Engineering the ATLANTIS in the Bahamas

So…

Add this onto the list of cool things I design!

My engineering firm may be designing a replacement for these huge “roof petals” over the top of a waterslide in Atlantis in the Bahamas! Apparently they're currently made of steel and rusting ALOT. The client would like to do it in welded aluminum (which won't corrode as bad near the water), but I have reservations as to how effective that would be. I also would want to see some cables or bracing added to the system; I'm not comfortable with hanging these 20-ft petals over everyone's head by themselves. Also, the biggest problem with aluminum is that when you weld it, it becomes weaker. This means the end product could be ALOT larger than the steel you see in these pictures. I would prefer to make it out of stainless steel, being sure to heat-treat the welds afterwards (stainless steel is composed of chromium, which is displaced when welded; heat-treating the weld restores the chromium which is stainless steel's main helper to resist corrosion).

Maybe I’ll just charge them a couple nights stay!!

Balcony Collapses and How to Avoid Them

The Causes

The main causes for deck collapses are either post and railing failures, or the connection of the deck ledger board to the house’s rim joist. The ledger board is usually a 2 by 8 or 2 by 10 pressure treated board that is bolted or screwed to the house and holds the joists of the deck. It is estimated that 80 percent of the deck collapses occur at the ledger to rim joist connection.

Deck ledgers should be screwed or preferably bolted to the building rim joists not merely nailed. Properly installed, bolts and lag screws will withstand the expansion and contraction of the wood. They can loosen over time, but will not pull out as nails would. You will notice a gap between the deck and the house is the bolted or screwed connection is loosening. If the ledger board has been nailed to the house, any lateral movement may cause the deck shift to the point of collapse.

Lag bolts and screws must be spaced properly to maintain the deck’s connection to the house. If bolts are spaced too far apart, they can bend if the deck becomes overloaded or is loose. Thirteen people died in a deck collapse in Chicago due to improperly spaced lag bolts.

If the connection between the ledger board and rim joist is not flashed properly, or if flashing was not installed at all, water can get behind the ledger board and cause the rim joist on the house to rot. This is because rim joist are not pressure-treated and will be susceptible to rot if they remain wet over long periods of time Flashing can be installed in the form of either a metal coil cut to the length of the ledger board and bent to fit under the siding and cover the top edge of the board, or in pre-formed flashing that comes in various lengths. Aluminum flashing should not contact pressure treated wood due to a corrosive chemical reaction of these two materials causing the aluminum to dissolve completely over time. New pressure treated wood uses alkaline copper quaternary (ACQ) chemicals for preservation, which is even more corrosive then previous treatments that were applied to wood. You should make sure that the correct anchors were used for attaching the posts to the pier and that cast aluminum porch post supports are not being used. This change in chemical treating also does not allow for making repairs to existing decks. ACQ in new wood will corrode existing connectors within three years. Decks need to be replaced in their entirety once decay has been observed.

Life Safety - Railings

Railings are one of the most obviously safety issues with decks and balconies because it is readily apparent when they are loose. There are many guidelines for railing design and requirements for installation which include height, grasping ability, design loads in both vertical and horizontal direction, space between railings, and even spindle location. Railings also need to be securely fastened to the deck framing with bolts that go completely through the post into a beam or joist. It is unacceptable to notch posts, so make sure that the deck’s support posts are separate from the railing posts. It is important that you inspect the railings at least twice a year and even a sound deck may have problems with the railings that need to be corrected.

The wood used to construct decks should last 20 to 25 years if properly maintained, but decks that are shaded can degrade faster as the wood does not dry out. This process is made worse if the wood has not been stained or painted. In cases such as this the wood loses it structural integrity as well as the ability to hold a fastener.

As wood ages it will expand and contract, causing graying, splintering, and cupping. This is also a sign of the wood losing its structural capabilities and nearing the end of its life cycle. The older the wood is, the more nails and screws will be pulled out from the decking and joists.

Make sure you have a qualified engineering firm that is competent in analysis of this kind, such as Engineering Express, to ensure appropriate protection and/or remediation.

FAU Needs Duct Tape! And Fast!

Wow. Amazing sometimes how you can look back at a bad idea. I'm sure whatever engineer designed this walkway at FAU's student apartments wouldn't sleep at night knowing this was hanging over the student's heads:

I'm personally surprised that the bearing key (the block of concrete holding the slab up) was made so small. Over time, the expansion and contraction (i.e. movement) of the walkway probably caused the cracking seen here. It looks like the contractor made it worse by putting just one set of steel shoring posts at the wrong place. The posts needed to help hold up the heavy slab that's causing the cracking, not the small key that's already busted! Hopefully Engineering Express will be called out to fix this mess; I wonder if other engineers would know how to fix this correctly.... and, no, NOT with duct tape!