Today, we had the opportunity to take a look at the shell of an old block building that has been in disrepair for many years. The building has several interesting masonry and structural elements. Today we’ll talk about some of those details and show some different visual aspects of the building which have implications in the structural and masonry construction.
Span and required trussworkThe building that we are looking at here in today’s article is a relatively large span. typical building spans of up to 18 ft can normally be accomplished or built with a supporting framework of lumber at regular interval spacing such as 16-in spacing. Short span buildings as low as 12 feet can be supported with roof or inter-floor type decks of relatively low thickness, but larger span buildings require larger thickness lumber to support the load. The relationship between the span and the thickness of the lumber is also impacted by the spacing of the joist or trusses between each joist or truss. In other words, if a building span is longer, in some cases the same size lumber, or boards, or trusses can accomplish the support structure for that same span if the spacing between those elements is closer. In most cases though, for practical purposes the smallest span built between framing members is about 12 in. Below that distance of about 12 in, the span is generally so tight that it makes it impossible to work within the space during construction. In many cases, with building framing, the span or space between structural elements such as joist, rafters, or wall studs, is intentional and beneficial because the void can be used to install needed elements. Some of the most common types of needed elements may include wiring, ductwork, cabling, piping, and insulation. The span in this particular building though is so large that the framework of trusses is built with steel for additional strength and a purlin system is set up spanning or running over each truss to support the lightweight corrugated metal deck roof above. Although the block and concrete walls of this building are particularly strong, even in comparison to historic or traditional historic brickwork, the lightweight deck of the roof significantly helps to decrease the load of the roof. at the center of each truss. Concrete opening headersTypical historic buildings in Capitol Hill, for example, can have door and window openings built with a variety of different materials, but overwhelmingly the most common type of opening header is a segment arch brick header. The historic brick headers in Capitol hill, particularly the segmented headers are often not self-supporting. Beyond a certain width of about 24 in, they require interior support, generally in the form of a wood beam that runs parallel to the segmented arch. By comparison Roman headers which are best described as a semicircle type of arch are generally self-supporting, indefinitely, with routine required upkeep such as repointing or masonry tuckpointing and historic mortar restoration. Here though these openings are built with concrete. There’s a few reasons why concrete works better in this type of arrangement. Concrete, with typical steel rebar or deformed reinforcement bars embedded into the concrete can support very wide spans. in a particularly large building, it’s much better to have large openings to allow fenestration and light from the outside to enter the inside of the building. Concrete elements, like historic brick elements, also require maintenance and restoration and upkeep, but they do not require the typical type of pointing a repointing needed for historic brick. When you look closely at the rough shape of the concrete header, you can see that the concrete header was cast in place and in this particular case an additional amount of concrete had to be added to the form in a partial separate batch. The fasteners temporarily used to support and hold the framework of the form together are left exposed on the face of the concrete header. This concrete header, though rough in finish quality, will be covered with a stucco or cementitious parge coat or some type of finish material to hide the imperfections in the concrete. The concrete was never intended to be an exposed finish in the building. Here though in this particular case we are getting the opportunity to look at this building in the middle of its original construction, frozen in time, a bit like a time capsule. Although these imperfections may look imperfect from an aesthetic perspective, they pose very little to no structural concern. Corbelled bearing header at postsCorbelled wall headers and corbelled brick footings are common in historic buildings, even corbelled precast concrete posts are relatively common in precast construction, but less common in cast-in place construction as shown here in the example. Below, we explain a few terms that are part of this understanding. Corbelled wall headers are common architectural elements found in Washington DC neighborhoods such as Capitol hill, Dupont circle, and Georgetown where the historic brick buildings were built over 100 years ago in most cases these elements are for architectural and decorative purposes only and provide little if any structural purpose. Corbelled brick footings, unlike wall headers or common in the historic buildings of Washington DC, particularly in row homes built around the early 1900s or late 1800s. at the time, when brick was beginning to be used for foundations, large footings would be buried underground and directly from the large footing which could be up to four to six bricks wide, a tapered increasingly thinner stem wall of the foundation would be built above that footing up until it reached an area just above grade. In most cases the historic foundation walls of Washington DC will be built triple wythe, meaning in most cases, at that point farther above grade up until a second or third floor level, the walls will be built in a double wythe assembly. Precast concrete can be found