small, I never use less than 1 kPa (20PSF) unless absolutely necessary, i.e. checking existing conditions. Therefore my total load is around 3.4 kPa unfactored. Much closer to MIStructE's 4 kPa.

 

[JedClampett]

I would be very careful in designing a balcony. In Chicago, there were at least two cases of balconies failing and killing multiple people. Once you've designed it, you can't control how it's used. Planters are one thing, but a drinking party, shoulder to shoulder, is worse. And attaching to brick (veneer; very bad, structural, just bad) makes it scary.
And you're not going to get nuclear level construction skill on a balcony.

 

[phamENG]

kissymoose - a lot of it comes from truss and TJI floors. Some of the building supply centers around here offer in house design for the floor systems. Somebody buys a plan from the internet and sends it to them, and without any guidance to the contrary the design for absolute code minimums and pay little attention to load paths, discontinuities, and the like. Several times, even though I called out specific floor loading requirements and arrangements to avoid some of the problems, the package that comes from them has their own "optimized" layout. House gets built, nobody realized the 10psf dead load assumption, and the tile floor is cracking, the island is sinking, the wood floors are buckling where the non-bear-wall is actually bearing and causing incompatible deflection.

Then there's the older houses that have started to creep. Walls that don't quite align with the foundation girders below, etc.

Most of these could be solved by more attention in design and construction, but there's little appetite here for paying an engineer to monitor residential construction. I've found that the easier way to head off many of these problems is to design the floor to be a bit stronger. It rarely causes any significant waste, and usually saves headaches when the inevitable mistake is made.

 

[kissymoose]

I whole-heartedly agree, thanks for the reply.