| This article focuses on the most misunderstood law | | | | So, what is the difference between the point load |
| of structural physics that all too frequently results in | | | | and the uniform load? The simple answer is:a lot! |
| very costly remediation and occasionally, can cause | | | | Without going into the engineering complexities of |
| complete collapse. The structural system here is the | | | | moments of inertia, section moduli, and even bending |
| simple post and beam system that occurs in nearly | | | | moments, suffice it to say that a point load is a lot |
| every building type whether you're building with | | | | harder on a beam than a uniform load. This is why if |
| wood, steel, concrete or any other structural | | | | you ever find yourself on thin ice, you should lay flat |
| material. | | | | out and distribute your weight evenly rather than |
| Let me set the stage here. You're out on a job site | | | | standing (and breaking through). |
| of a new construction wood framed two story | | | | Imagine a 10? length of 2×4 supported at each |
| office building that has a structural steel I-beam that | | | | end by cinder blocks. If I hang a 1 pound bag of sand |
| carries the load of the main level floor deck joists | | | | at each 1 foot interval, I would have 9 pounds of |
| along with the load from a second level floor deck | | | | sand hanging uniformly from this board in one pound |
| through a continuous bearing party wall on the main | | | | increments and it would bend a bit (I'm using 9 |
| level. The plans call out for a w10×45 steel | | | | pounds because the pound at the cinder blocks |
| I-beam in the main level floor deck to carry that load. | | | | doesn't really count). Now, if I were to take all 9 |
| The owner shows up and tells you that they leased | | | | pounds and hang them from the middle of the board, |
| two of the units together, and now we have to | | | | what do you suppose would happen? I encourage |
| create a 12? opening in that bearing party wall so the | | | | you to try it, but for those of you who just can't |
| spaces can be connected. The contractor calls the | | | | wait that long, I'll tell you: The board will deflect |
| lumber supplier and asks generally what size beam | | | | considerably more. In fact, it will deflect as if you had |
| would be necessary to carry a second floor deck | | | | 2 pound sacks distributed evenly at 1 foot intervals. |
| load and an 12? span. The beam is sized and the | | | | In simpler terms: double the effective load. |
| project continues moving forward. | | | | The physics behind this phenomenon are very |
| Does anyone see any problems with what was just | | | | extensive and there are a great many number of |
| described? Most framer's would say "that happens all | | | | variables involved. The point here is that it is |
| the time". It does happen all the time and there is a | | | | important to understand that the way you design |
| major concern here because by making that change, | | | | the structure in a building depends very heavily on |
| there is a better than 50% chance that you are now | | | | how that structure is loaded. This is one of the many |
| overstressing the steel I-beam below that was | | | | direct ways we are able to value engineer projects |
| designed to carry the uniform load from the upper | | | | as we design the architecture for them. It is also |
| and main level floors together. How can this be if the | | | | why remodel projects can be so difficult because the |
| resulting load to the foundation hasn't changed? | | | | forensic engineering required to determine the |
| The answer is in the load path. Different load paths | | | | existing load paths and how the new paths will affect |
| have different effects on the physical structure of | | | | them can easily eclipse the time it takes to actually |
| the beams in these buildings. On a simply supported | | | | prepare the drawings themselves. |
| beam carrying a uniform load (like a uniform layout of | | | | And most importantly, it is why you should never |
| floor joists for example), we calculate the load on | | | | make assumptions in the field about how to change a |
| this beam in terms of a linear loading of pounds per | | | | load path for those inevitable on-site changes. Always |
| linear foot. Once you have this calculation, you can | | | | involve the architect or engineer and if they can't |
| consult any number of tables or charts and size the | | | | feasibly visit the job site in time, explain everything |
| appropriate beam. However, if you have a point load | | | | that you're doing in detail from the top all the way to |
| from a load bearing column, you have to reduce that | | | | the bottom so they can assess every part of the |
| load into an equivalent uniform load so you can have | | | | system. The consequences of those assumptions will |
| an apples-to-apples comparison and use the same | | | | almost always be far more costly and no one wants |
| charts that you have for uniform loading. | | | | to be on that kind of thin ice. |