in small or large scale construction. In some cases small elements such as underground drain tubes or decorative elements can be built with precast concrete. These elements are assembled, cast into forms, stripped, and cured in a factory or plant off site. By comparison, cast in place concrete is brought or mixed directly on a job site or construction site in a wet form where it is hydrated and then poured into a field built form. In this case, because of the fasteners and lippage apparent in the exposed faces of the concrete, we are certain that this concrete was cast in place and not precast. Corbelled concrete headers in most cases, would be built in a precast plant and not built on site because the complexity of building the corbelled structural elements may be more cost-effective in a plant than on site, depending on several factors such as proximity and availability of options. In the picture below you can see a view, looking up the shaft of the cast in place concrete column with a corbelled header. At each trust tail, a corbell is located below the truss to support the weight of the truss which supports the purlins which support the roof deck. The corbelled header, in each of these cases is both a location which is wider than the remainder of the column for the purpose of mounting the truss tail, and this widened corbelled header also has the capacity for additional strength for support of the roof. From our visual observations of the building, alone, it’s not clear why the block work at the top of the wall, particularly the topmost two courses are different than the blockwork below. It’s possible that the building construction paused for a time between the construction of the walls and the installation of the roof system and only after the installation of the roof system, the contractor was able to then install the remaining block at the top of the wall. At that point in time, a separate point in the timeline of the project, the contractor may have chosen to change the vendor of the block or the supply chain and Associate availability may have changed such that the original block was not available any longer. this particular difference may not matter in any way to the finished product of the building, as the block wall will likely be covered completely. Concrete Post and BeamsAside from the large load path span and tie backs, the scale of the buildings can change significantly without necessarily changing much about the methodologies of construction. Except, in this case there is also one other big difference between the historic rowhome brick construction versus contemporary and / or newer large scale warehouse sized buildingsn such as the concrete posts and beam system between the blockwork. The concrete post and beam system which blends in, visually, with the adjacent block work, is the core structural framework to support the building. The block work also has a significant structural bearing capacity, but the post and beam system ties the frame of the building together in all areas. The vertical concrete post extends the structural load path from the footing, all the way up to the underside of the roof. the horizontal beams that run between each concrete post allow the block to be supported within certain vertical lengths so that excessive weight bears on no individual spot of block work. Historic masonry upkeep and preservationTo properly maintain, repair, and care for these historic buildings, a knowledge, interest and understanding of historic building principles is required. Here in Washington DC, historic masonry buildings are extremely expensive and the amount of financial loss caused by improper repointing and low quality construction is staggering. However, in addition to the direct financial value of the property, there is also a cultural loss when historic buildings are damaged. By comparison, consider neighboring poor cities, when historic buildings are damaged, it’s not just the loss of value to the property owner, there’s also a loss to all inhabitants and visitors of a city, present and future, who care about architecture, history, and culture. We encourage all of our clients, and all readers of this article and to our blog in general, to prioritize the historic built environment of Washington DC and neighborhoods such as Capitol Hill, Dupont Circle, and Georgetown and become educated on on the difference between proper historic preservation versus improper work which leads to significant damage to the historic fabric of a building. From a conservation and preservation perspective, several approaches can be taken to improve conditions related to deteriorated historic brick masonry. Primarily, lime mortar brick joints and low temperature fired soft red clay bricks should be inspected and checked on a routine maintenance schedule, either seasonally or at least annually. If brick masonry is kept in good condition, the life of embedded wood elements can be significantly extended. Hire a professional contractor which specializes, understands and appreciates historic construction elements and buildings. You can learn a lot more on our blog. Feel free to check it out. If you have questions about the historic masonry of your building in Washington DC, contact us or fill out the webform below and drop us a line. We will be in touch if we can help. <p>The post Case Study of a Large Block Masonry Building Shell – Part I of II first appeared on Infinity Design Solutions.</p> Via https://www.ids-dmv.com/masonry/case-study-of-a-large-block-masonry-building-shell-part-i-of-ii/
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About UsInfinity Design Solutions LLC (IDS) is a full service general contracting company in the heart of the Dupont Circle neighborhood of Washington, DC. We focus on repair and renovation of buildings and facilities in both historic designated neighborhoods and the commercial-zoned central business district of the city. Follow Us
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